No matter how the Supreme Court rules on Donald J. Trump’s challenge to a Colorado court ruling barring him from the state’s primary ballot, the case has already been a win for one group.

Oral arguments in the case on Thursday represent the culmination of a long and sometimes fitful evolution for Citizens for Responsibility and Ethics in Washington, or CREW, the watchdog group that initiated the case. The group helped find the plaintiffs who brought the case in Colorado and funded the lawsuit arguing that Mr. Trump is ineligible for the presidency because he engaged in an insurrection by promoting the storming of the Capitol by his supporters on Jan. 6, 2021.

Its pivotal role in the case comes amid intensifying pressure from donors for technically nonpartisan groups to take unambiguous and aggressive sides in a polarized Washington.

Since it was founded in 2002, CREW, which is registered under a section of the tax code for nonpartisan nonprofit groups, has been caught in a tug of war between Democratic donors who wanted it to wage political warfare and less partisan supporters who wanted to expose corruption and ethical lapses regardless of party. It often struggled to raise money.

Mr. Trump’s emergence as a political force — and his disregard for legal and ethical norms — fundamentally changed the equation. During his presidency, CREW was able to satisfy both sides of the internal debate by training its sights almost entirely on him and his allies as they flouted ethics rules.

CREW’s fund-raising soared, as donors poured cash into a host of left-leaning watchdog groups that came to be seen as a part of the resistance to Mr. Trump. CREW raised $28 million during Mr. Trump’s presidency, more than triple the $8.5 million it raised during President Barack Obama’s final term.

“You ever hear of CREW? A bunch of losers,” Mr. Trump told supporters at a rally in Iowa in November.

After the Colorado Supreme Court ruled in favor of CREW to remove Mr. Trump from the ballot, his campaign put out a statement declaring the group a “radical, left-wing activist group.”

CREW disputes that characterization, but after Mr. Trump left the presidency, the group quickly learned that some of its donors wanted the focus to stay on the former president.

Supporters balked when CREW filed a complaint against President Biden’s first press secretary, Jen Psaki, for appearing to endorse Virginia’s Democratic candidate for governor, Terry McAuliffe, from the White House podium. (Government officials are barred from using their offices to influence elections.)

“That’s not going to help you with people who are the normal contributors,” Albert J. Dwoskin, a longtime CREW board member and donor, said he warned CREW’s executive director, Noah Bookbinder, about the complaint.

“It’s not hard to find misbehavior in either party,” Mr. Dwoskin, a Virginia real estate developer and major donor to Mr. McAuliffe, said in an interview, but “you have to look at your donor base.”

Other major donors grumbled that the group should remain focused on Mr. Trump because they felt any wrongdoing by Democrats was trivial compared with the existential threat he posed to democracy, according to people familiar with CREW’s donor pool.

“It’s a lot easier to raise money when you are hyperpartisan,” said Louis Mayberg, a wealthy investor who was one of CREW’s founding board members.

“Even back in the earliest days, we had to walk that line to raise money because people were partisan and they wanted to give because they liked what you were doing that they agreed with,” Mr. Mayberg said. “But if they gave to you, and they didn’t like what we were doing, we were always dealing with the blowback.”

In an interview, Mr. Bookbinder acknowledged that in recent years his group has “had to back-burner a lot of other investigations into Republicans and Democrats.” He said that is because “there is one locus of real threats to the continuing viability of the democracy. That’s Donald Trump.”

But he said CREW had not been swayed from a commitment to calling out ethics issues on both sides of the aisle. In addition to the complaint against Ms. Psaki, which a federal agency confirmed as a violation of the Hatch Act, CREW criticized the White House’s ethics guardrails governing Hunter Biden’s art sales and called for the resignation of Senator Robert Menendez, Democrat of New Jersey, after he was charged with accepting bribes to help foreign countries.

“We’ve had donors feel like we’re too far to the right, and donors who feel like we are too far to the left,” Mr. Bookbinder said. “We’re not a political organization. We make decisions based on what we think is appropriate based on the facts and the law.”

The polarization is not confined to watchdog groups supported by Democratic donors.

The conservative group Judicial Watch, to which CREW has been compared, rose to prominence by exposing damaging information about the Democratic administrations of Bill Clinton and Mr. Obama, with Hillary Clinton once describing it as part of the “vast right-wing conspiracy.”

But the group also directed some scrutiny at Republicans, initially making efforts to obtain records related to Mr. Trump’s government spending, including at his properties.

Eventually, though, it came mostly to target Mr. Trump’s perceived enemies, including inside the federal government. Its fund-raising soared from nearly $135 million in the last four years of the Obama administration to $326 million during the four years of the Trump administration.

The group’s longtime president, Tom Fitton, said its approach was not driven by its donors’ partisan passions.

“We try to focus on issues, so it doesn’t matter,” he said. “We thought that the key issue during the Trump years was the government abuse of Trump.”

Last month, Judicial Watch and another conservative nonprofit group filed a brief with the Supreme Court opposing CREW’s ballot disqualification push. It’s a far cry from the days before Mr. Trump when right-leaning watchdog groups like Judicial Watch and left-leaning ones like CREW worked together to push for greater government transparency.

“CREW did some good transparency work, but they’ve become extremist recently,” Mr. Fitton said — an assessment that echoes the criticism of his group by left-leaning watchdog groups.

CREW used to routinely investigate powerful Democrats, and include them alongside Republicans on the group’s annual list of the most corrupt members of Congress.

At one point, Representative Nancy Pelosi, a frequent target of the group while she served as House Democratic leader and speaker, urged major Democratic donors to withhold funding from CREW, according to two people familiar with the situation.

It is not clear if Ms. Pelosi’s pressure dissuaded any donors. But the organization never raised more than $3 million in any given year during its first decade, or had a staff larger than 25 people, according to tax filings. Things got worse with the death in November 2013 of Peter B. Lewis, who had donated hundreds of thousands of dollars each year.

One of the leading big-money operatives in Democratic politics sensed opportunity.

David Brock, a self-described right-wing hit man who switched sides and became an enforcer for the Clintons, engineered a takeover of CREW in the summer of 2014. He incorporated it into a fleet of nonprofit groups and super PACs that he had created over the years to elect Mrs. Clinton and other Democrats, and to attack Republicans and their allies.

Under Mr. Brock’s stewardship, CREW pulled back from some criticism of Democrats. It stopped pursuing a Freedom of Information Act request for emails sent and received by Mrs. Clinton using a private server while she was secretary of state. At the time, the emails had become a liability for Mrs. Clinton’s presidential campaign, which was being supported by another group Mr. Brock had created.

Weeks after Mr. Trump’s victory, in an effort to allay internal concerns that the group was becoming too partisan, Mr. Brock left the board. But the group remained affiliated with his network of donors and nonprofits, which took on an even more aggressive and adversarial tone.

Within days of Mr. Trump taking office, CREW had sued him for violating the Constitution by allowing his company to accept payments from foreign governments for hotel rooms and other services. The lawsuit bounced around the courts until Mr. Trump left office and the Supreme Court effectively dismissed the case.

CREW also filed a litany of complaints that yielded findings against Trump administration officials for violating the Hatch Act, a federal law prohibiting executive branch employees from using their office to try to influence elections.

CREW stocked its board with Democrats and anti-Trump Republicans.

As fund-raising soared, the group tripled its staff. It cranked out more complaints against the Trump administration and its allies, which in turn likely fueled more fund-raising.

It still applied for federal assistance intended to help weather the Covid pandemic, securing more than $1 million in loans that were forgiven, according to records.

After Mr. Trump lost to Mr. Biden, CREW promised donors it would keep after the former president, according to internal documents.

A strategic plan for 2021 and 2022, marked “private and confidential,” boasted that CREW would pursue “appropriate consequences” for Mr. Trump and his allies, “potentially including jail time, civil fines, economic losses for complicit corporations and organizations, and the humiliation of being hauled into public hearings and written up in devastating reports.”

The goal, it said, was to “simultaneously damage their public standing and help restore Americans’ faith in government.”

Fund-raising in the first two years after Mr. Trump left office fell short of lofty goals listed in the internal documents, though the group continued raising more than it had before Mr. Trump took office.

CREW appears to be trying to reduce its reliance on major Democratic donors, and to sever some ties to the Democratic establishment. After attending Mr. Brock’s donor conferences in prior years, Mr. Bookbinder turned down an invitation in 2021.

CREW has begun investing more heavily in expanding its network of small donors, paying a digital fund-raising firm starting in late 2022. Late last year, it parted ways with Mr. Brock’s longtime fund-raiser, and Mr. Bookbinder resigned from a Biden administration advisory board.

Mr. Bookbinder dismissed the idea that the push to disqualify Mr. Trump was in any way linked to donor preferences, noting that not all donors agree with the effort.

“There are some who feel it’s helpful to Democrats to have Donald Trump on the ballot, or that this is going to excite Donald Trump’s base,” he said, while others feel it is anathema to democracy to take decisions out of the hands of voters.

“We are very comfortable that this is not just pro-democracy, but necessary for democracy,” he said, “and that’s something that I have explained to press, I have explained to partners, I have explained to donors.”