Republican megadonor and former Breitbart News investor Robert Mercer has all but vanished from the political donor scene after being one of President Donald Trump's leading supporters during his run for president in 2016.

Donors, strategists and Trump allies, who spoke on the condition of anonymity to speak openly about the situation, said the conservative financier has retreated from leading the way in financially backing the president's agenda.

Mercer and his family didn't make as many political contributions in 2018 as they had in previous cycles, especially in contrast to the 2016 cycle, when Trump ran for president. Political strategists believe Mercer's lack of involvement this year should sound the alarm for Republicans looking for assistance in the fundraising game when they fight for Trump's re-election in 2020.

Mercer still gave millions this year, but the sum didn't stack up with previous years' amounts. He gave $6.4 million to Republicans, the lowest amount since 2012, when he and his wife, Diana, contributed $5.8 million, according to the nonpartisan Center for Responsive Politics. The Mercers finished the 2016 election as one of the top 10 political donors, by giving just over $25 million to conservative causes.

Robert Mercer typically avoids public scrutiny, but his involvement with Trump thrust him into the political arena — an unwelcome development for him.

"They've disappeared," a prominent Republican bundler told CNBC, referring to Mercer and his daughter Rebekah, another prominent GOP backer. "I think they just don't like being in the spotlight," this person added.

"He's out," said a former associate of Mercer. "He's not going to play any major role going forward. They're tired of the attacks from the Democrats and the media constantly ganging up on them."

Notably absent from Mercer's 2018 contributions is any direct funding of Trump's campaign or his joint fundraising committees, which funnel donations to the Republican National Committee. The president's fundraising operation appears to have done fine without Mercer's help. Between his campaign and two joint fundraising committees, Trump managed to raise more than $100 million throughout this year.

Another leader in the GOP finance world said: "He really scaled back since all the Cambridge news."

During the presidential election, Mercer funneled millions of dollars into supporting Trump's run, including investing at least $15 million into the now-shuttered data gathering firm Cambridge Analytica, which was used to harvest private information of more than 50 million Facebook profiles.

Rebekah Mercer was on the board of the data-gathering company, which also boasted former Breitbart News boss and top Trump advisor Steve Bannon as an executive. The information that was gathered was used by the Trump campaign to create social media ads in an attempt to influence voters. The Department of Justice and the FBI are reportedly investigating the company.

Robert Mercer, through a spokesman, declined to comment on this story. A spokeswoman for Rebekah Mercer did not return repeated requests for comment.

A year before the midterms, Mercer stepped down as CEO of hedge fund Renaissance Technologies and sold his stake in Breitbart News, the conservative online news platform, to his daughters. In his letter to employees at the time, Mercer explained his reasoning for backing conservatives and his frustrations with the way he's been portrayed in the media.

"A society founded on the basis of the individual freedom that flourishes under a limited federal government has no place for discrimination," Mercer said at the time. "Of the many mischaracterizations made of me by the press, the most repugnant to me have been the intimations that I am a white supremacist or a member of some other noxious group."

The Mercers were also involved with propping up Trump's campaign through donations to Make America Number 1, a pro-Trump super PAC that was run by Rebekah Mercer. Her father was the PAC's top donor, with contributions ranging from $2 million all the way up to $11 million.

The PAC was named in a Federal Election Commission complaint by the nonpartisan watchdog group, Campaign Legal Center, in October 2016. The watchdogs claimed that Make America Number 1 broke FEC regulations by directly coordinating with the Trump campaign.

According to its latest filing, the PAC is still paying for legal counsel. The filing shows that the PAC paid high-profile law firm Greenberg Traurig LLP $118,580 in November for legal consulting. It's unclear whether the payment is related to the FEC complaint that was levied against the PAC two years ago.

When asked about the legal assistance being given to the PAC, the firm would not confirm whether their work has anything to do with any legal complaint.

"As you can see publicly, we have been their compliance counsel," a spokeswoman said.

Representatives for the FEC did not return requests for comment.

Most recently, another Mercer investment came under scrutiny as federal prosecutors in Manhattan reportedly opened an investigation into Trump's inaugural committee with a focus on the way it raised and spent its finances. Mercer contributed $1 million to the committee, which raised $107 million, almost double the amount brought in by Barack Obama's inaugural committee in 2008.

He recently wrote a check of $400,000 to a pro-Trump super PAC, the Great America PAC. Outside of that, however, he spent more to support the RNC, the National Republican Congressional Committee and super PACs helping Senate and House GOP candidates, including the Congressional Leadership Fund and Remember Mississippi.

Mercer also wrote a late $2,700 check to Sen. Ted Cruz, R-Texas, and another to Rep. Martha McSally who was seeking retiring GOP Sen. Jeff Flake's seat in Arizona. Cruz went on to edge Democratic challenger Rep. Beto O'Rourke, and McSally lost to the Democratic nominee, Kyrsten Sinema. McSally was recently named to fill the late Sen. John McCain's seat after Sen. Jon Kyl, who had been appointed to fill McCain's seat in September, announced he would be leaving Capitol Hill at the end of the year.

For the upcoming 2020 elections, political strategists argue that Mercer's attempt to stay out of the limelight and, in turn, limit his contributions, embodies a larger problem for the president and Republican lawmakers: an overall cutback in funding as the party and the president see their popularity wane. A recent Quinnipiac University poll gave Trump a 39 percent approval rating.

"Losing Mercer would be a great loss. Any party needs as many people like him as possible," political strategist Hank Sheinkopf told CNBC in an interview. "You don't have to be a great forecaster to see that the GOP are going to have a real problem in 2020 because Republican futures right now are not something you want to put a lot of cash behind."