Sen. John Thune (R-SD) after the weekly Senate GOP policy luncheon at the U.S. Capitol May 19, 2015. (Photo by Chip Somodevilla/Getty Images)

Sen. John Thune (R-S.D.), a popular conservative facing token opposition in November, said Wednesday he is donating $2 million in campaign cash to the Republican effort to keep control of the Senate.

At the same time, Sen. Charles E. Schumer (D-N.Y.), the would-be majority leader if Democrats win in November, made his own $2 million donation to the effort to rip control of the Senate from Republican hands, according to a source familiar with the move. Schumer also faces only nominal opposition in his bid for a fourth term in November.

Thune, the third-ranking leader who is chairman of the Republican Conference, said he made the announcement at a campaign-focused meeting of the GOP caucus at the National Republican Senatorial Committee a few blocks from the Capitol. The funds will go from Thune’s own re-election committee to the NRSC’s coffers, where it will be turned directly into campaign ads and other political efforts toward hanging onto the Senate majority — which observers think may be threatened by the performance of GOP presidential nominee Donald Trump.

“The stakes are high. I think the Senate is the last kind of check and balance,” Thune said in an interview after the closed-door meeting, echoing an argument Republicans are starting to make on the campaign trail if Hillary Clinton becomes president. “And, you know, I think we have to be prepared for however the presidential race goes and the Supreme Court’s hanging in the balance.”

That referenced Trump’s late-summer troubles — his campaign fortunes are intertwined with the Senate majority as almost 10 states are battlegrounds for both the presidential contest and Senate contests. Some Republicans have grown more optimistic about holding their majorities in Congress in recent days as Trump has avoided major gaffes and Democratic nominee Clinton has struggled with lingering questions about an FBI investigation into her email practices while serving as secretary of state.

[Democrat Chuck Schumer, one-man super PAC]

Schumer’s donation was announced by Sen. Jon Tester (Mont.), chairman of the Democratic Senatorial Campaign Committee, at the Democratic luncheon in the Capitol, the source said. A voracious fundraiser, Schumer had $27.5 million in campaign cash as of June 30, according to Federal Election Commission records.

Federal law allows candidates such as Schumer and Thune to declare funds as “excess” cash and transfer it to other party committees, but not directly to individual candidates.

With Senate Minority Leader Harry M. Reid (D-Nev.) retiring, Schumer will is likely to become the next leader by acclamation as no one is challenging his rise. The question is whether his new title will come with “majority” or “minority” in front of it.

Democrats need to pick up four seats if Clinton defeats Trump, leaving the Senate 50-50 and lead to Tim Kaine, as the new vice president, delivering the majority as the tie-breaker in their favor. They need a net gain of five seats if Trump wins the presidency.

Thune decided to go all-in to help his colleagues. His name has been synonymous with “rising star” since ousting the Senate Democratic leader, Thomas A. Daschle, in a hard-fought race in 2004. But, rather than make a risky leap in a presidential bid or angle for higher leadership posts, he has taken the slow-and-steady approach by rising up the ranks through lower-level leadership posts and at the committee level.

In addition to his leadership position, he now chairs the Commerce, Science and Transportation Committee, which has vast oversight of issues ranging technology and communications to rail safety.

Ever since that 2004 race, Thune has faced little to no opposition. South Dakota Democrats failed to field a candidate against him in 2010 and this cycle have a candidate, Jay Williams, who has raised just $36,000 for the longest of long-shot campaigns.

“I’m fortunate enough to be in a position to be able to help out,” Thune said. “I don’t need all the money I have, but other people do.”

He said he now has about $10 million left in his campaign fund.