WASHINGTON >> Wealthy Republican donors, viewing Donald Trump as a losing cause, are turning their firepower toward several key Senate races, threatening to bury Democrats under a deluge of negative advertising.

In North Carolina, the One Nation advocacy group just poured $1 million into an ad buy in support of Sen. Richard M. Burr, a Republican who has been buffeted by Trump’s difficulties in the state. That was on top of a $1.5 million infusion weeks before.

The group, which does not disclose its donors, is run by allies of Sen. Mitch McConnell of Kentucky, the majority leader, and Karl Rove. It has already spent at least $25 million on other close Senate races, including those in Pennsylvania and New Hampshire, Federal Election Commission records show.

The Freedom Partners Action Fund, a super PAC aligned with Charles G. and David H. Koch, has spent roughly $15 million on TV ads, a quarter of it in Pennsylvania, where Sen. Patrick J. Toomey is in a tough re-election fight, at least in part because of voters’ distaste for Trump.

The U.S. Chamber of Commerce has ignored Trump but spent around $15 million helping Republican senators in competitive races in New Hampshire, Pennsylvania and Ohio, as well as in places like Arizona, typically a safe Republican state.

“A lot of investors at our level are interested in a concrete plan and strategy,” said Steven Law, the president and chief executive of One Nation and the affiliated Senate Leadership Fund super PAC.

The race for the White House “is too volatile for them to feel comfortable investing in, and they see that,” Law said. “And there is a pathway for us to hold the Senate majority, and they are very eager to invest in that.”

The negative ad saturation has shown clear results in Ohio, where Sen. Rob Portman, helped by several outside groups, has been hammering former Gov. Ted Strickland, his Democratic challenger, for nearly a year. Portman holds a sizable lead in polls.

Democrats need to gain five seats to retake control of the Senate after the Republicans won it two years ago, or four if Clinton wins the presidency, giving Tim Kaine, her vice president, a tiebreaking vote.

Many Republican donors who either reject Trump or simply fear he will lose believe that maintaining control of the Senate is their only hope of standing up to major policy changes with Clinton in the White House.

“They decided that they couldn’t be for Trump,” said Sen. Chuck Schumer, D-N.Y., who will most likely be majority leader if Democrats take back the Senate.

“The Senate is their firewall, and they are going to do what they can to keep it. I wish we could change the rules, and I hope Citizens United is overruled,” Schumer said, referring to the Supreme Court decision limiting the government’s ability to restrict independent political expenditures. “But before that, we can’t unilaterally disarm them.”

One Nation and the Senate Leadership Fund reported raising $42 million in August — both an indication of Republicans’ increasing worry about the Senate and White House races and a clear sign that their attacks will be unrelenting.

“August is almost always a month where you sit by the phone like the Maytag repairman, waiting for a donor to call,” Law said. “What was remarkable was not only the fact we could reach donors, but they reached out to us. There was intense interest fueled by the post-convention drop at the top of the ticket.”

Groups supporting Democrats have not been silent. The Senate Majority PAC and Majority Forward have spent some $30 million combined on ads in competitive races, election commission records show. Democrats have also been aided by traditional allies: labor unions, environmentalists and supporters of abortion rights. But Republicans’ outside groups remain significantly ahead in ad spending.

Democrats’ stronger candidate spending narrows some of the gap. Candidates can purchase advertising at a lower rate than PACs and other outside groups. But in exchange, they are subject to fundraising limits of $2,700 per contributor per election, while outside groups can raise and spend unlimited funds.

Some donors to groups supporting Republican Senate candidates are clearly in the camp that opposes Trump’s candidacy. For instance, two major Senate Leadership Fund donors — Warren Stephens, an investment banker, and Paul E. Singer, who runs a hedge fund — previously gave $2.5 million each to an anti-Trump super PAC, according to reports filed with the FEC.

Portman, whose courtly demeanor runs counter to the tenor of his attacks on Strickland, demonstrates what money and old-fashioned negative advertising can do. He amassed a large campaign fund and turned toward negative campaigning more than a year ago, attacking Strickland early in 2015 for his support of the Iran nuclear deal and never letting up.

Republicans tagged Strickland as “Retread Ted,” turning his age and experience against him and criticizing the state of Ohio’s economy on his watch as governor. Strickland has not been able to emerge from the barrage to redefine his candidacy in a more positive light.

Portman has had other advantages: a giant grass-roots team with over 1,000 college interns; the support of the popular sitting governor, John Kasich; and a strong focus on issues like the state’s opioid abuse problem.

But the piles of money and negative ads have clearly put Portman in a commanding position. Last month, the Democratic Senatorial Campaign Committee said it was holding off on any further spending in Ohio, essentially abandoning Strickland.

While Republicans played a tough offensive game in Ohio, they are racing to play defense in North Carolina as Burr has stumbled. The Senate Leadership Fund just announced an $8 million ad buy in the state. Groups are also spending money in Indiana, where Democrats recently recruited former Sen. Evan Bayh to challenge Rep. Todd Young for an open Senate seat that seemed safe for Republicans six months ago.

Republicans in Florida, Nevada and Missouri can expect additional PAC money for television, digital and mailed advertising, as well as other campaign support.

How much will the outside groups spend at the end of the day? “As much as we can raise,” Law said.

Democrats laugh nervously, hoping that antipathy toward Trump will serve as a counterweight.

“No matter how much special interest money Republicans dump into states to try and save their weak incumbents, they cannot escape Donald Trump at the top of the ticket,” said Shripal Shah, a spokesman for the Democrats’ Senate Majority PAC. “We’re on offense across the map and are well positioned to win this November.”