Hundreds of millions of dollars that Republican groups had been poised to spend in the 2016 presidential election are now increasingly likely to move into Senate and House races, as many big donors look to distance themselves from the party's presumptive nominee, Donald J. Trump.

These groups and their Democratic counterparts have already spent more than $25 million on advertising in Senate general election races alone, according to Kantar Media/CMAG, significantly outpacing both the 2014 and 2012 campaigns in outside spending. And more than $134 million in advertising for Senate races alone has been reserved by groups for the general election.

"There's at least uncertainty about what the impact of Trump's candidacy would be down ticket, and there's also a sense that investing in the Senate is an important defensive play," said Steven J. Law, the president of American Crossroads, a Republican "super PAC[1]" that was one of the biggest spenders[2] in the 2012 campaign. "It's an insurance policy against whatever might happen at the presidential level, keeping in check the potential for a President Clinton or keeping in check whatever a Trump presidency might mean."

Mr. Law added that many of the donors to his group, which was founded by Karl Rove, the Republican strategist, are still eager to play a role in the 2016 election, but do not necessarily feel comfortable yet supporting Mr. Trump.

"A lot of our donors were investors in either campaigns or super PAC activity on behalf of specific presidential candidates who are no longer in the race, and that investment opportunity has been taken away," he said.

Crossroads, which in early 2015 created a new group devoted solely to protecting the Republican Senate majority, is in a wait-and-see crouch, with officials saying that they currently have no plans to buttress Mr. Trump's candidacy and that they expect the Senate contests to remain the focus.

And the vast political network of Charles G. and David H. Koch, the billionaire brothers who have provided critical support for conservative causes, which early last year pledged to spend nearly hundreds of millions of dollars on the 2016 election[3], has spent money only on Senate races so far. Its three most prominent groups — Freedom Partners Action Fund, Americans for Prosperity and Concerned Veterans for America — have already invested $12.4 million in five different states.

The Koch network announced on Tuesday that it had reserved an additional $30 million in digital and broadcast advertisements in crucial Senate swing states for September alone.

And Republican groups that spent more than $25 million trying to stop Mr. Trump's march to the nomination are also directing their funds elsewhere. The Club for Growth, which ran a series of negative ads against Mr. Trump, has been spending aggressively in Senate and House races in Georgia, Indiana, North Carolina and Pennsylvania, among other states. (The group is frequently a target of criticism in the stump speeches of Mr. Trump, who calls its members "crooked as hell.")

Larry McCarthy, a Republican ad maker who worked for both Right to Rise, the group that supported Jeb Bush, and Our Principles PAC, an anti-Trump group, said he expected that as money from donors slows to a trickle for the "Stop Trump" movement, those donors who might be reluctant to flip and support the candidate will turn to the places where they think their money will have an impact.

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"If the traditional G.O.P.[4] donors and financial infrastructure is not enthusiastic about the presidential race, then they will circle the wagons around the Senate," Mr. McCarthy said.

He noted the "tightrope" that the Republican senatorial candidates can expect to walk with Mr. Trump at the top of the ticket, saying that most of the ads for them will "feature copy lines of 'independence' and 'delivering for their home state.' "

The investment by outside groups, which was expected to be used primarily in swing states once thought to be competitive, is being deployed to some states where Republican incumbents now find themselves in increasingly tough races. This month, The Cook Political Report shifted its projections in 13 states[5], almost all in favor of Democrats.

Wisconsin, where Senator Ron Johnson is seeking a second term, was among those states, moving from "Toss Up" to "Lean Democratic." Around the same time, Freedom Partners Action Fund, part of the Koch network, aired an ad[6] attacking his opponent, former Senator Russ Feingold, over problems at a local Veterans Affairs hospital that he was accused of ignoring (a claim his campaign has denied[7]). The group is putting $2 million behind the ad.

The ad followed a $1.2 million campaign[8] by Americans for Prosperity, another Koch-aligned group, in March. The Democratic Senatorial Campaign Committee has reserved $2 million for Wisconsin in the fall and could spend more.

In New Hampshire, Senator Kelly Ayotte, locked in a tight race with Gov. Maggie Hassan, has received support from 10 different Republican groups, who, combined, have given about $5.8 million, nearly double what Democratic groups have spent on the race.

But Democratic groups are also poised to spend a lot more. The Democratic Senatorial Campaign Committee's independent expenditure arm has nearly $50 million in advertising reservations for both digital and broadcast ads. Senate Majority PAC, which supports Democratic candidates, also has reserved $7.5 million in New Hampshire, $9.5 million in Ohio and $8.5 million in Nevada.

Mel Immergut, a prolific fund-raiser for Mr. Bush's 2016 campaign, said he was watching the presidential race closely, but focusing his energy on the Senate, especially the races of Ms. Ayotte and Senator Rob Portman of Ohio.

"I am getting very interested in a number of Senate contests because I think no matter what happens in the presidential election, it's going to be crucial for the Republican Party to keep control of the Senate," Mr. Immergut said. "If Donald Trump wins the White House, obviously having Republican control of the Senate with a new president is important, and that's even more the case if Hillary Clinton were to win."

To help Republican candidates navigate the fine line they may have to walk, ad targeting firms are looking for ways to reach voters they had not sought before: "Hillary Republicans" and "Trump Democrats," split-ticket voters who could help Republicans win down-ballot contests.

Deep Root Analytics, a Republican data firm specializing in television advertising, has been experimenting with models built around the type of voters who might support Mrs. Clinton at the top of the ballot because they could not bring themselves to vote for Mr. Trump, but who would vote Republican the rest of the way. Or those Democrats drawn to Mr. Trump rather than Mrs. Clinton, who then might also be swayed down ballot.

"Two important political audiences this year are going to be the Hillary Republicans and Trump Democrats," said Brent McGoldrick, the chief executive officer of Deep Root Analytics. "That will be the most marketed to, advertised, sought after audience in the free world over the next six months. Those slivers of the electorate who can't bring themselves to vote for their own candidate. Republicans in swing seats or seats that are safe that now that map has expanded."