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” that was one of the biggest spenders 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.