WASHINGTON — Senate majority leader Harry Reid’s relentless attacks on the billionaire Koch brothers are having an unforeseen impact: spurring other wealthy GOP donors to give more money to groups that keep their supporters’ names secret.
Several prominent pro-Republican advocacy groups say they are benefiting from a burst of cash as some donors — fearful of harsh public attacks such as those aimed at the Kochs — turn away from political committees required under federal law to reveal their contributors.
The trend can be seen at the prominent GOP super PAC cofounded by strategist Karl Rove, American Crossroads, which discloses its donors to the Federal Election Commission.
The group, which hauled in $117 million during the 2012 election, has raised $9 million so far this cycle — including just $266,000 in April.
At the same time, group officials said, donors are more interested than ever in supporting Crossroads GPS, a sister organization with a tax-exempt status that allows it to keep its donor list private.
The two groups recently kicked off a $10 million television advertising campaign against vulnerable Senate Democrats — $8 million of which was paid for by Crossroads GPS.
Paul Lindsay, a Crossroads spokesman, said the spotlight Reid and others have put on major political funders on the right has driven more contributors to the nonprofit. Democrats are ‘‘dragging private citizens through the mud, and that’s led to a shift of sorts for donors,’’ Lindsay said.
Politically active tax-exempt groups are on track to play a larger role this cycle than in 2012, which saw $310 million in reported election spending by non-disclosing groups.
These organizations are set up as ‘‘social welfare’’ groups or business lobbies under the tax code, designations that give them wide leeway to engage in campaigns without having to register as political committees and disclose donors to the FEC.
Political nonprofits have become major players in elections since the Supreme Court’s 2010 Citizens United decision paved the way for unlimited political spending by corporations and unions.
The Obama administration moved late last year to crack down on political nonprofits, proposing a new Internal Revenue Service rule that could have limited their election-related activities.
But after the draft rule triggered protests from the left and the right, the agency confirmed last week that it plans to rework the regulation, delaying the process indefinitely.
The delay allows the clout of these politically oriented nonprofits to grow, at least in the near term.
‘‘We’ll see more groups abusing the tax laws, calling themselves social welfare groups, when they really are political groups,’’ said Miles Rapoport, president of Common Cause, a nonpartisan group seeking to reduce the influence of money on politics.
Industrialists Charles and David Koch have given millions of dollars to conservative super PACs, which are required to disclose donors.
They are also the backers of an extensive network of political nonprofits, which raised at least $400 million in 2012 from unknown donors. This year, groups in the Koch network such as Americans for Prosperity and Freedom Partners are spending millions of dollars on ads against vulnerable Senate Democrats, a barrage that could help the GOP retake the chamber.
The Kochs have defended the right to spend money anonymously, noting that many donors across the political spectrum seek privacy. They point to the threats they have weathered since gaining a public profile.