WASHINGTON — Republicans are struggling to make the $1.5 trillion Trump tax cuts a winning issue with voters in the midterm congressional elections, but the cuts are helping the party in another crucial way: unlocking tens of millions of dollars in campaign donations from the wealthy conservatives and corporate interests that benefited handsomely from it.

Billionaires and corporations that reaped millions of dollars in tax cuts are pumping some of that windfall into the Congressional Leadership Fund, a “super PAC” closely aligned with Speaker Paul D. Ryan that is flooding the airwaves and front porches of swing congressional districts with increasingly sharp attacks on the Democratic candidates vying to wrest control of the House.