House Republicans investigating the liberal fund-raising organization ActBlue have asked its chief executive to testify next month to explain how the group vets its donors, a significant escalation in public pressure on the Democratic Party’s biggest financial engine.

Hours after the Republican request on Thursday, ActBlue pushed back in a letter dismissing the inquiry as a “partisan attack on a political opponent at a pivotal moment in the electoral cycle.”

The Republicans’ request for testimony in a public hearing — and the response from ActBlue — represented a significant escalation between the two sides.

First, Representative Bryan Steil of Wisconsin, the chairman of the House Administration committee, asked Regina Wallace-Jones, the ActBlue chief executive, in a letter on Thursday to appear at a public hearing on May 19 to answer questions about the organization’s fraud prevention efforts.

Then, ActBlue’s lawyer, Vincent Cohen, responded in a letter urging the committees to “reconsider their current approach” and dismissing questions about how exactly ActBlue had vetted donors from abroad. People who are not United States citizens or permanent residents are prohibited from donating directly to federal candidates or political action committees.

“In 2025, in response to this partisan investigation, ActBlue also cut off all donations from American citizens abroad,” Mr. Cohen wrote.