Senate Democrats had sought to shut down the inquiry, noting that it was based on claims that Russia has fanned about Mr. Biden and Ukraine.
Senator Chuck Schumer of New York, the minority leader, noted during one such attempt last week that the United States had just imposed sanctions for election interference on a Ukrainian lawmaker with ties to Russian intelligence, Andriy Derkach, who was peddling edited tapes purporting to show improper acts by Mr. Biden in Ukraine. The tapes were part of a “a covert influence campaign,” the Treasury Department said this month, bent on “spurring corruption investigations in both Ukraine and the United States designed to culminate prior to Election Day” — like the one Mr. Johnson conducted.
Mr. Derkach claimed that he had provided information to Mr. Johnson and Mr. Grassley, though the senators said they never sought or received anything from him. (They have worked with Andrii Telizhenko, another Ukrainian, despite law enforcement officials sharing concerns with the committee that he, too, could be spreading the same Russian misinformation.)
On Wednesday, Mr. Schumer said the report read “as if Putin wrote it, not United States senators.”
Mr. Johnson forcefully denied that his report was based on any disinformation. But he also said a claim should not be off limits merely because bad actors were amplifying it.
“If there is somebody in Ukraine and somebody in Russia also publicizing the fact that Joe Biden and Hunter Biden created this massive conflict of interest,” Mr. Johnson said, “is that something we are just supposed to take a hands-off attitude toward?”
Democrats were not the only ones taking issue. Last week, Senator Mitt Romney of Utah, the 2012 Republican presidential nominee, criticized Mr. Johnson for pursuing what he called a “political exercise.”
“It’s not the legitimate role of government for Congress or for taxpayer expense to be used in an effort to damage political opponents,” Mr. Romney said.
Catie Edmondson contributed reporting.