LONDON — Russia’s influence in British politics has resurfaced as a potentially explosive issue in the country’s general election, with Hillary Clinton joining a parade of critics in castigating Prime Minister Boris Johnson for withholding a secret parliamentary report on Russia until after Britain goes to the polls.

Mr. Johnson’s decision to delay the politically sensitive report until after the Dec. 12 election has not prevented the leak of unsavory details, which, if confirmed, would paint a damning portrait of Russian oligarchs funneling money to Conservative Party politicians.

By adding her voice, Mrs. Clinton, whose presidential bid in 2016 was undermined by systematic Russian interference, could thrust the dispute over Russia to the center of the campaign.

“Who do they think they are that they would keep information like that from the public, especially before an election?” Mrs. Clinton said in an interview with The Guardian newspaper. “Well, I’ll tell you who they think they are. They think that they are the all-powerful, strong men who should be ruling.”