Speaker Mike Johnson on Wednesday abruptly canceled a vote on his initial plan to avert a government shutdown, as opposition to the six-month stopgap funding measure piled up in both parties.

It was a bruising setback for Mr. Johnson coming only a few weeks before a Sept. 30 deadline Congress faces to fund the government or face a shutdown.

In short remarks just off the House floor around midday on Wednesday, Mr. Johnson told reporters that he would not go ahead with the planned afternoon vote on the spending plan, saying that “we are in the consensus-building business” and adding that he was working with a slim majority.

As early as Monday, it appeared clear that the measure was doomed on the House floor. Resistance from an array of Republican factions bubbled up almost immediately after lawmakers returned to Washington from a lengthy summer recess earlier this week. And Democrats rejected it in part because it contained legislation they oppose to require proof of U.S. citizenship when registering to vote, a policy they say is unnecessary and could make it more difficult for eligible voters to register.

Mr. Johnson’s abrupt announcement threw a wrench into efforts to keep the government funded beyond Sept. 30. The Democratic-controlled Senate and White House have said they would reject the short-term spending bill even if Mr. Johnson could muster the votes for it in the House.

But his decision not to go ahead with the vote on Wednesday indicated that efforts to whip Republicans into supporting the legislation were not yielding results, potentially sapping Mr. Johnson of any negotiating leverage he might have had.