President Trump on Thursday threw his support behind a bipartisan effort to allow proxy voting for new parents in Congress, dealing a major blow to Speaker Mike Johnson’s attempts to kill the measure.

“You’re having a baby, you should be able to call in and vote,” Mr. Trump told reporters flying to Florida with him aboard Air Force One. “I’m in favor of that.”

The president’s endorsement of the plan came after he spoke with Representative Anna Paulina Luna, Republican of Florida, who has been pushing for a resolution that would allow lawmakers to vote remotely for up to 12 weeks after the birth of a child. There is no parental leave from voting for congressional lawmakers.

And it appeared to force Mr. Johnson’s hand to make some concession to Ms. Luna, which he had previously been unwilling to do. On social media, Ms. Luna wrote that the speaker called her after Mr. Trump’s comment.

“We discussed limiting the vote to just new moms who cannot travel because of health concerns,” she said. Ms. Luna appeared ready to compromise, as well. “This is smart,” she wrote, opening the door to an agreement on a policy that was more narrow than the one that a majority of House members had already said they would support.

Ms. Luna has been leading the charge to allow proxy voting since she gave birth last year. She used a maneuver called a discharge petition — a demand signed by 218 members of the House, the majority of the body — to force consideration of the bipartisan measure to change the chamber’s rules.