By Clare Foran, Annette Choi and Haley Talbot, CNN

Washington (CNN) — Speaker Mike Johnson’s historically narrow majority just shrunk even further, creating a major challenge for congressional Republicans as they seek to enact President Donald Trump’s agenda.

At the start of the 119th Congress, Johnson was already facing the narrowest House majority in nearly 100 years.

The margin is now even smaller after Mike Waltz, a Florida Republican, resigned from the House on Monday to serve as Trump’s national security adviser, a role that does not require Senate confirmation.

That brings the partisan breakdown to 218 Republicans and 215 Democrats, one of the thinnest House majorities in history.

And the margin is soon expected to shrink even further.

Trump has named GOP Rep. Elise Stefanik of New York as his pick for US ambassador to the United Nations, a role that is subject to Senate confirmation.

If Stefanik is confirmed as expected, Republicans’ House majority would drop down to 217 to 215.

At that point, House Republicans would not be able to afford a single defection to pass legislation along party lines until the vacancies are filled.

Republicans won 220 House seats in the November elections, while Democrats won 215, the most narrowly divided House majority since the outset of the Great Depression, almost a century ago.

At the start of the new session of Congress, however, the partisan breakdown stood at 219 to 215, because former GOP Rep. Matt Gaetz of Florida opted not to return to Congress.

Passing a bill in the House requires a majority of all members present and voting. The magic number is 218 if every member shows up to vote and all 435 seats are filled, but that can change if there are vacancies or absences. A tie vote in the House is a fail.

If Republicans aren’t in lockstep, then House GOP leaders will need Democrats to pass legislation.

How the House majority ranks in history

The last time a minority in the House held 215 or more seats was after the 1930 elections, when Republicans won 218 seats, Democrats won 216 and the Farmer-Labor Party won one.

The 72nd Congress — which took place in the early years of the Great Depression era — officially started in March 1931, but did not actually convene to conduct legislative business until months later, in December 1931.

At the official start of that term, in March, the House margin had narrowed even further — to 217 seats for Republicans to 216 for Democrats with one seat for the Farmer-Labor party and one vacancy as a result of the death of one Republican.

In an unusual turn of events, however, the partisan breakdown changed significantly by the time Congress convened when a series of additional deaths and ensuing special elections flipped control of the chamber to Democrats, though the margin remained narrow.

According to House historical records, the 65th Congress had the closest party split in American history, but in that case, the partisan division was so narrow that neither party secured an outright majority in the House based on election results, which left Republicans with 215 seats and Democrats with 214. As a result, a handful of third-party lawmakers played a decisive role when the House convened to elect a speaker.

It is not yet clear when a confirmation vote for Stefanik will take place, but Senate Republicans have pledged to hold votes on nominations as soon as possible.

Vacant seats in the House don’t remain vacant forever. Special elections have already been set to fill the Florida congressional seats held by Gaetz and Waltz, but that won’t happen immediately and the seats are expected to remain vacant for several months.

With Republicans in control of Washington, there will be intense pressure on GOP leaders to swiftly enact Trump’s agenda.

But the extraordinarily tight margin gives any rank-and-file lawmaker the ability to exert outsized influence by making demands on the speaker.

Johnson has already had to contend with that – frequently in the form of pressure from his right flank, a dynamic that now set to intensify.

Challenges ahead after speaker election

Johnson won the speakership in a nail-biter of a vote at the start of the 119th Congress, a sign of the difficulty of overseeing such a narrow majority with little room for error.

To be elected speaker, a candidate needs to win a majority of members who vote for a specific person on the House floor.

The election took place with the majority at 219 to 215, which meant that Johnson could only lose a single Republican votes if every lawmaker voted and all Democrats voted against him.

After the initial roll call vote, Johnson had come up short, with three GOP defections. Rep. Thomas Massie of Kentucky had voted for Rep. Tom Emmer of Minnesota; Rep. Ralph Norman of South Carolina had voted for Rep. Jim Jordan of Ohio; and Rep. Keith Self of Texas had voted for Rep. Byron Donalds of Florida.

But after everyone had voted, the vote was held open for an extended period of time — meaning that it was not gaveled to an official close — as Johnson worked to lock down the votes. Then, in an abrupt turn of events, Self and Norman flipped their votes in support of Johnson, effectively handing him the gavel.

This story and headline have been updated with additional developments.

CNN’s Zach Wolf contributed to this report.

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