House Democrats will add to their ranks after Adelita Grijalva won Arizona’s special election on Tuesday.
That will bring the partisan breakdown in the House to 219 Republicans and 214 Democrats, with two seats still vacant.
With his historically narrow majority, Speaker Mike Johnson can only lose two Republicans on any party-line vote.
The outcome of the election is also expected to give GOP Rep. Thomas Massie and Democratic Rep. Ro Khanna the 218 signatures they need to force a House vote on the release of more files related to the Jeffrey Epstein case.
At the start of the 119th Congress, Johnson was already facing the narrowest House majority in nearly 100 years. The tight margin has created a major challenge for congressional Republicans as they seek to enact Trump’s legislative priorities, leaving little room for error.
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.
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.
Challenges created by a narrow majority
Johnson won the speakership in a nail-biter of a vote at the start of the 119th Congress.
The election took place with the majority at 219 to 215, which meant that Johnson could only lose a single Republican vote if every lawmaker voted and all Democrats voted against him.
The partisan breakdown in the House has shifted several times since then as a result of vacancies that have arisen due to lawmaker deaths and resignations.
Johnson has at times had a bit more breathing room and congressional Republicans have gone on to pass major legislation – most notably Trump’s massive tax and spending cuts package in July – but not without challenges uniting their conference.
GOP leaders lost two Republican votes on final passage of the tax and spending cuts bill in the House. Reps. Thomas Massie of Kentucky and Brian Fitzpatrick of Pennsylvania voted alongside Democrats against the measure.
The next major legislative fight facing Congress will be over government funding with a shutdown deadline looming at the end of September.
This story and headline have been updated with additional developments.
CNN’s Zach Wolf contributed to this report.
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