Nearly a month after the general election, one congressional race has yet to be called — in a razor-thin contest that will determine how much of a majority Republicans will have in Congress.

Out of 435 House races, California’s District 13, located in the state’s agriculture-heavy Central Valley, is notable as the final uncertain race of the 2024 election. The Republican incumbent, Representative John Duarte, is fighting to keep his seat against a Democratic former state lawmaker who lost to Mr. Duarte by only a couple hundred votes in 2022, Adam Gray. 

This time around, Mr. Gray appears to be ahead, although his narrow lead has been tightening this week. As of Tuesday morning, Mr. Gray leads by less than 200 votes — 105,331 to 105,188, or 50.03 percent to Mr. Duarte’s 49.97 percent.

Although Republicans are heading into 2025 with a trifecta — winning the White House, Senate, and House, the narrow majority in the House means every seat will count for President-elect Trump to push his proposals through. While Republicans have secured 220 seats in the House to Democrats’ 214 seats, several members of Congress have been tapped by Trump for administration roles, leaving three vacancies in the House that had been filled by Republicans.

Congresswoman Elise Stefanik has been nominated for United Nations ambassador, while Representative Mike Waltz has been picked for national security advisor. Congressman Matt Gaetz resigned his seat in Congress after being nominated for attorney general — but he stepped away from the nomination after his alleged sexual misconduct blew up nationally. 

Although the three congressional vacancies are in solidly Republican districts, the special elections to replace Trump’s nominees won’t take place for months, so Republicans face an extremely thin majority in the House as Trump begins his second term. 

That means California’s District 13 race will be pivotal to determine how large of a majority Republicans hold. The Golden State has a notoriously slow ballot counting process, as ballots continue to be tallied for weeks following the election. County elections officials are required to complete the final official results by December 5, and the Secretary of State must certify them by December 13. 

In 2022, the race between Mr. Gray and Mr. Duarte in the 13th district wasn’t called until December 2 and also fell within a margin of only a few hundred votes.