Senate Republicans huddled behind closed doors Tuesday to plot the path forward for the unified control of government they won in the November elections, though GOP senators said afterward a very narrow House majority will likely determine how sweeping their policy proposals will be.

Republicans in the U.S. House of Representatives will hold a slim 220-215 majority when the next Congress gets underway next month after Democrats picked up a seat in California, according to Edison Research on Wednesday.

The final race to be called in the 2024 House of Representatives elections has been called for a Democratic candidate, meaning Republicans’ margin of control in that chamber of Congress will be among the most narrow in U.S. history.

The narrow majority leaves Republicans with even less margin for error as they try to enact President-elect Trump’s agenda.

Republicans will have a narrow majority in the House, with Democrats flipping one final seat in California, leaving GOP leaders with even less margin for error.

House Republicans are headed into next year with an even slimmer majority that will temporarily shrink to a zero-seat advantage in the early months of President-elect Donald Trump’s presidency, imperiling his ambitious agenda.