WASHINGTON — Republicans won control of the House on Wednesday, returning the party to power in Washington and giving conservatives leverage to blunt President Biden’s agenda and spur a flurry of investigations. Yet a threadbare majority will pose immediate challenges for GOP leaders and complicate the party’s ability to govern.

More than a week after Election Day, Republicans secured the 218th seat needed to flip the House from Democratic control. The full scope of the party’s majority may not be clear for several more days — or weeks — as votes in competitive races are still being counted.

But they are on track to cobble together what could be the party’s narrowest majority of the 21st century, rivaling 2001, when Republicans had just a nine-seat majority, 221-212 with two independents. 

That’s far short of the sweeping victory Republicans predicted going into this year’s midterm elections, when the party hoped to reset the agenda on Capitol Hill by capitalizing on economic challenges and Biden’s lagging popularity.

Instead, Democrats showed surprising resilience, holding on to moderate, suburban districts from Virginia to Minnesota and Kansas.

The results could complicate House GOP leader Kevin McCarthy’s plans to become speaker as some conservative members have questioned whether to back him or have imposed conditions for their support.

The narrow margins have upended Republican politics and prompted finger-pointing about what went wrong. Some in the GOP have blamed President Trump for the worse-than-expected outcome. The former president, who announces a third White House bid on Tuesday, lifted candidates during this year’s primaries who struggled to win during the general election.

Despite its underwhelming showing, the GOP will still see its power in Washington grow. Republicans will take control of House committees, giving them the ability to shape legislation and launch probes of Mr. Biden, his family and his administration.

There’s particular interest in investigating the overseas business dealings of the president’s son Hunter Biden. Some of the most conservative lawmakers have raised the prospect of impeaching Mr. Biden, though that will be much harder for the party to accomplish with a tight majority.

Any legislation that emerges from the House could face steep odds in the Senate, where the narrow Democratic majority will often be enough to derail GOP-championed legislation.

With such a slim majority in the House, there’s a potential for legislative chaos. The dynamic essentially gives an individual member enormous sway over shaping what happens in the chamber. 

That could lead to particularly tricky circumstances for GOP leaders as they try to win support for must-pass measures that keep the government funded or raise the debt ceiling.