Having won control of the Senate despite a map stacked against them, Democrats early Wednesday clinched the 218 seats needed to take control of the House of Representatives, according to The Associated Press, and appeared on track to capture many more.

With final results still outstanding, it was unclear exactly how large Democrats’ majority would be, a factor that will shape their ability to set an agenda and muscle bills through the Senate, where Republicans retained the ability to block major measures from the minority.

But in a thrashing for the G.O.P., Democrats had won decisive victories across the country, including in solid conservative bastions, in what was projected early Wednesday morning to be the largest popular vote victory for either party since 1976.

The election results promised a new era of divided government in Washington and a freshly assertive Congress after two years in which Republicans generally ceded power to Mr. Trump and reduced the legislative branch’s role to that of a mere bystander. It capped a fiercely fought midterm election cycle that Democrats entered chastened from the 2024 presidential election but finished buoyant and with strong political winds at their backs.

“The American people have spoken and made clear they’ve had enough of the extremism, enough of the corruption,” Representative Hakeem Jeffries, the Democratic minority leader who is now in line to be speaker, told supporters in Washington. “They want a Congress that’s going to deliver for the American people.”

Senator Chuck Schumer, the New York Democrat and the longtime leader who was deeply involved in his party’s uphill battle for the majority, vowed a swift change in direction.

“We have five words for the American people,” he told reporters in Washington. “Relief is on the way.”

With control of Congress and its committees, Democrats are expected to deploy their oversight powers to aggressively investigate Mr. Trump and his administration over allegations of corruption, aggressive immigration enforcement tactics and the president’s pardons.

Representative Robert Garcia, the California Democrat who is poised to be the chairman of the Oversight Committee, the main investigative panel in the House, issued a warning in a statement on Tuesday night.

“Lawyer up,” he said.

Democrats have also promised a shock-and-awe legislative sprint in which they plan to push through bills that would restore health care subsidies and Medicaid funding, address the increased costs of food and fuel and seek to force Mr. Trump to roll back his tariffs and end the war with Iran.

Democratic control of the Senate will also give the party veto power over Mr. Trump’s judicial and administration nominees, as well as the leadership of the committees that will have the authority to subpoena administration officials and others.

Mr. Trump, who appeared stunned by the results even as he deflected any blame for the losses, insisted he would still have the upper hand over a Democratic Congress.

“I have total veto power, so they have to come to us,” he said in an interview with The New York Times as the Democratic victories piled up. “And if they have some good ideas, I’m certainly open to working.”

Despite their losses in the last presidential election, Democrats had been consistently bullish about their ability to win the House majority. The party out of power typically gains seats in midterm elections, and Republicans have held only a slender majority in the House since they took over in 2023.

They were less optimistic about their prospects in the Senate, where the political map was stacked against them with seats in states that had become solidly Republican during the Trump era.

But as voters who backed Mr. Trump in 2024 began souring on the president and his agenda, Democrats saw an opportunity to drastically expand their political map. They won in longtime G.O.P. bastions like Kansas and Texas, reversed years of losses in Iowa and Ohio, and flipped both North Carolina and South Carolina.

Mr. Schumer recalled that when he first began talking about reclaiming the majority in 2025, “people rolled their eyes.” Democrats, he argued, prevailed because they heeded voters who were frustrated with how things were going in Washington and felt ignored by Republicans.

“We listened; they didn’t,” he said. “And the American people reduced the red brick wall to a pile of bricks.”

Their victories were powered by Americans’ growing dismay over the rising costs of goods and fuel linked to Mr. Trump’s tariffs and the war. Democrats painted the G.O.P. as beholden to the president and indifferent to the financial struggles of everyday Americans.

Those arguments were bolstered by growing concerns among voters about corruption in Washington and the cuts to Medicaid and health care subsidies that Republicans enacted as part of their signature tax cut and domestic policy package last year.

Their strength became evident early in the night, when Democrats surged in crucial House battlegrounds and had notched victories in competitive races in Florida, Pennsylvania and North Carolina. The party needed a net gain of only three seats to win the House.

Aware of the historical trend, Republicans had taken efforts to stem their expected losses with a frenzy of redistricting in red states that was meant to increase the number of G.O.P.-leaning districts and force Democrats out of what had been safe seats.

But those moves fell short in the face of Mr. Trump’s plummeting approval ratings. In Florida, Democrats won in three districts that Republican state lawmakers redrew to favor their party, including some where Mr. Trump won by more than 10 percentage points in 2024.

Even with many races uncalled, Speaker Mike Johnson effectively acknowledged his party’s defeat on Tuesday night.

“We knew that to keep the House and sustain our majority, we would have to defy more than a century of midterm history,” Mr. Johnson said. “It’s very, very difficult to overcome. And it looks like history will be kept tonight.”

The drubbing appeared to leave many Republicans reeling on Tuesday evening, and raised questions about the direction that the party would take in the final two years of Mr. Trump’s second term, a lame-duck period that has bedeviled many presidents.

Senator John Thune of South Dakota, the Republican leader, said his party would work to thwart Democrats at every turn.

“As we head into 2027, Senate Republicans will not stand idly by as the far-left, socialist wing of the Democrat party attempts to unwind everything we’ve done,” Mr. Thune said in a statement early Wednesday. “Thankfully, we still have President Trump in the White House, and we’ll work with him to stop the worst impulses of Democrat extremism. We cannot let their fringe positions become the mainstream.”

During the first half of his second term, Mr. Trump has enjoyed near-total control of Congress, with lawmakers showing little appetite for imposing guardrails on his actions.

But Senate races in particular suggested that Mr. Trump had lost his once dominant sway over Republican voters. The president won both North Carolina and South Carolina in all three of his presidential elections. But in both cases in this election, his handpicked candidates trailed significantly.

And with Mr. Trump losing ground even in Ohio and Iowa, two former battleground states that he transformed into G.O.P. strongholds, it was unclear whether he would exhibit the same hold on Republican lawmakers that has characterized his second term.

Sensing a political shift in the final weeks of their campaigns, many vulnerable lawmakers sought to put some distance between themselves and his policies, particularly his signature tariffs. But the pivot appeared to have come too late.

Annie Karni and Robert Jimison contributed reporting from Washington.