The vaunted blue wave that Democrats had hoped for failed to fully materialize on Tuesday night, but the days of one-party control in Washington are now over. President Trump’s strength in rural areas kept the Senate in Republican control, but voters in urban and suburban districts across the country sent the White House a clear message: They want a check on the president.
When the new Congress is sworn in this January, Democrats will be able to curb Mr. Trump’s legislative ambitions and, armed with subpoena power, flex their oversight muscles to initiate investigations into allegations of misconduct by the president and his administration. If the special counsel, Robert S. Mueller III, finds substantial evidence of illegal conduct during the 2016 election, he now will have a receptive wing of government to pursue his findings.
“Tonight, the American people have demanded accountability from their government and sent a clear message of what they want from Congress,” Representative Jerrold Nadler, Democrat of New York and the new chairman of the House Judiciary Committee, said on Twitter. The president “may not like it, but he and his administration will be held accountable to our laws and to the American people.”
But after eight years in the minority, Democrats hoping to reclaim the White House in 2020 will also have to prove they are interested in governing — and temper the liberal ambitions of the party’s most ardent left-wingers.
“It’s like being the rescue team at an 88-car pileup: Who knows where to begin?” asked Representative Jamie Raskin, Democrat of Maryland. “I think the key principle is that we’ve got to make progress on the real problems of the country.”
Democratic leaders have already said they plan to use their first month in the House majority to advance sweeping changes to future campaign and ethics laws, including outlawing the gerrymandering of congressional districts and restoring key enforcement provisions to the Voting Rights Act. They also intend to press for infrastructure investment and legislation to control the climbing costs of prescription drugs — initiatives that will test whether Mr. Trump is willing to work with them.
Those measures, they believe, will be broadly popular.
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But without overwhelming numbers, Democrats will not have the strength to push many of the initiatives their left flank ran on: a single-payer health care system, boldly expanded college access and an Immigration and Customs Enforcement agency that’s at least reined in.
Democrats will also have to balance legislative ambitions with their efforts to satisfy the desires of their base to investigate the president. That could lead to gridlock.
“The expectation is that we will behave as a real branch of government and not just a supplicant to Trump, which this current Congress has been for the last two years,” said Representative Raúl Grijalva, Democrat of Arizona. “There’s an expectation that we’re a check and a balance so that means a stalemate.”
Midterm elections are always a referendum on the president, and never more so than in 2018, when Mr. Trump told voters across the country that he was on the ballot. Historically, the party out of power picks up seats in the first midterm of a presidency, and Democrats followed that pattern this year.
Unlike the midterms of 2006, when President George W. Bush declared that Democrats had delivered “a thumping,” or 2010, when President Barack Obama described Republicans’ victory as “a shellacking,” the Democrats did not score an overwhelming victory Tuesday night. Republicans are likely to expand their majority in the Senate, and Democrats lost some governorships that they badly wanted, especially in Ohio and Florida.
But they do have a lot to celebrate. Democrats not only won the districts they were favored in, but locked up many where they were not. In New York, Max Rose, a health care executive and Army veteran, ousted Representative Dan Donovan, the only Republican member of New York City’s congressional delegation, in a race that analysts had said leaned Republican.
In Texas, Democrat Colin Allred, a former N.F.L. player and civil rights lawyer, defeated the incumbent Republican, Pete Sessions. In Illinois, Lauren Underwood beat Representative Randy Hultgren, a Republican who won by 19 points in 2016.
And in Virginia, Democrat Abigail Spanberger, a former C.I.A. official, unseated Representative Dave Brat, a Tea Party darling who himself scored a huge upset four years ago when he defeated his predecessor, Eric Cantor, the Republican leader, in a Republican primary.
In a year when Mr. Trump put racial divisiveness on the ballot, Democrats ran and won with a diverse set of candidates who are infusing the party with new energy. Many are from the party’s progressive wing. Newcomers like Alexandria Ocasio-Cortez, a self-described democratic socialist from New York, and Ayana Pressley of Massachusetts will almost certainly take the lead in pressing for a liberal agenda.
Democratic leaders plan to satisfy those demands by using their newfound majority to push through long-stalled initiatives that they say have broad support within the electorate. High on their list of priorities is gun safety legislation, legislation to offer a pathway to citizenship for the undocumented young people known as Dreamers, and a bill to extend broad civil rights protections to gays, lesbians, bisexuals and transgender people.
At the same time, Democrats must be careful not to overreach. Progressives will almost certainly clamor for Mr. Trump’s impeachment — a push that Democratic leaders have indicated that they will resist, at least until Mr. Mueller releases his findings.
“Everybody understands that we have to choose our battles very carefully now,” Mr. Raskin said. “We have to make sure that we are advancing a common sense majority platform that America wants, and let’s hope that we can bring Republicans aboard with us.”
That may be overly optimistic, especially with Republicans in control of the Senate. Senator Mitch McConnell of Kentucky, the majority leader, has shown little inclination to cooperate with Democrats, and is unlikely to do so without a nudge from Mr. Trump. And Mr. Trump himself may be in no mood to cooperate once Democrats start demanding documents — including, perhaps, the president’s tax returns.
Mr. Trump is already anticipating as much — and gave a hint earlier this week of how he will react to Democrats’ demands.
“I don’t care,” he told reporters. “They can do whatever they want and I can do whatever I want.”