The Democratic victory is also a sign of political health, to the extent it is a form of pushback against the excesses, rhetorical and in terms of policy, committed by the Trump administration and propounded by President Trump during this fall’s campaign. Turning against the dominant party in Washington even in a moment of economic prosperity, voters from Key West to Kansas refused to accept the continued degradation of their nation’s political culture. Republicans retained control of the Senate, where the map this year favored their defense. But voters nationwide refused Mr. Trump’s invitation to vote on the basis of fear of immigrants; they did not respond to his depiction of his opposition as dangerous enemies.

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Now the House will be in a position to investigate any number of potential administration transgressions and demand accountability: the awful separation of migrant children from their parents; the dubious decision to add a question about citizenship to the 2020 Census; the president’s harassment of special counsel Robert S. Mueller III’s investigation.

The new majority also has an opportunity to offer a positive legislative agenda. The Democrats achieved their victory Tuesday night in large part by promising to protect health-care coverage, especially for Americans with preexisting conditions. Though effective in winning over moderate voters, the campaign did not establish a clear mandate for much beyond that — eminently valid — objective. And of course, even if the Democrats set forth a list of specific proposals for the House, before or after Election Day, the Senate and Mr. Trump’s veto pen could block it.

Still, the party can outline an alternative policy direction for the country. It can begin with measures to shore up the Affordable Care Act but then move to reforms of federal gun laws. Where the Republican majority has denied science, the Democrats can offer an approach to climate change. They can propose relief to the “dreamers” and, ideally, other undocumented immigrants, along with generous but not unlimited opportunities for future legal immigration. They should propose to restore the United States to its rightful place as a welcomer of refugees; to end the disgraceful denial of congressional representation to citizens in the District of Columbia; to repeal the most egregious giveaways to the rich in the 2017 tax bill.

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Tuesday was a good day for Democrats. It may also be a good day for Republicans, if they take the lessons of their House defeat to heart and reconsider the devil’s bargain they have made with Mr. Trump. Indeed, if the results help lead to a reemergence of that party’s better angels, then it will have been good day for America as a whole.