CONTINUING UNCERTAINTY over a few races does not obscure the clear result of the Nov. 6 election: The Democrats have recaptured decisive control of the House of Representatives. Less clear, however, is on behalf of precisely what policy agenda they have recaptured it. In part, this is a consequence of the party’s decision to campaign on a relatively amorphous message, other than the pledge to defend health-care coverage for people with preexisting medical conditions. Democrats banked instead on the public’s desire for a Congress that could check President Trump.

Democrats should swiftly fill in the blanks of their agenda for the House. The surest way to lose power again would be to exercise it on behalf of nothing in particular; our interest, however, is not what’s best for them, but what’s best for the country. And it so happens that the country needs precisely what the Democrats in the House are now positioned to provide: tough oversight of the Trump administration and a clear alternative to the priorities the Republican Party has pursued during the past two years of all-GOP government.

Taking the second point first: Democrats obviously cannot overcome the 60-vote rule in the Senate, much less Mr. Trump’s veto. They can, however, use the legislative process to address concerns that the GOP has unreasonably ignored or actively made worse in the past two years. Republicans have responded not at all to mass shootings; Democrats should pass an assault weapons ban, modeled on the federal law in force between 1994 and 2004, to show that they, by contrast, understand the need for action. The GOP slashed taxes for the rich without regard for exploding federal debt; a restoration of the pre-Trump estate tax would show that fiscal responsibility and greater equality of wealth can go together. In a society where millions fear for their most basic political rights, legislation to enhance protections under the Voting Rights Act, and to grant the District voting rights in Congress, would reassure. And, of course, they must keep their health-care promises.

Oversight should not be allowed to detract from substantive policy changes but rather should be pursued where it can do the most to support them. That would argue against any attempt to impeach either the president or Justice Brett M. Kavanaugh. It would argue in favor of investigating the Department of Homeland Security’s indefensible treatment of families at the border, as well as a thorough probe of the Environmental Protection Agency’s apparent attempts to undermine climate science and environmental regulation more generally.

In her post-election news conference, potential House speaker Nancy Pelosi (D-Calif.) wisely said there will be no “scattershot freelancing” when it comes to committee investigations, which should be an easy promise to keep, because the Trump administration would present a target-rich environment even to relatively restrained investigators. Ms. Pelosi also acknowledged that her party had not campaigned directly against Mr. Trump’s fearmongering and bigotry, because “in order to get in a position to fight it, we had to win on the issues that strike right at the financial security of America’s working families.” She spoke as well of a willingness to work with Mr. Trump on matters such as prescription drugs and infrastructure. That’s fair enough, but tens of millions of voters are also counting on her and her colleagues to stand up to the president and send the world a very different signal about the values that Americans hold dear.

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