In the wake of the stunning victory by Doug Jones in Alabama, the Democrats have a very real prospect of taking over both the House and the Senate in 2018. Taking over the House requires winning only 24 seats, which, with the generic ballot favoring Democrats by double-digits and consistent outperformance across 2017's special elections, is a more likely outcome than not, even considering the impact of partisan gerrymandering.

Taking over the Senate is a heavier lift, requiring Democrats to defend vulnerable seats in red states like Missouri and Indiana, North Dakota and West Virginia, as well as in purple states like Florida and Minnesota. But Democrats have real shots to win open seats in Arizona and Tennessee, and to topple GOP Sen. Dean Heller in Nevada. And if Stephen Bannon gets his way, they may have opportunities in states like Nebraska. Even Texas Sen. Ted Cruz should be at least a little bit worried. And the Democrats only need a net pickup of two seats to take the chamber.

So in a little less than a year, the Democrats could have control of both houses of Congress. What will they do then?

It's worth asking that question now.

The surge of support for Democrats so far has been powered primarily by the extraordinary unpopularity of both individual Republican candidates and the GOP agenda. African-American voters in Alabama turned out in record numbers to stop someone who would like to repeal every post-Civil War amendment to the Constitution; many white Republican voters stayed home rather than vote for an extremist who was also a credibly accused molester of teenagers.

But opposition to President Trump and the burgeoning wave of Bannonite trolls does not constitute a governing agenda. And that's something the Democrats need to have — and to sell — before their big victory, even if they don't need it to win.

Why? First of all, because once they've taken power it will probably be too late. If a new Democratic Congress doesn't come in with a clear agenda, the agenda will be set by events — or by the president's incendiary tweets. Jockeying for 2020 will begin distracting everyone with presidential dreams, along with everyone up for re-election that presidential year, almost the minute the votes from 2018 are counted.

Meanwhile, the Republicans, as soon as they've lost power and its attendant responsibilities, will comfortably revert to scorched-earth opposition mode, firing up the negative partisanship machine that has served them so well in elections past. If the Democrats do have an agenda, but haven't defined it well in the public's mind before the election, then the Republicans will define it for them. And the Democrats won't find that characterization flattering.

Democrats are rightly outraged by the Republican tax cut. But they should run in 2018 not merely on the fiscal recklessness and unfairness of the bill, but on an alternative use of $1.5 trillion. Promise to cancel the tax cut and spend every dollar on essential infrastructure, from old-fashioned stuff like repairing bridges to forward-looking projects like upgrading the electrical grid. Dare the president to veto the very thing he touted as making him different from traditional Republicans.

And do it now, before the president can argue that the Democrats are just obstructing the administration's own, still vague infrastructure proposals.

The same goes for the other outrages of the last year, the attempts to wreck the health-care exchanges, to cripple the Consumer Financial Protection Bureau. And it goes in spades for the ongoing Republican efforts to make it more difficult for people to vote. There's probably no more fundamental issue for the voters who made the biggest difference in Alabama, and probably no issue that is more important to maintaining the Democrats' ability to win elections — but this is not an issue that needs to divide voters on racial lines. The defense of the franchise, like the defense of free speech and freedom of religion, matters to every American.

It might be objected that all of the above and more are included in the package of Democratic proposals that was rolled out to great fanfare last summer. And that's a fair point. But in the wake of a string of impressive victories, are Democrats proclaiming that their message is getting through, and promising to push forward with an agenda that is winning adherents in red states as well as blue? Or are they talking about demographics?

Demographics matter, enormously. You can't win an election if you don't know where the votes are. And identity politics — paying attention to how policies actually affect different communities; speaking to those communities in ways that let them know you understand them, hear them, and share their concerns; and making sure that remains true by recruiting from those communities, at every level — is, in the final analysis, just what politics is.

But the ultimate political identity we share is as fellow Americans. Any successful national politics needs to connect every other aspect of identity to that unifying one. And winning a statewide election in Alabama — with a candidate who did not trim his sails in order to appeal to its conservative electorate — is just about the best possible time for Democrats to be making that connection.

Democrats can win in 2018 on the basis of negative partisanship — Alabama proves that if nothing else. And the criminality, incompetence, and extremism of both the administration and many Republican candidates will provide plenty to run against.

But there's a chance to win much more than an election. I hope the Democrats seize it.