In normal times, there’s good reason to run on this issue. Millions of families are plagued by inadequate insurance coverage. If you’re trying to win a swing voter in Arizona, it’s a bread-and-butter issue that has appeal.

But the Democratic campaign is inadequate to the current moment. It offers no counternarrative to Trump, little moral case against his behavior, no unifying argument against ethnic nationalism. In politics you can’t beat something with nothing. Democrats missed the Trumpian upsurge because while society was dividing into cultural tribes, they spent 2008 through 2016 focusing on health care. Now that the upsurge has happened, they are still pinioned to health care.

Worse, the Democratic strategy simply revives the old 1980-2008 playbook. It’s Democratic spending promises versus Republican tax cuts. This familiar, orthodox argument pushes left and right back into their normal categories. It destroys any possibility of a realignment.

We’ve learned a few things about the Democratic Party. First, it’s still fundamentally a materialist party. The Trumpian challenge is primarily a moral and cultural challenge. But the Democrats are mostly comfortable talking about how to use federal spending to extend benefits. Some Democrats want to spend a lot more (Medicare for all, free college education), and some want to spend less, but their basic instinct is that national problems can be addressed with more federal money. Their basic political instinct is that you win votes by offering material benefits.

Second, we’ve learned that when Democrats do raise a moral argument, it tends to be of the social justice warrior variety. The core argument in this mode is that the oppressive structures of society marginalize women, minorities and members of the L.G.B.T.Q. communities.

It turns out that if your basic logic is that distinct identity groups are under threat from an oppressive society, it’s very hard to then turn around and defend that society from authoritarian attack, or to articulate any notion of what even unites that society. You can appeal to women as women and to ethnic groups as ethnic groups, but it’s very hard to make a universal appeal to Americans as Americans, or defend the basic American norms that Trump calls into question. It’s a messaging vulnerability that Democrats have imposed upon themselves.

Democrats still seem likely to win the House, because Trump is so effective at driving away voters. But Democrats are blowing the political opportunity of a lifetime. They seem to be getting little traction in red states and now may end up losing ground in the Senate. Instead of drawing disaffected voters away from the G.O.P., they seem to be pushing Republicans back to Trump.