All of which makes it striking that somehow this issue is dominating the Democratic presidential debates.

Almost all of these candidates are experienced politicians who know that if they spend a lot of time campaigning on Medicare-for-all, the absolute best-case scenario is that they’ll actually get elected and then spend the first year of their presidency reenacting the Obamacare wars. Otherwise, it’s a losing proposition, yet that’s the territory upon which they’ve chosen to fight out their campaigns.

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Except, of course, when they venture onto even more treacherous ground, such as reparations for slavery, giving free health insurance to illegal migrants, or decriminalizing unauthorized border crossings. Those issues all poll extremely badly outside the progressive left, yet they have commanded considerable support on the primary debate stage.

Democrats have plenty of policy ideas that are more popular, simpler to explain and easier to implement. A $15 minimum wage, free college, higher taxes on wealthy people and corporations … you don’t need to support that agenda to recognize that Democrats would be better off campaigning where most of the voters are. Why are they crowding onto the battlefield’s left flank?

Well, because the base wants it. Democrats are discovering what Republicans learned eight years ago: that a grass-roots activist movement they initially hailed as a powerful new political weapon was actually nothing of the sort. The tea party, it turned out, wasn’t so much a sign that the electorate was moving against Republicans’ opponents as one that the GOP’s core voters were breaking away from the mainstream — and were happy to turn their fire on any Republican who tried to steer them toward the middle. Now the Democrats may be reenacting the Republican fracture.

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One sign of that is a Democratic primary where most of the top candidates are running not merely to the left of the general electorate, but to the left of the average self-identified Democrat on issues such as health care and immigration. Another sign is in the House, where the impeachment rumbling has lately grown appreciably louder.

Two-thirds of Democratic voters support impeaching President Trump — which frankly seems surprisingly low. But if a third of your own voters oppose impeachment, that’s a warning sign. A more dire warning: Among the broader electorate, impeachment support is just 37 percent, according to a Politico/Morning Consult poll taken in late July after former special counsel Robert S. Mueller III’s congressional testimony. Fully half of the public opposes impeachment. It isn’t a matter of ignorance; more than 70 percent of people say they watched, read or heard about Mueller’s testimony.

Nonetheless, support for impeachment keeps growing among blue-state Democratic representatives, and it’s not clear how long House Speaker Nancy Pelosi (D-Calif.) can restrain them. The progressive base wants to see Trump impeached and, like the tea party, its members tend to be isolated in politically homogenized districts. Mistakenly thinking that their outlook is more widely shared than it really is, they believe that their views are the party’s ticket to electoral victory.

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Democrats apparently hope to both assuage and inspire them by performing a sort of slow-motion impeachment dance, without actually going all the way. That’s what Republicans tried to do to President Bill Clinton in 1998, and there’s a big risk that this will end the same way — with the impeachment inquiry taking on a life of its own, breaking loose from the control of its creators and ultimately helping the very man they’re trying to take down.

Unfortunately for Democrats, there is one important way in which progressives aren’t like the tea party at all: They dominate only a thin geographic slice of the country. In a system tilted toward representing geography rather than population density, that leaves Democrats more dependent than Republicans on swing districts that won’t want to go where Democrats seem to be heading.

So there’s a real risk that the progressive left’s rise may mirror the tea party in another important way: It will end up handing more power to the most extreme and intransigent elements of the Republican right.

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