After the 2018 midterm elections, Gallup asked Democrats and Republicans where they hoped their respective parties would move ideologically. The results are instructive — about the trajectories of both the impeachment debate and the Democratic contest for president.

When Democrats and Democratic-leaning independents were asked if they would rather see their party become more liberal or more moderate, 54 percent said more moderate, 41 percent said more liberal. Republicans, by contrast, think their already radicalized party is still not conservative enough: Fifty-seven percent said they wanted the party to become more conservative; only 37 percent said more moderate.

The results added to the mountains of evidence showing how the political polarization we talk about so much is asymmetric: Republicans have not only moved much further to the right than the Democrats have moved left, the GOP wants to keep moving toward the outer edges.

The relative moderation of the Democrats explains why there was not a majority in the House of Representatives to impeach President Trump until his effort to pressure Ukraine to smear former vice president Joe Biden became public. Only then did the Democrats’ significant moderate contingent decide that dealing with Trump’s abuses could not wait until Election Day.

Republicans, on the other hand, remain in lockstep behind Trump no matter how much he parrots Vladimir Putin’s views because they know their party’s primary voters will punish any deviationism.

You would think that the shocking (but not surprising) Post report about Trump’s acceptance of the Russian dictator’s tutelage would move at least some GOP politicians to reconsider. Trump, The Post reported, told a senior official that he believed Ukraine, not Russia, interfered in the 2016 election because “Putin told me.” But Republicans, with a careful eye on their primary electorate, seem happy to join the president in parroting the Moscow line.

The relative moderation of Democrats also explains the shape of the party’s presidential contest at year’s end. The bottom line: A lone progressive would be favored for the nomination, but there is not room for two. If Sens. Elizabeth Warren (D-Mass.) and Bernie Sanders (I-Vt.) continue to be more or less equally strong, they’ll block each other’s way.

Warren briefly gained ascendancy this fall by consolidating the left and beginning to win acceptance from the party’s center. A mid-October Quinnipiac poll showed her leading the field with 30 percent overall — and with a commanding 50 percent-to-14 percent lead over Sanders among Democrats who said they were very liberal. It also showed her at 34 percent among somewhat liberal Democrats and 19 percent among those who called themselves moderate or conservative.

Quinnipiac’s December poll found Warren hemorrhaging across the board. Her lead over Sanders among very liberal Democrats had been cut to five percentage points, while her support in the two other groups had been cut by more than half.

Yet Warren has shied from criticizing Sanders because she will need his supporters if he falters. So instead, she has targeted South Bend, Ind., Mayor Pete Buttigieg. During Thursday’s debate, she sent the words “wine cave” viral by referring to the venue of a high-end Buttigieg fundraiser. Sensing an opening, Warren’s backers went on a wine cave offensive, though Buttigieg gave as good as he got in the exchange.

Her choice of adversary reflected the second divide in the party, along class lines: Warren and Buttigieg are both favorites of the party’s educated middle and upper-middle class, while both Sanders and Biden are popular among less upscale Democrats — younger ones in Sanders’ case, older ones in Biden’s.

Buttigieg is a ripe target because he has been surging in the Iowa and New Hampshire polls. He also stands in the way of Warren on the class front and Sen. Amy Klobuchar (D-Minn.) on the ideological front. So the moderate Klobuchar targeted Buttigieg, too, while both she and Warren reminded debate watchers that they were the only women onstage.

And, by the way, Andrew Yang earned himself a Cabinet position. Secretary of commerce, maybe?

What stands out is Biden’s durability, and he helped himself with his best debate performance to date. He will stay formidable as long as he can maintain his strength among three overlapping groups: moderates, white working-class voters and African Americans. The test would come if he suffered twin defeats in Iowa and New Hampshire.

Thus: The math suggests room for two moderates but only one champion of the left, and this will dictate much of what happens on the Democratic side next year.

As for Republicans, they are stuck with Trump. And, to their shame, even politicians who know how profoundly flawed he is will have to keep pretending they like him.

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