Mike Segar / Reuters

Supporters of Senator Bernie Sanders insist that their guy can eject Donald Trump from the White House. The more insistent question, though, is whether Sanders will cost Democrats the House of Representatives.

Democrats won the House in 2018, riding a surge of anti-Trump voting from a constituency that’s been scared by the Sanders campaign: older, college-educated, conservative-leaning women. Such voters tipped into the Democratic column the congressional seats once held by George H. W. Bush, Newt Gingrich, and Eric Cantor.

In 2018, upmarket districts voted to reprimand the president’s language and behavior. A Sanders nomination invites those districts to vote in 2020 to raise their taxes and replace their health insurance. That may be a tougher sell.

The Democratic House majority is new, fragile, and dependent on voters who are more conservative than the median Democrat.

In 2018, Democrats flipped districts like New Mexico’s Second, which stretches across the bottom tier of the state. Trump carried the district by 10 points in 2016, but Xochitl Torres Small won it by two points in 2018, thanks in part to an ad that showcased her skill with a bird gun. “New Mexicans,” said the ad copy, “don’t care which party gets the credit or the blame. We just want someone to deliver.”

Democrats won South Carolina’s First, which stretches from the posh coastal towns of Hilton Head and Beaufort to the Charleston suburbs. The district was formerly held by the very conservative Mark Sanford. Trump won it by 13 points in 2016; freshman Representative Joe Cunningham nabbed it by only one point in 2018.

Democrats won New York’s Twenty-Second, which extends from the university city of Binghamton to postindustrial Utica. Trump won there by a staggering 15 points in 2016; Anthony Brindisi defeated by two points a Republican incumbent prone to incendiary comments like—in a radio interview shortly after the Parkland school shooting—“It's interesting that so many of [the] people that commit the mass murders end up being Democrats.”

Altogether, 31 of the congressional districts won by Trump in 2016 are now held by Democrats, 21 of them freshman. Only three Hillary-won House districts are represented by Republicans.

Democrats succeeded in Trump country because the Democratic Party attracted a broad coalition of moderates and liberals. The Sanders campaign aims first and foremost to reinvent the Democratic coalition as a narrower ideological movement, in much the same way that the once-broad Republican coalition has been transformed. But the difference between the two is that many fewer Americans identify as “progressive” than as “conservative.” Worse for Democrats: Not only does Sanders propose to break the cookie in such a way as to leave his party with the smaller piece, but he also does so in a political context that already disfavors them.

Democrats hold virtually every one of the urban and academic districts that will rally to progressive politics. But thanks to enterprising candidates who keep in touch with their districts, they also hold Minnesota’s Seventh, a 90 percent white district running north-south adjacent to the two Dakotas. It’s represented in Congress by Collin Peterson, a pro-life Democrat who chairs the House Agriculture Committee. In 2018, a Democrat won the country’s richest congressional district, the Virginia Tenth, which has a median household income of more than $127,000. Democrats now represent all of the country’s 10 richest districts.

Sanders supporters take as an article of faith that Sanders will win votes from working-class voters who swung to Trump in 2016. This idea is based on a single data-point: Some 10 to 12 percent of those who voted for Sanders in the 2016 primary then voted for Trump in the general election. If Sanders could have held all those primary voters in a general election, and also if he had won everybody who voted for Hillary Clinton in the primary, then he would have defeated Trump. But once you state the two ifs, you see the problem.

The political scientist Brian Schaffner, who closely studied these Sanders-Trump switchers, finds that they were older white voters with conservative racial views. As compared with other Sanders voters, the Sanders-Trump switchers were much more likely to deny that white people enjoy special advantages in American society. They were also much less positive about President Obama than were Sanders voters who did not switch to Trump.

No Democrat, including Sanders, is likely to outbid Trump for these voters in a general election.

Meanwhile, it’s very hard to identify congressional districts where the hypothetical return of Sanders-to-Trump voters to the Democratic column would swing the district—and it’s easy to identify many where discomfort with Sanders could swing the district back to the Republican column.

In 2018, Democrat Lizzie Fletcher won Texas’s Seventh, a wealthy district in and around Houston. The district had been held continuously by Republicans since 1966, when it was won by George H. W. Bush. In 2016, Republican John Culberson got almost 144,000 votes; Democrat James Cargas, 112,000. In 2018, the Democratic vote improved to 128,000; the Republican vote fell to 116,000. Yet the district remains Republican +7 according to “The Cook Political Report.” What happens to Lizzie Fletcher if Bernie Sanders wins the nomination on a message of higher taxes, no private health insurance, and admiration for Fidel Castro? Do you think a Republican House member cannot recover to 144,000 running against that?

Bernie Sanders is sometimes compared to George McGovern, the liberal Democrat who lost every state except Massachusetts and the District of Columbia to Richard Nixon in 1972.

Defenders of Sanders correctly point out that we live in a more polarized and partisan era, and that anti-Trump feeling will surely put a political floor under Sanders well north of McGovern’s 37.5 percent of the vote. And that’s probably true. It’s hard to see Sanders losing California or New York, as McGovern did.

But in an important way, Sanders represents an even greater danger to Democrats than McGovern did. McGovern ran in an age of ticket-splitting. In that same election where McGovern did so disastrously, Democrats lost only 12 seats in the House. They actually gained two in the Senate and also won a governorship.

That pattern will not repeat itself in 2020. If Sanders loses badly as moderate voters swing away from Democrats, he will take with him a big clutch of House Democrats and Democratic Senate hopefuls. It will be a loss up and down the ticket, a loss that could not only reelect Trump, but also enable him, by preserving his elected bodyguard in the Senate and restoring his majority in the House. The question to weigh before Super Tuesday is thus not only Sanders versus Biden or Sanders versus Bloomberg. It is whether you prefer Speaker Pelosi or Speaker McCarthy, and Chairman Schiff or Chairman Nunes. The hopes of congressional Democrats hang in the balance in the fateful week ahead.

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