When a party chooses its presidential candidate, it also chooses its party leader in the election. This year the Democrats face an unusual situation. Bernie Sanders isn’t just an outsider to the party establishment; he’s not even been a member of the party, and has long excoriated it in unsparing language. Although the media haven’t much focused on this history, the early signs suggest it could become a problem for Sanders in getting the nomination—and a problem for the party if he does get it.

According to the entrance polls at the Iowa caucuses, there was a 30-percentage-point split between self-identified Democrats and independents in their support for Sanders. Hillary Clinton won 56 percent of self-identified Democrats but only 26 percent of independents, while Sanders won only 39 percent of Democrats but 69 percent of independents.

This difference will loom large in primaries and caucuses limited only to registered Democrats. To vote in closed primaries, voters first have to register their affiliation with the party, usually about a month before election day. All the February caucuses and primaries are effectively open. (Nevada is described as “closed,” but participants can pick a party on the day of the caucus.) Thereafter, according to the nonpartisan organization FairVote, Democrats are holding closed primaries in three of the largest states (Florida, New York, and Pennsylvania) as well as in 13 others (Colorado, Connecticut, New Jersey, Maine, Maryland, Kentucky, Alabama, Louisiana, New Mexico, Arizona, Oregon, Wyoming, and Nebraska).

California law allows the parties to limit participation in what will be the last major primary in June. For 2016, however, the California Democrats decided to open up their election, while the Republicans are keeping theirs closed. That could prove to be a fateful decision if the Democratic battle is still undecided. Some of the anti-establishment, independent vote that might have gone into the Republican primary could go to Sanders (also a possibility in other open-primary states now that Rand Paul has dropped out).

Party opposition to Sanders is especially strong among Democratic officeholders. In FiveThirtyEight’s endorsement primary, Clinton has a lead over Sanders of 466 to 2. Although endorsements themselves may not matter much in swaying voters, they’re an indication of the depth of party support, which will likely translate into a substantial edge for Clinton among the 712 superdelegates at the Democratic convention unless she stumbles badly in the primaries.

Sanders and his supporters see the party support for Clinton as evidence that “the establishment” is against him. But there are two other interpretations. What party leaders necessarily care about is winning the next election. They look at the electability of the presidential candidate as it affects the electoral prospects of candidates at all levels, including their own. The endorsement primary is a symptom of deep anxiety about what Sanders would do to the entire party’s fortunes in November.

The lack of support for Sanders among elected Democrats may also reflect his lack of support for them. During 2015, Clinton raised $18 million for other Democratic candidates, while Sanders did no fundraising for them at all. Those are just last year’s numbers. The difference in party fundraising between them going back decades would surely be even more dramatic. After all, before this campaign began, Sanders was emphatic that he was not a Democrat.

Sanders has left a long trail of denunciations of the Democratic Party. He began on the revolutionary left; in 1980, he served as an elector for the Socialist Workers’ Party, founded by Leon Trotsky and committed to nationalizing major industries. In 1989 he said the Democrats and Republicans were “in reality, one party—the party of the ruling class.” That year he wrote an op-ed in the New York Times describing the two parties as “tweedle-dee” and “tweedle-dum” since both subscribed to what he called an “ideology of greed and vulgarity.” As the Republican Party has moved to the right, Sanders has said the Democrats are better, but he has refused to run as a Democrat and continued to insist—as late as the 2012 election—that he is not a Democrat because the party fails to support the interests of workers.

Though he refers to “Wall Street” and “big corporations” in his current campaign rather than to “the ruling class,” his attacks on Democrats are basically the same as before. They’re just focused on Clinton now. But what he says about her he could just as easily say about most Democrats running for Congress or in the states—and they surely know it.

To people on the left who have long attacked both parties, Sanders’s disdain for Democrats may not be a problem. But it would be a remarkable and difficult situation for any party to go into an election with a presidential nominee so at odds with its other candidates. As the journalist Michael Tomasky has argued, many of them would run on their own, keeping their distance from Sanders. And if Michael Bloomberg runs as an independent, the party could face a schism.

If Sanders had conspicuously changed his positions as well as his rhetoric at some time in the past, his early history might not have posed as serious a problem for the party as it does. But he’s still talking about a revolution in the name of socialism, and, let’s give him credit—that’s not just rhetoric.

The taxes Sanders is calling for are in a different league from any peacetime Democratic candidate in history.  The single-payer health plan alone, according to the estimates of the Sanders campaign, requires raising as much revenue as the federal government collected in 2013 through the individual income and estate taxes. In other words, we’d need to double that revenue. To do so, Sanders proposes the following: a 6.2 percent increase in payroll taxes; a 2.2 percent increase in income taxes on everyone; higher estate taxes; taxing capital gains and interest as ordinary income; limiting tax deductions for the rich; and higher income-tax rates on the upper brackets.

To be sure, Sanders says that Americans would save money because they wouldn’t be paying private insurance premiums. But his plan has no limits on the scope of coverage and no patient cost-sharing—it transfers all private health spending into the Treasury, without any clear means of cost restraint. Free can be expensive. The plan would create winners and losers: The losers will surely fight it, and many of those who might be winners won’t trust the federal government enough to go along with it.

The taxes for Sanders’s health plan, combined with other tax increases on wages he’s proposed, would raise the top marginal federal rate to 77 percent, as Dylan Matthews shows at Vox. In addition to those taxes on earnings, Sanders is calling for an increase in the capital-gains tax, a new financial-transaction tax, and a new carbon tax.

The last of these, the carbon tax, is a good idea and a hard sell in itself. But the total package is not a platform that Democrats can run on—it’s a platform they’re going to run from. And it is a fantasy-come-true for Republicans. For decades, they’ve been falsely accusing Democrats of favoring huge tax increases. In this case, it would be the truth.

So far, many Democrats have hardly registered the full significance of what Sanders’s candidacy would mean for their party. But I don’t think Republican strategists have missed the possibilities. A Sanders nomination would be their opportunity to capture decisive control of all branches of the federal government from a divided and weakened Democratic Party. Among other things, Republicans would be able to consolidate a Supreme Court majority for a long time to come. As those who supported Ralph Nader in 2000 should recognize now, the costs of purity can be heavy indeed.