With Hillary Clinton’s strong performance on Tuesday night, the Democratic primaries are effectively over. Barring an unforeseen catastrophe, Clinton will be the Democratic nominee. And it’s increasingly likely that her opponent will be Donald Trump, who won a clean sweep of five states on Tuesday and only seems to be getting stronger. Both parties must now gear themselves for a Clinton-Trump match-up in the fall.

Clinton indicated as much in her victory speech last night. As in earlier speeches, she made a play of Trump’s name and his penchant for racism, declaring, “Love trumps hate.” This slogan is an early clue as to how Clinton will frame the election, presenting herself as an inclusive advocate of national unity and Trump as an avatar of prejudice and divisiveness. Yet such an election poses unique problems that go beyond normal politics, and Clinton’s team may not yet appreciate how difficult this fight will be.

If winning the presidential election were all that mattered, Trump would be Clinton’s dream opponent. The Democratic front-runner struggles with poor approval ratings—55.6 percent unfavorable, according to Huffington Post’s aggregation of the polls—which means she needs to compete against someone who is even less popular than her. Trump fits that bill handsomely, standing at 63.6 percent unfavorability. Furthermore, Trump’s racism and misogyny are likely to motivate the very voters that Clinton most needs to attract: people of color, single women, and young people. And not surprisingly, in head-to-head polls, Clinton enjoys a hefty lead over Trump, even as she trails behind the less-polarizing John Kasich and enjoys a significantly smaller lead (of roughly 5 percentage points) over Ted Cruz.

Yet there are reasons why the real estate mogul should be a far greater cause for fear than Cruz or Kasich. Cruz might be a political extremist, further to the right than any serious presidential candidate since at least Barry Goldwater. But the Texas senator is still bound by the rules of normal politics, still beholden to donors and constituencies that serve as a check on what he can say or do. Cruz would be a predictable opponent in that he’d follow a hyper-conservative script and make largely ideological arguments. Trump, in contrast, is not predictable in that manner and has no loyalty to traditional Republican causes. He could, as he has in the primaries, present himself as an opponent of the Iraq War and interventionism, a supporter of Planned Parenthood in non-abortion funding, an enemy of free trade pacts, and a defender of Social Security and Medicare.

And Trump’s unpredictability goes far beyond policy. He is wealthy enough not to worry about donors, and his core supporters have shown every sign that they will stick with him no matter what. Rather than being repulsed by his excesses, they thrill at Trump’s subversion of the rules of political decorum. This makes Trump a potentially more destructive opponent on a personal level, because he could do considerable reputational damage to his Democratic opponent.

Two of President Obama’s campaign masterminds, David Axelrod and David Plouffe, took up the special problems Trump poses during a recent episode of the Axe Files podcast. Both came to the surprising conclusion that Cruz, not Trump, is the candidate you’d prefer to fight against.

Axelrod noted that Trump “can make incursions in places you don’t expect,” and “plays by no rules and will hit you with punches that no other politician would throw. I would think that would be a little bit unsettling.”

Plouffe agreed that Trump would be a “very unsettling” opponent. “With Trump,” he said, “I also think execution and prosecution on the campaign day to day would be gruesome. Anything Donald Trump says is legitimate news if he’s the Republican nominee. You know they had a little dustup last December where he threw a brushback pitch about old news regarding the former president. I think a lot of people were shocked by that. My guess is that’s just a taste of what we would see.”

The “old news” Plouffe was alluding to was Trump’s reference in December to Bill Clinton’s “terrible record of women abuse.” That comment is surely only an appetizer for the no-holds-barred personal attacks that Trump will unleash if he’s the nominee. Material will be ready at hand: Roger Stone, a Trump crony and notorious dirty trickster, is author of a book called The Clintons’ War on Women, which rehashes in lurid detail allegations of sexual abuse on Bill Clinton’s part and the supposed role Hillary Clinton played as accomplice. Stone, it’s worth remembering, was the likely source behind the National Enquirer report on Cruz’s alleged extramarital affairs. Running against Trump would put the Clintons back in the tabloids.

Axelrod and Plouffe are exactly right: Trump is a very dangerous opponent, one who can derail the political process even if he loses.

To be sure, any Republican opponent is likely to try to make hay of the various scandals and pseudo-scandals that surround the Clintons. Cruz, whose campaign manager is widely known for vicious attacks, would certainly dredge them up. The difference with Trump is that he’s unusually ruthless about such attacks, and rarely hides behind surrogates. He doesn’t engage in Bush family–style underhandedness—there will be no whisper campaigns and Swift Boaters. Trump might use Roger Stone, but he’s likely to give some sanction under his own name, as he did when he tweeted an unflattering photo of Cruz’s wife.

When Trump has gone birther against Barack Obama or Ted Cruz, he’s done so personally. This means that his attacks enter the mainstream of political discourse more quickly and stay there permanently. Clinton will end up facing the same dilemma that hurt Jeb Bush, Marco Rubio, and the other Republican candidates: If she responds to Trump’s attacks, she’ll sink to his level, but if she ignores them she may look weak or evasive.

This is where Trump becomes Clinton’s most dangerous opponent: Even if he loses, his toxicity will linger on to poison Clinton’s administration. She might start her presidential term with a large chunk of the Republican Party believing, for instance, that she is complicit in sexual assault. Trump has already shown he’s an ugly winner, denigrating his opponents even when he’s besting them at the polls. This suggests that Trump will be an even uglier loser—especially for the ultimate political prize. He won’t accept the prospect of defeat graciously. Instead, he will be tempted to imitate Samson, to pull the pillars of democracy down so that he destroys his opponents as well as himself.

In the wake of defeat, Trump is also likely to claim he was cheated, which is what he did after the Iowa Caucus. Even if Trump loses by a landslide, his campaign could generate millions of supporters who reject the legitimacy of a Clinton presidency. Re-integrating bitter Trumpkins into the polity will be a major challenge.

Last night, Clinton offered a message of inclusiveness and warmth, declaring that “love” binds the country together. This positive message might be a way of answering Trump without descending into the pigsty. But Clinton might also try to find ways to marginalize Trump so that he starts to be seen as a fringe figure. A little bit of mockery could add some needed energy to her message. Love, in other words, might need a little assistance if it is to truly trump hate.

Donald Trump is no ordinary political candidate. The Republicans have already learned how difficult he can be to defeat, or even to challenge. In the general election, because the demographics are much broader and more favorable, Clinton will have an easier time of vanquishing him. But then she’ll still have to face the other problem of running against Trump: his ability to contaminate everything he touches.