Analysis:: A Trump-Clinton general election poses a question: Which one does America hate less?

Reporting from Philadelphia —  Momentous victories in Tuesday’s primaries drove Hillary Clinton and Donald Trump ever closer to a November face-off in which the strongest argument each can make for election is the threat posed by the other.

Together, Clinton and Trump are the two most unpopular presidential candidates in memory, and both are moving to improve their images for the general election.

But they are so well-known, and operating in such a polarized political environment, that their efforts may only serve to tinker around the edges.

Both candidates issued calls to unity within their parties in their victory speeches Tuesday night, then aimed at each other. That suggested a general election battle of brutal negativity, which threatens to alienate voters and further increase partisan polarization.

The winner may be the candidate America hates the least.

Clinton rebuked Trump throughout her speech by inference with her call to “build an America where we can all rise together, an America where we lift each other up instead of tearing each other down.” And she criticized Trump’s denunciations of varied groups.

Trump returned the favor in his speech, declaring that Clinton “will be horrible, absolutely horrible” on the economy and foreign policy.

“She will not be a good president. She doesn’t have the strength; she doesn’t have the stamina,” he said.

By winning delegates resoundingly, Clinton and Trump already have demonstrated that they have substantial support in their own parties. The problem for both is they need broader backing to win in November, and reaching for different groups is significantly harder in a nation where people have surged to the ideological poles. Fewer voters remain in the middle than in the past. And persuading any of them will be tough for the two front-runners because of who they are.

In an April NBC News/Wall Street Journal poll, only about a quarter of Americans had a favorable view of Trump. That would mean doom for any other candidate — and may still, except that Clinton had a favorable rating among only about a third of the country. Majorities of Americans, in other words, dislike both of them.

“It’s going to be an extraordinary challenge for either campaign to move their numbers into positive territory,” said Neil Newhouse, a Republican pollster who came up against the same difficulty in 2012 when working for GOP nominee Mitt Romney.

Absent a national crisis that illuminates an unknown facet of the candidates’ behavior, “it’s going to take a hell of a lot more than millions of dollars in campaign ads to change the image of these two,” he said.

To boost her image among reluctant Democrats and others considering her, Clinton has nearly stopped criticizing her challenger, Vermont Sen. Bernie Sanders, and on Tuesday night offered her most lavish praise of him and his supporters to date.

She has also begun airing ads with more emotional punch, and she has laid out specific plans and demanded in recent speeches to be held accountable for them — a way to establish a fresh base line for a candidate whose trustworthiness has been doubted even by her own party.

Her challenger, Sanders, remains in the race through the June primaries. But he suggested in a statement late Tuesday that his forces would head to the convention to influence the party platform, not battle for the nomination.

Trump on Tuesday offered his familiar two-step, insulting his adversaries while pledging to expand the nation’s jobs and improve its economy, still the most important topic to voters eight years after the recession.

But he, too, will seek wider appeal. On Wednesday, he will begin to fill in gaping holes in his policy proposals and political experience with a speech on foreign policy in Washington. In an olive-branch meeting scheduled for May, he will sit down for a lengthy television interview with Megyn Kelly, the Fox News broadcaster he’s spent months belittling.

Trump faces continued battling with Texas Sen. Ted Cruz, who is fighting him not only at the ballot box but in state-by-state delegate skirmishes; and Ohio Gov. John Kasich. The GOP challengers struck a deal to allow Cruz free rein in next week’s Indiana primary and Kasich in later races in Oregon and New Mexico.

But Trump’s five-of-five sweep on Tuesday was a sobering rebuke of the two other candidates that puts intense pressure on them to win big—and fast. Even then, neither can win the nomination outright before the convention.

Tuesday’s results substantially extended Trump’s margin among delegates, lessening the chances of a successful insurrection against him at the summer convention.

At this point in any primary campaign, where there are clear front-runners if not official nominees, a race can change its tone as primary animosities are replaced by general election fissures. The looming presence of November can also force voters into a new mindset. For both Trump and Clinton, that is their best hope for a fundamental shift in how they are viewed.

“Once you have a general election contest, it’s very clarifying,” said Democratic pollster Anna Greenberg. “Many of the people lukewarm on Trump in the Republican Party and Hillary in the Democratic Party are going to be forced to decide who they support. ... It will be good versus evil on both sides.”

Trump appears to have the bigger task. Clinton must draw out young voters who have flocked to Sanders and formed much of the voting base for President Obama. But she has the rest — women, African American voters, Latinos. And she’s not Trump, a man with whom those who have formed the winning Obama majority have little in common.

Trump, however, has to balance delivering the raw and angry diatribes that appeal to the voters he has with the more presidential advances desired by the voters he needs.

For weeks, his team has promised the arrival of a new, more presidential Trump. But the candidate himself made clear in his Tuesday night remarks that he doesn’t actually want to shift personas, fearing the impact on the voters who come by the thousands to his rallies and week after week deliver him victories.

“He has a bigger internal consolidation challenge than she does, and he has to rehabilitate himself more,” said Greenberg. “Hillary Clinton has not gone around insulting people, entire ethnicities.”

The advertising that filled the airwaves in Tuesday’s states spoke to another distinction between the candidates and how they seek to define themselves toward November.

In his ad, Trump stood alone in a skyscraper, the city below him, and talked about building a wall on the Mexican border, building the military, increasing jobs and fighting terrorism. The image was of straight talk and, more than that, blunt toughness meant to appeal to voters who feel their lives are increasingly frightening and unpredictable. It suggested that Trump alone was enough to reverse the bad times.

Clinton aired ads speaking of toughness, but also ones in which she high-fived crowds, embraced the sorrowful mothers of slain children and beamed as she hugged a little girl. In her closing ad, Clinton looked ebullient, but never alone.

That ad was called “Love and Kindness.” In the long election to come, those words may seem fleeting.

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Follow me on Twitter: @cathleendecker. For more on politics, go to latimes.com/decker.

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