Hillary Clinton tried to draw a sharper contrast with Donald Trump on Sunday, saying the central message of her campaign was "We're stronger together," even as a new poll showed the two candidates in a statistical tie.

The new Washington Post/ABC News survey found the presidential frontrunners in "a virtual dead heat" among registered voters.

Never in the poll's history, The Post said, had two major party nominees been viewed as harshly, with nearly 6 in 10 voters holding negative impressions of both candidates.

That underscored a growing sense of urgency in the Clinton campaign to define her both more clearly and positively -- a task made harder by her inability to shake off her tenacious Democratic rival, Bernie Sanders.

Clinton, in an appearance on NBC, challenged Trump's "Make America Great" slogan, suggesting that "he seems to be particularly focused on making himself appear great."

The former secretary of state said her campaign would be "demonstrating the hollowness of his rhetoric, and the danger of a lot of what he has said."

Asked to sum up her own message, Clinton said, "We're stronger together."

She said Americans needed to unify and work together to improve the economy. "We're stronger together when we have a bipartisan, even nonpartisan foreign policy that protects our country. And that provides the kind of steady, strong, smart leadership that the rest of the world expects."

But the new poll seemed to show that if Americans are united about anything, it is in their dissatisfaction with the two leading candidates.

What is shaping up to be the nastiest American presidential campaign in recent memory is beginning with voters expressing historically deep and practically identical levels of discomfort with both Trump and Clinton.

This has shaken up some in the Democratic camp, who assumed Trump would be an easy target, and it has provided encouragement to Sanders, the Vermont senator who trails Clinton but keeps winning primaries.

Appearing on Sunday talk shows, Sanders pleaded with the hundreds of the party's so-called super-delegates who support Clinton to reconsider their allegiance ahead of the Democratic nominating convention in late July.

A frequent critic of the Democrats' selection process, he said the party was headed for an "anointment" of Clinton.

As the race for the White House grows closer, Clinton supporters and some Democratic Party leaders have made increasingly insistent calls for Sanders to step aside and let Clinton turn her full attention to Trump.

But Sanders vowed yet again on Sunday to stay in the race at least until California, the most populous state, stages its primary on June 7.

The Post/ABC poll found the two leading candidates are in a statistical dead heat, with 46 percent of registered voters favoring Trump and 44 percent supporting Clinton.

That amounted to an 11-point shift in the Republican's favor since March.

Clinton's net negative rating among registered voters was a minus 16, virtually the same as Trump's minus 17, though his negatives were significantly higher among all adults.