Donald Trump could draw the United States into nuclear war, Hillary Clinton warns. Clinton would plunge the country into a constitutional crisis, he says.

As the caustic presidential race lurches toward the finish line, each candidate is aggressively casting the other as a catastrophic choice for the White House. Making an affirmative case about his or her qualifications and vision has become a secondary priority.

It's an ugly conclusion to a contest featuring two of the most unpopular presidential candidates in modern American politics. The sexual assault accusations that have trailed Trump in the race's closing weeks and a new FBI review into Clinton's email habits seem likely to only reinforce the public's negative perceptions, leaving the candidates to essentially argue to voters that they're the best of two unappealing options.

"I would rather be here talking about nearly anything else," Clinton said Tuesday during a rally in Florida where she leveled a series of attacks on Trump's character and preparedness for the White House. "But I can't just talk about all of the good things we want to do."

Indeed, Clinton's speeches in this final full week of campaigning have overwhelmingly focused on Trump. On Monday, she warned against giving Trump the authority to order a nuclear attack, bringing along a former nuclear launch officer to bolster her point.

"Imagine his advisers afraid to tell him what he doesn't want to hear, racing against his legendarily short attention span to lay out life-and-death choices too complex to be reduced to a single tweet," Clinton said Monday in Ohio. "Then imagine him plunging us into a war because somebody got under his very thin skin."

After spending much of the summer and fall tearing Trump down, Clinton had planned to close the campaign on a more positive note. She talked about giving Americans something to vote for, not just against. And with public opinion polls showing her with solid leads in most battleground states, she started talking about healing divisions and unifying the country after the election.

But her advisers say they saw polls tighten even before the FBI launched its new email review. The campaign now believes she needs to make a last push to define Trump as an unacceptable choice in order to seal the deal with persuadable voters.

On Tuesday, Clinton focused on Trump's demeaning and predatory comments about women, calling him a "bully." This time she brought with her former Miss Universe Alicia Machado. Trump criticized Machado for gaining weight after winning the 1996 contest.

Trump's campaign rhetoric has always been dark, full of searing depictions of a crumbling nation, and he has not been shy about going negative on Clinton. He routinely calls her "Crooked Hillary" and "the most corrupt person ever to run for the White House."

But Trump, too, has stepped up his broadsides after the last weeks of October handed him a pair of potentially potent political gifts: the projected "Obamacare" premium rate hike and FBI Director James Comey's letter revealing that agents are reviewing emails that may be connected to Clinton's private server.

His campaign sees the latter in particular as an opportunity to reinforce questions about Clinton's trustworthiness and remind voters that sending Clinton to the White House could lead to the return of the scandals that trailed Bill Clinton's presidency in the 1990s.

"She would be under protracted criminal investigation and probably a criminal trial, I would say," Trump said during a rally in Michigan on Monday. "So we'd have a criminal trial of a sitting president."

Campaigning Tuesday in Pennsylvania, a state in which Trump has directed an abundance of time and resources, he and his running mate Mike Pence delivered their most full-throated takedown yet of President Barack Obama's health care law.

Though barely mentioning Clinton's name, the typically fiery Republican somberly warned that electing Clinton would "destroy American health care forever."

Clinton's and Trump's closing campaign advertisements reiterate the race's sharply negative tone.

Her campaign has several commercials out that directly question whether Trump would launch a nuclear attack. The ads feature clips of him saying he likes to be unpredictable and would "bomb the (expletive) out of them."

She's also doubled down on her argument that Trump's offensive comments about women, as well as his boasts about touching women without their permission, disqualify him from the White House. A 60-second ad that features Trump in his own words over the years concludes: "Anyone who believes, anyone who says, anyone who does what he does, is unfit to be president."

Meanwhile, Trump's ads reinforce his message that the country risks doom if it doesn't change directions by electing him. "Hillary Clinton will keep us on the road to stagnation," a narrator says in one of his latest ads.

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Lemire reported from Philadelphia. AP writer Julie Bykowicz in Washington contributed to this report.

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Follow Julie Pace at http://twitter.com/jpaceDC and Jonathan Lemire at http://twitter.com/JonLemire