The outcome of the 2016 White House race could come down to which candidate successfully appeals to voters as the lesser of two evils – and not just who does the better job of driving party faithful to the polls.

There’s little argument over the deep voter distaste for Democratic front-runner Hillary Clinton and presumptive Republican nominee Donald Trump -- with polls essentially showing them the most unpopular presidential candidates in modern U.S. history.

The RealClearPolitics.com polling average pegs Trump’s unfavorability rating at roughly 65 percent, and Clinton’s at about 55 percent. A fresh Quinnipiac poll shows voters in key swing states have sour feelings toward both candidates. In Tuesday's Nebraska GOP primary, four in 10 voters still chose someone other than the party's presumptive nominee. And in West Virginia, exit polls showed a third of Democratic primary voters vowing they'd vote for Trump over Clinton. 

But in those numbers may lie a unique opportunity: a chance for the likely general election foes to poach disenchanted voters from each other’s party. This raises the possibility of a frenzied scramble for crossover votes going into the fall -- as Trump appeals to Democrats to vote against Clinton, Clinton appeals to Republicans to vote against Trump, and both play big to independents.

Republican strategist Rob Burgess pointed to a long history of Americans “claiming to vote for the [lesser] of two evils.” However, he says such lore could be reality in 2016, especially if Trump continues to harp on Clinton’s legacy of “shady record-keeping.”

“As long as Secretary Clinton is being investigated for the improper handling of our country's classified documents, voters will pause before casting a vote for her," he said.

Clinton has struggled with voters who question her trustworthiness amid an FBI investigation into her use of private emails while secretary of state and other controversies.

Likewise, Trump’s controversial comments and views on everything from Muslims to women to illegal immigrants – and fluid positions on a range of key issues -- have made him plenty of foes on the Republican side.

There already are signs Clinton’s team wants to peel off disaffected Republicans – and GOP donors. The day after Trump won a landslide victory last week in Indiana, a top Clinton fundraiser reached out to a Republican fundraiser with an email, obtained by FoxNews.com, that included the subject line: “We want you on the Hillary team!!!”

“Definitely let me know if there's even a slight chance of getting you on our team,” the email read. “Trump will be a disaster for the country if he wins the presidency.”

Trump has an uncanny appeal for many voters, which helped him vanquish 16 GOP rivals and increase party turnout “by millions and millions” compared with 2012, as he frequently boasts.

Still, some party elders and leaders are reluctant to get behind him, including House Speaker Paul Ryan. Former Presidents George H. W. Bush and George W. Bush also have no plans to endorse Trump, though no prominent Republican has publicly vowed to instead vote for Clinton.

Perhaps the most high-profile defector to Clinton is Mark Salter -- a former, long-time adviser to 2012 GOP presidential nominee John McCain.

“I’m with her,” he tweeted after Trump attacked then-primary rival Texas Sen. Ted Cruz based off a National Enquirer story.

Fox News exit polls from several recent primaries suggest a significant number of Democrats and Republicans wouldn’t vote, respectively, for Clinton or Trump in a general election. For instance, in Pennsylvania, a majority of John Kasich voters said they wouldn’t vote for Trump; in Maryland, nearly 80 percent of Bernie Sanders backers said they wouldn’t vote for Clinton.

However, whether they vote against their party, stay home or come around to their nominee in the end remains to be seen.

Trump, while appealing to anti-Clinton sentiments, also is reaching out to Democrats with a populist message on jobs and trade – suggesting supporters of Democratic underdog Sanders should agree with his message.

The billionaire businessman’s assault on international trade deals, blamed for moving hundreds of thousands of jobs overseas, has a ready audience among organized labor -- a reliable Democratic voting bloc. Union leaders have mounted a preemptive counter-attack.

“There is no denying that Donald Trump sees our members as his path to the presidency,” Richard Trumka, leader of the AFL-CIO, which represents roughly 12.5 million workers, warned a gathering of steelworkers last month in Washington, D.C. “He says he’ll bring back steel. He says he’ll bring back coal. Baloney. … Donald Trump is a dangerous, delusional demagogue.”

While Trump has made his pitch from the start of the election cycle to voters of all political stripes, Clinton seems to have made a decision to wait until Trump emerged as the presumptive GOP nominee. 

On Sunday, Clinton told CBS’ “Face the Nation”: “I am asking people to come join this campaign. … And I’ve had a lot of outreach on Republicans in the last days who say that they are interested in talking about that.”