Rarely have two U.S. presidential front-runners needed more fromrunningmatesthan HillaryClintonand DonaldTrump.

The Democratic and Republican leaders in the 2016 White House race are unusually unpopular nationally, polls show, despite their success with voters in their respective parties.Clintonhas struggled to generate excitement about her candidacy and assuage voter concerns about her trustworthiness, whileTrumpfares badly among women and minorities in polls.

Their weaknesses only increase the importance of their vice presidential pick, who couldpotentially help them rally support in key demographics ahead of the Nov. 8 vote - if each gets their party's nomination.

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Hispanic advocacy groups are lobbyingClintonto choose a Hispanicrunningmate. The person most often mentioned - Julian Castro, the housing and urban development secretary who is one of the party's rising stars - is opposed by liberal activists who accuse him of favoring Wall Street firms in the sale of distressed mortgages.

Some of the groups have started an online petition against Castro's candidacy, which has angered the Latino Victory Fund, a nonpartisan advocacy group that has been pushing for a Latino vice-presidential nominee.

“These attacks against Secretary Castro are completely unfounded, short sighted, and only serve to pit us against each other,” said Cristobal Alex, the fund’s president. The U.S. Hispanic Chamber of Commerce has called onClintonto choose Castro.

Democratic strategist Joe Velasquez, aClintonsupporter, said a Hispanicrunningmate could make a difference in swing states like Florida, Colorado, Nevada and Virginia, which have large Hispanic populations. Polls show Hispanic voters overwhelmingly dislikeTrump, in part because of his comments likening illegal immigrants to criminals.

U.S. Labor Secretary Tom Perez has also been mentioned as a possible Latino pick.

ButClinton’s decision isn’t clear-cut.

GivenTrump’sunpopularitywith some moderate Republicans, she may want a vice-president who can wrest away some of those voters. Someone like Tim Kaine, a senator from Virginia who personally opposes abortion, or New Jersey Senator Cory Booker, who has close ties to the financial sector.

There was no immediate comment from Castro, Kaine, Perez or Booker.

Clintonthough could give a higher priority to winning over liberal democrats, who see her as too pro-Wall Street and have flocked to her rival, Senator Bernie Sanders of Vermont.

Sherrod Brown, a senator from Ohio who is a fierce opponent of global trade deals, and Senator Elizabeth Warren of Massachusetts, a fierce critic of the banking industry, have both been named as possible vice president picks.

Brown and Warren did not respond to requests for comment.

Democratic strategist Chris Kofinis is skeptical thatClintonneeds to spend much time appeasing the liberal wing of her party if she wins the nomination.

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“As much as people have made out this race betweenClintonand Sanders to be an ideological death match, DonaldTrumpwill do more to bring the Democratic Party together than anyone has ever done,” he said.

Trumpis viewed unfavorably by 80 percent of Democrats, according to a recent Reuters/Ipsos tracking poll.

But other Democrats fear the increasingly bitterClinton-Sanders battle, has fractured the party. Jim Manley, a former top aide to Senate Minority Leader Harry Reid, saidClinton's pick "has to be viewed through a prism of, among other things, their ability to bring on Sanders voters.”


Both theClintonandTrumpcampaigns refused to comment on the vice-presidential speculation, saying they remain focused on winning their respective nominating contests.

Clintonholds a commanding lead among the party delegates who will pick the party's nominee, but Sanders has pledged to keep fighting.Trumpmay not have the Republican nomination locked down until the summer, if he can beat off challenges by Texas Senator Ted Cruz and Ohio Governor John Kasich, who isrunninga distant third in the Republican race.

Typically, vice-presidential selections are vetted for weeks after a candidate has emerged as the party’s nominee and are announced shortly before the party conventions in the summer.

Trump’s polarizing candidacy could make his job of finding arunningmate more difficult. For much of the campaign season, many in the Republican establishment, along with well-funded political committees, have been looking for ways to keepTrumpfrom winning the Republican nomination.

Trumphas stated his preference for an established office-holder rather than a political neophyte like himself, specifically listing Wisconsin Governor Scott Walker, Florida Senator Marco Rubio, and Kasich.

Walker, who has endorsed Cruz, laughed when he was asked about the prospect, but has been noncommittal. Rubio has repeatedly rejected the idea. Kasich last week said there was “zero chance” of his playing second fiddle toTrump, despite mounting speculation that he would be an ideal candidate because of his deep government experience and Ohio’s crucial role as a general election swing state.

A source close to Kasich told Reuters the governor has no interest in working withTrumpas the two don't agree on much.

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Cruz, too, said this week he would not beTrump’srunningmate, while telling reporters in Hollywood, Florida on Wednesday that "aTrump-Kasich ticket loses to HillaryClinton."

Tony Fratto, a Republican strategist, saidTrumpmay be forced to turn to “guys at the end of their careers, not people who believe they have a future. Someone in their last act who has nothing to lose by accepting an offer fromTrump.”

That might include New Jersey Governor Chris Christie, who has been one or the few establishment politicians to campaign forTrump, or Florida Governor Rick Scott, who has endorsedTrump. Both are in their final terms in office.

Christie was asked about the prospect in an interview with a New Jersey radio station on Thursday.

"The way I think about these things, you never say 'never,'" he said.

Other names linked toTrumpinclude former New York Mayor Rudolph Giuliani, Alabama Senator Jeff Sessions, aTrumpadviser, and Ben Carson, the retired neurosurgeon and former presidential candidate.

There was no immediate comment from Sessions, Scott and Giuliani. Armstrong Williams, a close adviser to Carson, said the former White House hopeful doesn’t want the slot.

"At this point he has no interest in being in politics ... none,” Williams said. “I thinkTrumpis looking for someone with government experience, someone who is more political."