WASHINGTON - Ironically for the Republican Party, the candidate who seems headed for the nomination is the one who is least likely to beat Democratic front-runner Hillary Clinton in the 2016 presidential race.

US Republican presidential candidate Donald Trump speaks during "Stop the Iran Deal" rally at West Lawn of the Capitol in Washington D.C., capital of the United States, Sept. 9, 2015. [Photo/Xinhua]

Bombastic billionaire Donald Trump is leading the rest of GOP candidates by a whopping 15 points, according to the latest Real Clear Politics' polling average. His lead even expanded to 21 points in a CNN/ORC poll.

But while Trump has grabbed much media attention, he does not stack up as well against Clinton.

Indeed, Clinton leads Trump by six points in Real Clear Politics' polling average. While that is not as big as Trump's lead against other GOP hopefuls, it is still a clear lead. And that could well spell trouble for Republicans.

What's more, Trump seems perhaps one front-runner who would have the toughest time beating Clinton. In Real Clear Politics' head-to-head matchups, Florida Senator Marco Rubio leads Clinton by a sliver, essentially making it neck-in-neck between him and Clinton.

Clinton leads New Jersey Governor Chris Christie, Texas Senator Ted Cruz, former brain surgeon and political outsider Ben Carson and former Florida Governor Jeb Bush by less than two points, which is essentially a tie. That means that if Trump fails to clinch the GOP nomination, the White House will be up for grabs in 2016.

"Trump is behind Clinton in the polls because he has taken positions that have alienated large groups of Americans," Brookings Institution's senior fellow Darrell West told Xinhua.

"His views on immigration have upset Latino voters. His bellicose language on ISIS (Islamic State) and foreign policy have created anxiety among women and other voters concerned about the United States going to war in the Middle East," he said. "Educated voters find his lack of policy substance upsetting."

West added that Clinton matches up well with Trump because she is knowledgeable on many issues, understands the need for moderation in foreign policy, and is inclusive in how she thinks about social issues.

Public opinion surveys show that Trump polls very poorly among Latino voters. That is a crucial swing group and Republicans need to get more than one-third of that group in order to win the presidency. Many Latino voters are concentrated in key states such as Florida, and that gives them unusual influence in the campaign.

Trump does not play well among female voters because they worry about his war talk and sharp rhetoric on many issues. Many women see him as too quick to condemn other people and having positions that divide the country. He has very high negative ratings among women in general, West added.

Trump's lack of popularity among Latinos and women voters will work in Clinton's favor.

While Democrats tend to get most of the single women's and Latinos votes, Trump will be unlikely to peel off very much of Clinton's support among those groups if the two candidates face off. Clinton is also very popular among African Americans, and she will more than likely clinch the majority of that crucial group.

"Given what Trump has said in the GOP primary, there are groups -particularly Hispanics and women - who will find it nearly impossible to vote for him," Dan Mahaffee, an analyst with the Center for the Study of the Presidency and Congress, told Xinhua.

Furthermore, seeing polls that show that 50 percent of Americans would be embarrassed by a Trump presidency does not bode well for Trump's prospects in a general election, Mahaffee said.

Finally, if Trump were the GOP nominee, "there may be many centrist Republicans or center-right independents who decide to cross the aisle and vote for Hillary over Trump," he added.

All this bodes well for Clinton, and a Trump nomination as the GOP candidate would be the best chance of a Clinton win in the 2016 race to the White House.

Meanwhile, Clinton continues to lead in the polls against Democratic rival Bernie Sanders. While Sanders has drawn much media attention, he trails Clinton by a cavernous gap of 26 points, according to Real Clear Politics.

All told, Clinton seems more than likely to gain the Democratic nomination.