Vice President Joe Biden, who is not a declared candidate for 2016, is the Democratic Party's best hope for keeping the White House, a new poll says. Reuters

The Democratic Party's best hope for winning the White House in 2016 is Vice President Joe Biden, not former Secretary of State Hillary Clinton, a new Quinnipiac University national poll says. Biden isn't a declared candidate for the party's 2016 presidential nomination, but he has reportedly been weighing a potential run while the once seemingly inevitable Clinton has been slipping in polls around the country.

As for the Republican Party, real estate mogul Donald Trump has continued his dominance in the polls, seeing his share of likely voters jump to 28 percent from 20 percent at the end of July. It's the highest share of the potential vote any Republican has received, but also the widest lead over competitors so far in the election cycle. Trump's nearest challenger is retired neurosurgeon Ben Carson (12 percent), followed by former Florida Gov. Jeb Bush, Texas Sen. Ted Cruz and Florida Sen. Marco Rubio (all with 7 percent).

But while Trump may be dominating the GOP field, he doesn't do quite as well when stacked against the top-polling Democrats. Biden beats Trump 48 percent to 40 in a hypothetical election, Clinton beats him 45 percent to 41, and Vermont Sen. Bernie Sanders tops Trump 44 percent to 41, the poll says. Against Bush and Rubio, Democrats win in every scenario -- except for a Sanders-Rubio matchup, where Sanders is down 1 percent. Biden has the biggest leads.

Former U.S. Secretary of State Hillary Clinton has been sliding in the polls lately. Will she slide far enough to convince Vice President Joe Biden to enter the presidential race? Reuters

Biden has said he will decide by late September whether to launch a run for president. Political strategists say he is paying close attention to whether Clinton's candidacy will continue to be damaged from ongoing questions about her use of a private email server while secretary of state [2009 to 2013] and contributions made to her family's charitable organization from countries she was working with while in the office. If it looks like she'll continue to be dragged down, chances are better that Biden will enter the race. Despite her recent troubles, Clinton remains the front-runner among Democrats, with support from 45 percent of likely voters, followed by Sanders at 22 percent and Biden at 18.

Clinton's vulnerabilities seem more obvious by the day. Her 45 percent pull is down from 55 percent a month ago. In New Hampshire, Clinton lost her edge over Sanders this week in an average of polls conducted in the state. Sanders, an independent who caucuses with Democrats in the Senate, is still viewed as a long shot for the White House, but has steadily gained on Clinton in the months since he declared his candidacy.

The Quinnipiac poll was conducted Aug. 20-25. Pollsters contacted 1,563 registered voters nationwide for live interviews on landlines and cell phones. Those contacted included 666 Republican voters and 647 Democrats. Overall, the poll had a margin of error of plus or minus 2.5 percent. For the Republican portion, there was a margin of error of plus or minus 3.8 percent. For the Democratic portion, there was a margin of error of plus or minus 3.9 percent.

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