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Photo by Jeff Swensen/Getty Images Photo by Jeff Swensen/Getty Images

Joe Biden is up and Hillary Clinton is down in a new Monmouth University national poll of Democrats and Democratic-leaning voters.

Support for the vice president is building as speculation grows about his potential entry into the 2016 presidential race, with 22 percent saying they'd back him. That's ahead of Senator Bernie Sanders of Vermont, who was picked by 20 percent. The difference is within the poll's plus or minus 5.3 percentage point margin of error.

Clinton, who continues to confront questions about her use of a private e-mail server while secretary of state in the Obama administration, has the support of 42 percent, down from 52 percent a month ago. Biden's number is 10 percentage points higher than a month ago, while Sanders has seen a 4-point increase during that time.

“For a guy who is not running for president, Biden sure is making headway against the frontrunner,” Patrick Murray, director of the West Long Branch, N.J.-based Monmouth University Polling Institute, said in a statement. “There also seems to be an opening with the more liberal Sanders voters, if Biden plays his cards right.”

Slim majorities of Clinton and Sanders voters say they would be at least somewhat likely to consider switching their support to Biden if he jumps into the race. The vice president isn't expected to make a decision until at least the end of September.

Biden’s favorable rating among Democrats and those leaning that way has moved a few percentage points higher and now stands at 71 percent, up from 67 percent a month ago.

As her nomination support has dropped, Clinton continues to be viewed favorably by the vast majority of Democrats, with her favorability rating unchanged from a month earlier, at 71 percent. For Sanders, his rating stands at 41 percent, with 14 percent viewing him unfavorably and 45 percent still having no opinion of him.

The telephone poll was conducted Aug. 31 to Sept. 2 with 1,009 U.S. adults. This release is based on a voter sample of 339 registered voters who identify themselves as Democrats or lean toward the Democratic Party.

One prominent conservative seemed to be egging Biden on as he contemplates entering the race. Fox Chairman Rupert Murdoch tweeted on Monday that it's “very likely” the vice president would win the nomination and that he'd “be hard to beat” in 2016.

Looks like Biden already running. Very likely he wins nomination and be hard to beat. — Rupert Murdoch (@rupertmurdoch) September 7, 2015