One quarter of Americans who are registered Democrats or lean that way say Vice President Joe Biden is now their top choice for president. The findings of a national Bloomberg Politics poll released Wednesday represent a notable achievement for an as-yet undeclared candidate, suggest concerns about Hillary Clinton's candidacy, and raise the prospect of a competitive three-way race for the Democratic presidential nomination. 

Clinton, once the prohibitive front-runner, is now the top choice of 33 percent of registered Democrats and those who lean Democrat, the poll shows. Biden places second with 25 percent and Vermont Senator Bernie Sanders is at 24 percent. The other three Democratic candidates combined are the top choice for less than 4 percent of that base.

Recent polls in states that will host the earliest contests of the presidential campaign, Iowa and New Hampshire, have shown Sanders closing in on and, in some cases, surpassing Clinton with Biden in a more distant third place. As Clinton's numbers have dropped, the vice president has been openly mulling a late entry into the race. But he is delaying a decision to allow himself and his family an opportunity to grieve the death of his son Beau to brain cancer.

Read the questions and methodology here.

The latest Bloomberg survey shows the vice president at par with Sanders in terms of Democratic support nationally. Adding to the good news for the vice president: His favorability ratings are on the rise. Since the last Bloomberg poll in April, Clinton's favorability ratings have dropped 10 points, from 48 percent to 38 percent. Biden's 49 percent favorable score represents a 3-point uptick. He was the only one of a dozen national political figures and entities whose approval rating improved over the summer. 

“He's been a very good, positive influence on the Obama administration,” said Karen Hood, a 63-year-old consulting engineer from Houston, Texas, who said that Biden is her first choice. “I think he would help carry on what President Obama has started.”

Clinton has been holding onto first place thanks to the the support of women Democrats who are registered voters, said J. Ann Selzer, whose firm conducted the poll. Among women, Clinton gets 35 percent support compared to 31 percent among men. Sanders, meanwhile, enjoys a reverse gender gap: 30 percent of men support him, compared to 20 percent of women. Among male Democrats, Clinton edges Sanders by a single point, 31 percent to his 30 percent. Biden, who has worked hard to win women's support by emphasizing his work on issues such as the Violence Against Women Act, draws support about equally from both genders.

Sanders is still unknown to about one in three registered Democrats. Only six percent say they don't know Biden.

The survey also asked U.S. adults across party lines whether they think Biden, 72, who has twice run unsuccessfully for president, should get into the race or stay out. Forty-seven percent said he should run, while 37 percent prefer for him to stay out and 15 percent aren’t sure.

A closer look at the sample of the Democrat and Democrat-leaning poll respondents shows that nationally, Clinton is better liked by women than men—74 percent to 64 percent—while Biden's favorability is closer to gender-neutral, with 81 percent for women and 79 percent for men. The survey also found those who are married or have children under the age of 18 were more likely to give Biden favorable ratings than Clinton.

Opinions on President Barack Obama are fairly evenly divided: 46 percent of survey participants said they approve of the job he's doing and 47 percent don't. Obama is underwater on most issues, with 47 percent approving of his handling of the economy and 44 percent approving of his work on health care, but his ratings are much lower on his handling of the Republican-controlled Congress and the nuclear deal with Iran. He gets just 35 percent approval on each of those issues.

The poll of 1,001 adults, including 375 registered Democrats and Democrat-leaners, was conducted Sept. 18-21 for Bloomberg Politics by Selzer & Co. of Des Moines, Iowa. The overall sample has a margin of error of plus or minus 3.1 percentage points, while the Democratic sample has a margin of error of plus or minus 5.1 percentage points. 

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