Former Secretary of State Hillary Rodham Clinton’s lead in the race for the 2016 Democratic presidential nomination has shrunk over the past month amid increased support for both Vice President Joseph R. Biden Jr. and Sen. Bernie Sanders of Vermont, according to a poll released Tuesday.
Mrs. Clinton still had the support of 42 percent of Democrats and Democratic-leaning voters nationally in the Monmouth University poll, but that was down from 52 percent a month ago.
Mr. Biden, who has not yet declared his 2016 intentions, was at 22 percent in the survey — a 10-point increase from a poll released in early August. Mr. Sanders, who has emerged as Mrs. Clinton’s chief rival on the Democratic side in the race for the White House, was at 20 percent — up 4 points from a month ago.
“For a guy who is not running for president, Biden sure is making headway against the front-runner,” said Patrick Murray, director of the independent Monmouth University Polling Institute in West Long Branch, New Jersey. “There also seems to be an opening with the more liberal Sanders voters if Biden plays his cards right.”
Majorities of current Clinton and Sanders supporters — 56 percent from each — said they would be at least somewhat likely to consider switching their support to Mr. Biden if the vice president jumps into the race.
Mr. Biden recently met with Sen. Elizabeth Warren of Massachusetts, who has emerged as a hero among liberals. Twenty-three percent said they would be more likely to support Mr. Biden if he promised to tap Ms. Warren as his running mate, though 66 percent said that prospect wouldn’t affect their decision.
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But 43 percent of Sanders supporters and 18 percent of Clinton supporters said adding Ms. Warren to the ticket would make them more likely to support Mr. Biden, according to the poll.
Seventy-one percent said they have a favorable view of Mr. Biden, versus 9 percent who said they have an unfavorable one — slightly better than a 67 percent/14 percent split from a month ago.
Seventy-one percent also said they have a favorable view of Mrs. Clinton, compared to 17 percent who said they have an unfavorable one, the same as a month ago. Mr. Sanders had a 41 percent/14 percent favorable/unfavorable split, with 45 percent of Democrats who still said they have no opinion of him. Mr. Sanders had a 42 percent/12 percent split a month ago.
The survey taken from Aug. 31 to Sept. 2 was of 339 registered voters who identified themselves as Democrats or lean toward the Democratic party. The margin of error was plus or minus 5.3 percent.