Former vice president Joe Biden lost ground this summer to two of his more liberal challengers, Sens. Elizabeth Warren and Bernie Sanders, according to a national Democratic primary poll from Monmouth University released Monday.

The mid-August poll found a virtual three-way tie between the top candidates. Biden’s support among Democratic voters for the party’s nomination fell to 19 percent from 32 percent in June, while support for Warren (D-Mass.) and Sanders (I-Vt.) each rose slightly to 20 percent, from 15 percent and 14 percent, respectively.

The poll comes a week after Biden released a television ad that cited recent polling to boast of his own electability against President Trump. “All the polls agree Joe Biden is the strongest Democrat to do the job,” the ad’s narrator declared, citing recent hypothetical general election matchup polls between Biden and Trump.

Most recent national polls have shown Biden with a significant lead, and the Monmouth poll is the first major national survey to show Biden without at least a slight edge in primary preference — a fact that the Biden campaign pointed to in arguing that the results did not accurately reflect the state of the race.

“Sometimes there are going to be outliers. We kind of all forget that five out of 100 polls are just going to be wrong. It’s statistics,” said John Anzalone, a pollster for the Biden campaign. “It’s like one of the ‘Sesame Street’ songs, ‘One of these things is not like the others.’”

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The Monmouth survey, which is based on a smaller voter sample than many other national polls, found Biden’s support fell broadly across demographic groups, among liberals, moderates, white and nonwhite voters.

“The main takeaway from this poll is that the Democratic race has become volatile,” said Patrick Murray, director of the independent Monmouth University Polling Institute. “Liberal voters are starting to cast about for a candidate they can identify with. Moderate voters, who have been paying less attention, seem to be expressing doubts about Biden.”

The share of Democratic voters who have a favorable view of Biden has also steadily declined. His net favorability margin — the share of voters who have a favorable view of him minus those who have an unfavorable view of him — fell to 41 in August from 57 in May and 71 in January.

At the same time, the net favorability margin for Warren rose to 52 in August from 46 in May and 40 in January. Sanders support has been more steady — 40 in August, dipping from 44 in May and 49 in January.

The Monmouth poll contrasts with other national surveys this month showing Biden holding a steady advantage. A CNN poll released last week found 29 percent of Democratic-leaning voters supported Biden, compared with 15 percent for Sanders and 14 percent for Warren.

The Democratic National Committee has recognized the Monmouth survey as a qualifying poll for the next two debates, but the results released Monday had no effect on who will make the September debates. Spiritual author Marianne Williamson was the only candidate who has not qualified for that event to score 2 percent or more. She needs to receive three more polls at 2 percent or higher by Wednesday to make the debate stage.

Financier Tom Steyer needs one more poll, Rep. Tulsi Gabbard (D-Hawaii) needs two more polls and Sen. Kirsten Gillibrand (D-N.Y.) needs three more polls.

The poll, conducted between Aug. 16 and 20 by telephone, surveyed a sample of 298 registered voters who identify as Democrats or lean toward the Democratic Party. It had a margin of error of plus or minus 5.7 percentage points.

Scott Clement contributed to this report.