Sen. Kamala D. Harris (Calif.), who has tangled with Biden on the debate stage, takes 11 percent in the new poll, up slightly from April, while support for Sen. Bernie Sanders (Vt.) has dropped to 9 percent from 16 percent four months ago.

Iowa voters have become much more familiar with their party’s candidates since the spring, with several establishing heavily positive reputations. Warren stands out the most on this front, with 76 percent of likely caucusgoers holding a favorable impression of her, compared with 14 percent unfavorable. Those figures mark an improvement from her favorable-unfavorable margin of 67 percent to 20 percent in April.

Former Texas congressman Beto O'Rourke listens as Sen. Elizabeth Warren (D-Mass.) speaks during the Democratic presidential primary debate on July 30 in Detroit. (Paul Sancya/AP)

Biden is also widely popular, with 73 percent rating him favorably and 19 percent unfavorably, although his positive rating dipped five percentage points in the past four months.

Pete Buttigieg, the mayor of South Bend, Ind., garners a 68 percent favorable mark, up sharply from 45 percent in April, a point when nearly one-quarter of voters had not heard of him. Despite the improvement, Buttigieg’s 8 percent support among likely caucusgoers is little changed from 9 percent in April.

The poll finds Sanders has become less popular since the spring, coinciding with his drop in support. His favorable rating declined from 67 percent in April to 58 percent in the new poll. One-third of those surveyed now have an unfavorable view of Sanders, the highest among candidates measured in the survey — and significantly greater than the share who give unfavorable marks to Biden (19 percent) and Warren (14 percent).

Support for Sanders’s nomination in latest Monmouth poll matches his 9 percent support in a July USA Today/Suffolk poll following the first candidates’ debate in June, but is lower than the 19 percent support he received in a mid-July CBS News/YouGov poll. Together, the surveys underscore the uncertainty of candidates’ support at a stage when voters are still evaluating the field, as well as the bursts of coverage surrounding debates.

Warren’s rise has been fueled by increasing support from liberal Democrats and college graduates, according to the poll. Her support among liberals overall has grown from 12 percent in April to 27 percent in the latest survey. Warren has a clear advantage among the smaller segment of “very liberal” Democrats, standing at 37 percent support compared with 19 percent for Sanders and 7 percent for Biden. The former vice president is buoyed by moderate-to-conservative Democrats, among whom he garners 36 percent support to Warren’s 9 percent.

Younger voters were Sanders’s base group in 2016, but the Monmouth poll shows he faces significant competition for this group in Iowa. Sanders garners 18 percent support among voters ages 18-49, similar to 19 percent for Biden and 20 percent for Warren. Sanders receives a far smaller 5 percent among voters ages 50-64, and just 1 percent support among seniors. Warren’s support is fairly constant across age groups, while Biden is particular strong among older voters, garnering roughly one-third of voters ages 50 and older.

The poll suggests that Biden may have more difficulty than Warren mobilizing his supporters to attend in-person caucuses, which are anticipated to decide 90 percent of the delegates. Biden garners 26 percent support among those who plan to attend in-person caucuses, compared with 37 percent who plan to attend virtual caucuses, which are expected to award 10 percent of the delegates.

Warren, by contrast, fares much better among voters who plan to attend caucuses in person (20 percent) rather than a virtual caucus (11 percent).

Notably, 32 percent of likely Democratic caucusgoers think half or more of the two-dozen Democratic candidates could beat President Trump in November 2020, while 35 percent think fewer than half of them could, and 30 percent think only one or two of them could.

“Biden is the main beneficiary of voters who think that the field is pretty weak against Trump,” Patrick Murray, director of the Monmouth University Polling Institute, said in a statement. “Among those who feel only one or two candidates have a shot, 41% currently support Biden, compared with just 13% who are for Warren, 11% for Harris, 10% for Sanders, and 5% for Buttigieg.”