The poll also suggests Democrats may have a difficult time uniting behind a candidate after their increasingly contentious primary fight.
One-quarter of voters supporting Sanders say they wouldn't back Clinton in a general election, the new McClatchy-Marist poll released Wednesday finds. About 69 percent of Sanders supporters say that they would support Clinton if she were to become the party's nominee.
Sanders supporters least likely to support Clinton if she wins the nomination include independents, moderates and men, according to the poll, while his backers who are most likely to support the former secretary of State include nonwhites, Democrats, women and people ages 18 to 44.
Only 14 percent of those supporting Clinton say they wouldn't back the Vermont Independent senator in a general election, with 79 percent saying they would support him.
Overall, the poll found Sanders leading among those ages 29 and younger, Hispanics and independents. Clinton leads among those 60 and older, African-Americans and Democrats.
The poll was conducted with 1,297 people from March 29-31 and has a margin of error is 2.7 points.
Sanders has won seven of the last eight primary contests, including the Wisconsin primary Tuesday. Despite his hot stretch, Clinton still leads the overall race for pledged delegates, 1,279 to 1,027.
According to the RealClearPolitics average of polls, Clinton has a 5.9-point lead over Sanders nationally.