A new poll that shows presidential hopeful Bernie Sanders inching ahead of rival Hillary Clinton nationally also indicates that fully one in four supporters of the Vermont senator say they would not back Clinton should she nab the Democratic nomination.

"Right now, the Sanders voters are more reluctant to support a Clinton candidacy," said Lee Miringoff, director of the Marist Institute for Public Opinion in New York, which conducted the McClatchy-Marist national poll.

According to McClatchy, 25 percent of Sanders supporters said they would not vote for Clinton in a general election, while just 69 percent said they would back her.

"By comparison," McClatchy reports, "Clinton supporters are considerably more open to supporting Sanders should he overtake her large lead in delegates and win the nomination. Just 14 percent of Clinton supporters would shun him in the general election, while 79 percent would support him, the poll found."

Nationally, the poll find Sanders beating Clinton 49 to 47 percent. The survey of 1,297 adults was conducted March 29-31 with a margin of error of +/- 2.7 percentage points.

It comes on the heels of Sanders' double-digit win in Wisconsin on Tuesday.

The results reflect ambivalence on the part of a controversial subset of Sanders supporters, whose concerns about Clinton—her ties to Wall Street and the fossil fuels industry, her establishment politics, her flip-flopping stances on trade deals—run deep.

Actress Susan Sarandon, for example, said in an interview late last month that she could see why people would be reluctant to support Clinton in November. "If you think that it's pragmatic to shore up the status quo right now," Sarandon said, "then you're not in touch with the status quo. The status quo is not working."

Indeed, a separate Wall Street Journal/NBC News poll last month indicated that one-third of Sanders' supporters couldn't see themselves voting for Clinton in November.

Furthermore, USA Today wrote on Wednesday, "Wisconsin exit polls underscore Clinton's longer term challenges in exciting Democrats to back her. Just six in ten say she's honest and trustworthy. Sanders also ran even with Clinton among female voters, who've carried her in other races. He won both higher and lower-income voters, including union households, and broke even with her with moderates."

All this speaks to the critical question of electability, which has become a theme of the 2016 campaign season.

As Sanders himself told a crowd of 2,000 in Wyoming on Tuesday night: "Momentum is that when you look at national polls or statewide polls, we are defeating Donald Trump by very significant numbers, and in almost every instance our margin over Trump is wider than Secretary Clinton's."