About 20 percent of likely Democratic voters say they would buck the party and vote for Republican presidential candidate Donald Trump Donald John TrumpObama calls on Senate not to fill Ginsburg's vacancy until after election Planned Parenthood: 'The fate of our rights' depends on Ginsburg replacement Progressive group to spend M in ad campaign on Supreme Court vacancy MORE in a general election, according to a new poll.

The willingness of some Democrats to change sides could be a major problem for Democratic front-runner Hillary Clinton this fall.

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The new figures were released by Mercury Analytics, a research company with clients that include MSNBC and Fox News, as the result of an online poll and dial-test of Trump’s first campaign ad.

A smaller number of Republicans say they’d vote for Clinton — about 14 percent.

Ron Howard, the company’s CEO and a Democrat, said he expected Trump’s first campaign ad to have “little impact — or in fact negative impact” on Democrats and independents. But the ad actually resonated with Democrats more than he expected.

About one-quarter of Democrats said they "agree completely" that the ad raises good points, with another 19 percent agreeing at least "somewhat."

“The challenge to Hillary, if Trump is the nominee and pivots to the center in the general election as a problem-solving, independent-minded, successful 'get it done' businessman is that Democrats will no longer be able to count on his personality and outrageous sound bites to disqualify him in the voters' minds,” Howard said, according to U.S. News & World Report.

The company surveyed about 916 likely voters this week. The poll has a margin of error of plus or minus 3.5 percent.