Nearly half of Republicans believe millions of people voted illegally in the last presidential election a claim that President Donald Trump has repeatedly made, even though neither he nor anyone else has produced concrete evidence to show its true.

Forty-eight percent of Republicans said they believe between 3 million and 5 million people voted illegally in 2016, while 17 percent said they do not, according to a new HuffPost/YouGov poll . Another 35 percent of the GOP said they were unsure.

Just under one-quarter of Democrats said they believed the allegations that millions of votes were cast illegally, while 51 percent said they didnt and 26 percent said they were unsure.

The continued belief that millions voted illegally in 2016 underscores the way that Trump has been able to undermine confidence in the American electoral system without producing proof of widespread fraud. Overall, 28 percent of Americans said they believed between 3 million and 5 million people voted illegally in 2016, while 37 percent said they did not. Thirty-five percent said they were unsure.

Last year, a Quinnipiac poll found that 29 percent of voters believed millions of illegal votes had been cast in 2016, with Republicans by far the most likely to subscribe to the theory. In that survey, which didnt give respondents an explicit prompt to say they werent certain about the plausibility of the theory, more than 60 percent said they didnt believe that widespread voter fraud had occurred.

Since November 2016, Trump has said he would have won the popular vote had it not been for millions of people who voted illegally. He convened a presidential commission to investigate voter fraud, which failed to turn up evidence of illegal voting and was disbanded within a year. Still, Trump has continued to say millions voted illegally.

Theres no evidence to support this claim, and several studies have shown that voter fraud is not a widespread problem . The political science professor whose work Trump may have extrapolated from to say he really won the popular vote testified in court earlier this year, saying he could not support the conclusion that voter fraud is a widespread issue.

Lorraine Minnite, a political science professor at Rutgers University and the author of The Myth of Voter Fraud, said Trumps panel to investigate voter fraud helped maintain a public impression that illegal voting is a major issue.

The trick or the tool of using investigations in general, its a problem because you can investigate anything. You can accuse people of anything. And then you can throw resources at that, she told HuffPost. An investigation, when you hear about that, you think, Well, they must have some reason to launch an investigation. If theres no problem, why would they be investigating it? Its a PR stunt.

Despite their willingness to believe millions of illegal votes were cast, Republicans were also more likely to say they were confident in the election system. Seventy percent said theyre at least somewhat confident that votes were accurately counted across the country last year, compared to 39 percent of Democrats who said the same.

The lingering belief about voter fraud may be having concrete consequences. New Hampshire officials have said there is no evidence of voter fraud in the state, but they cited the perception of it to justify legislation that would tighten residency requirements to vote there. Trump has claimed without evidence that he would have won the state had it not been for people bused in from neighboring states to vote there.

The HuffPost/YouGov poll also illustrated a clear partisan divide on obstacles to the ballot box. Republicans said by a 56-point margin that voter fraud is a more widespread problem than eligible voters being prevented from voting. Democrats said by a 23-point margin that legitimate voters being unable to cast a ballot was the more widespread problem.

The difference between Trump and Hillary Clinton supporters was even starker. Trump voters said by a 68-point margin that voter fraud was more widespread, and Clinton voters said by a 51-point margin that there were more qualified voters being blocked.

The HuffPost/YouGov poll consisted of 1,000 completed interviews conducted May 17-20 among U.S. adults using a sample selected from YouGovs opt-in online panel to match the demographics and other characteristics of the adult U.S. population.

HuffPost has teamed up with YouGov to conduct daily opinion polls. You can learn more about this project and take part in YouGovs nationally representative opinion polling. More details on the polls methodology are available here .

Most surveys report a margin of error that represents some, but not all, potential survey errors. YouGovs reports include a model-based margin of error, which rests on a specific set of statistical assumptions about the selected sample rather than the standard methodology for random probability sampling. If these assumptions are wrong, the model-based margin of error may also be inaccurate. Click here for a more detailed explanation of the model-based margin of error.