Attendees at the Conservative Political Action Conference this week picked Bernie Sanders as the biggest threat to stop Donald Trump from being reelected in November.

Organizers at the annual conference conducted a straw poll that measures aspects of the president’s performance, including predictions for the next election. As Fox News reported, attendees said that they believed the Vermont senator had the best chance of denying the president a second term in office.

“Thirty-four percent of attendees, meanwhile, ranked self-described democratic socialist Sen. Bernie Sanders, I-Vt., the current front-runner in the crowded Democratic field, as the toughest for Trump to beat in November, with 23 percent of CPAC attendees saying former New York City Mayor Mike Bloomberg would be the toughest, and 9 percent picking former Vice President Joe Biden.”

Both Sanders and Biden have led Trump in head-to-head polling, though results have been close.

Sanders had snatched the mantle as Democratic primary frontrunner from Joe Biden after wins in two of the first three contests, but Biden turned in a strong performance in Saturday’s South Carolina primary. That latest victory gave Biden the lead for popular vote nationally and closed the gap in pledged delegates. Sanders still leads the delegate count and has polled well in many of the states that will vote on the upcoming Super Tuesday, including the delegate-rich California.

Though the CPAC attendees saw Sanders as the biggest threat to Trump, the results were mixed. Trump spoke at the conference on Saturday, at one point instructing the crowd to cheer loudest for the Democratic candidate that they thought he should run against and would give him the best chance to beat. The crowd cheered loudest for Sanders, giving a standing ovation.

Trump and allies have already begun to make some attacks on Sanders, especially his self-identification as a Democratic socialist. Vice President Mike Pence spoke out against socialism in his CPAC speech, saying that the election would show the stark differences between Trump and Sanders.

“We’ve got to tell the truth about socialism and the benefits of freedom,” he said, via Fox News. “When many in this rising generation speak of socialism, I believe they think of better health care or better education or a cleaner environment — but the reality of socialism is very different.”

In his speech at CPAC, Trump also said that he plans to attack Sanders over the Vermont senator’s proposal to allow all prisoners to vote in elections. Trump pointed out that this would give the Boston Marathon bomber the right to vote in the election.