Vermont Sen. and Democratic presidential candidate Bernie Sanders has joined calls for an apology from Republican front-runner Donald Trump for his handling of a question at a town hall that asserted President Barack Obama was a Muslim and not an American. Trump's failure to challenge the questioner prompted widespread condemnation from those who said the comments were both untrue and hateful.

"Trump must apologize to the president and American people for continuing the lie that the president is not an American and not a Christian," Sanders tweeted Friday.

The claim was made by an unidentified attendee of a Trump rally in Rochester, New Hampshire. “We have a problem in this country, it’s called Muslims," the man said. "You know our current president is one. You know he’s not even an American.”

Rather than correct the individual, Trump, who has previously questioned Obama’s birthplace, responded: “We need this question.”

The man then asked what could be done to stem the growth of supposed Muslim training camps, and how America could “get rid” of them. Trump responded by saying he would look into many different options as president. When asked by reporters why he didn’t challenge the statements about the president’s identity, Trump declined to comment. His supporters have since said that he did not hear that part of the question.

Former Secretary of State Hillary Clinton, the Democratic front-runner, also tweeted that Trump's silence following the statements was "disturbing, & just plain wrong."

Nearly 30 percent of Americans believe Obama is a Muslim, a recent CNN/ORC poll found. That figure rises to more than 40 percent among Republicans.

When U.S. Sen. John McCain, who ran against Obama in 2008, was told during a similar town-hall event by a woman in the audience that Obama was an Arab, he immediately corrected her, something Trump's critics have pointed to as an example of how he should have handled the questioner.