Mr. Sanders’s campaign has always said he was being attacked by the Democratic establishment. But now he is actually the target of a significant negative advertising campaign.

Backed by more than $680,000 from the group Democratic Majority for Israel’s political action arm, a new negative ad will air in Iowa through the caucuses. That’s a significant buy in a state that has not seen Democrats aim a negative attack ad at any Democratic candidate during this cycle.

The message: Featuring six Iowa voters, the ad argues that Mr. Sanders can’t defeat Mr. Trump. The voters cite the senator’s heart attack last year and his “socialist” ideology — he calls himself a democratic socialist — as evidence that he would not match up well against Mr. Trump, particularly in “Michigan, Pennsylvania and Iowa.”

The takeaway: The ad comes amid growing concern among some Democrats that Mr. Sanders could capture the nomination, and that if they don’t mount an effort now, he could be unstoppable after Super Tuesday. But it also opens up a new line of attack against him that doesn’t quite ring true; though the voters in the ad argue that he couldn’t beat Mr. Trump, Mr. Sanders narrowly leads the president in many polls.

Nonetheless, in a race that hasn’t seen a single negative ad on television, it remains to be seen how impactful this one can be.