A Pro-Israel Democratic super PAC has spent hundreds of thousands of dollars promoting a negative ad campaign in Iowa which targets 2020 presidential hopeful Senator Bernie Sanders ahead of next week's caucus.

The ad, which is funded by the Democratic Majority for Israel, shows voters from Iowa arguing why Sanders will struggle to beat President Donald Trump in November's general election.

"I like Bernie, I think he has great ideas, but in Michigan, Pennsylvania, Iowa, they're just not going to vote for a socialist," a voter says.

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The 30-second spot, which first appeared on Wednesday, also takes aim at the senator's left-wing ideology and his recent heart attack.

"I do have some concerns about Bernie Sanders’ health, considering the fact that he did have a heart attack," another voter says.

The ad is scheduled to appear on CNN, MSNBC and ESPN until Monday's Iowa caucus.

Democratic Majority for Israel, which was formed in January 2019, has repeatedly targeted the senator as it seeks to stamp out anti-Israel voices in the Democratic party.

Last year, the group's president said he found it "deeply disturbing" that Sanders had surrounded himself with "endorsers who hate Israel" - a reference to Representative Ilhan Omar, who had just endorsed Sanders, and Linda Sarsour, the Palestinian-American activist who is an official surrogate for the Sanders campaign.

Super PACs have become a fixture of the US campaign finance system. They are funded by American individuals, corporations and unions, who are legally allowed to donate as much money as they want to supposedly independent organisations which are linked to a particular candidate.

According to The Nation, it's unclear who's funding the group, with the Super PACs spokesperson brushing off the question in a previous interview.

The Super PAC has also denied having any links with AIPAC, America's leading Israel advocacy group. But according to The Nation, 11 of its 14 board members have worked at, volunteered with, donated to, or spoken at AIPAC.

Billionaire class is 'getting nervous'

Mark Mellman, the president of the Democratic Majority for Israel, told The New York Times that the ads aimed to show that Sanders was in a "uniquely bad position" to defeat Trump.

"For many months people were saying he really didn't have much of a chance, but you have to look at the data now and say he does have a realistic chance of winning Iowa and potentially the nomination," Mellman said.

"This is the point at which there could be room for someone else to make their case and have a better candidate emerge."

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The Times reported that ads would appear in the cities of Cedar Rapids and Des Moines, and would also appear on cable, including on CNN, MSNBC and ESPN.

Politico, citing Advertising Analytics, reported that the anti-Sanders spots cost more than $680,000 to produce.

Faiz Shakir, Sanders' campaign manager, sent an email to supporters on Tuesday warning that an "outside spending group" was targeting the Vermont senator.

He also said that the campaign was aware of a Super PAC placing $1 million worth of ads in Iowa this week in support of former Vice President Joe Biden.

"We have a small lead in Iowa heading into Monday's caucus," Shakir wrote. "But outside groups are on the attack and hoping to stop us. Bernie needs us all if we're going to fight back and win."

Anticipating blowback from the ad, Sanders released a counter video late on Tuesday, arguing that the billionaire class was "getting nervous" that his campaign was gaining ground.

"It's no secret that we're taking on the political establishment and the big money interests, who are now running attack ads against us in Iowa," Sanders tweeted. "But we have the people, and our grassroots movement will prevail."

The attack ads come as a Real Clear Politics (RCP) poll shows Sanders leading his Democratic rivals ahead of the Iowa caucus.