(CNN) A pro-Israel Democratic super PAC is launching a new television ad on Wednesday in Iowa that questions Sen. Bernie Sanders' electability and raises the specter of his health following a heart attack last fall.

The 30-second spot, which will run throughout the week, comes in response to Sanders' rise in the polls and marks a late acknowledgement from some within the Democratic establishment that the Vermont senator is a legitimate contender for the party's nomination in 2020.

The Democratic Majority for Israel's political arm is spending $681,000 on the ad, which features a string of voters expressing concerns over Sanders' general election prospects and health. It does not address Israel or Sanders' foreign policy positions.

"I do have some concerns about Bernie Sanders' health considering the fact that he did have a heart attack," one of the Iowans featured in the ad says. Another suggests that, while he likes Sanders, voters in Michigan, Pennsylvania and Iowa are "not going to vote for a socialist."

Sanders was hospitalized for more than two days after he suffered a heart attack in October while on the campaign trail. He had two stents inserted after doctors found an arterial blockage.

Sanders campaign co-chair Rep. Ro Khanna, a California Democrat, tweeted out a blistering response to the ad late Tuesday, saying the focus on Sanders' health crossed a line.

"For a Democratic Super PAC to attack @BernieSanders health is beyond the pale of decency," Khanna said. "Imagine the outrage if a Dem group had run attack ads about FDR's polio. Every Presidential candidate must denounce this & call for the ad to be taken down."

If elected, Sanders -- who in 2016 became the first Jewish candidate to win a major party primary contest when he won in New Hampshire -- would be the first Jewish president. He is to the left of the Democratic field on most key issues surrounding the relationship between US and Israel, but does not support the controversial Boycott, Divestment, Sanctions movement.

In an interview Tuesday night, DMFI president Mark Mellman said the group was moved to put hundreds of thousands of dollars behind the ad in an effort to blunt Sanders' momentum in Iowa and other early states.

"I think for many months now, the consensus has been that he really didn't have much of a chance at winning the nomination," Mellman said. "We looked at the situation a couple of weeks ago and decided that assumption was no longer valid -- that he really does have a chance to win the nomination. And we thought that would be bad for Democrats, in terms of our ability to defeat Donald Trump, and bad for the issues that we care about."

Mellman also criticized Sanders for his past statements about Israel and on a number of policy positions, including the question of whether to maintain Obama-era levels of aid to Israel. DMFI, like Sanders, supports a two-state solution in the region.

"He's voted right on a lot of legislation, but at the same time, he uses hyperbolic and vituperative language designed to stigmatize Israel," Mellman said of Sanders, who has been critical of the Israeli government led by Prime Minister Benjamin Netanyahu. In his role as a pollster, Mellman has worked on campaigns opposing Netanyahu, including on behalf of Israel's "Blue and White" party, in recent elections.

On Sunday, former New York City Mayor Michael Bloomberg jabbed Sanders, who spent time in Israel on a kibbutz, or communal farming settlement, during the 1960s.

"Now, I know I'm not the only Jewish candidate running for president. But I am the only one who doesn't want to turn America into a kibbutz," Bloomberg said, without naming Sanders, during a speech in Miami.

Asked if DMFI would commit to backing Sanders if he won the nomination, the group's communications director, Rachel Rosen, did not offer a direct response.

"We don't expect that to happen," Rosen said. "But we are unalterably opposed to Donald Trump and his policies."

Mellman dismissed the progressive backlash to the ad and critics who have voiced concerns that it could hurt Sanders, if he wins the nomination, in a potential general election showdown with Trump.

"I honestly don't think Iowans saying -- in Iowa, in January -- that Sanders can't win is going to actually change people's minds about who to vote for in November," Mellman said. "I just think that's a ludicrous proposition."

The group does not currently have plans in place for additional ad buys, Mellman told CNN, but didn't rule out the possibility.

In an email to supporters on Tuesday, Sanders campaign manager Faiz Shakir referenced the ad without discussing its content or naming the organization behind it.

"This morning we learned that an outside spending group placed $700,000 worth of negative ads in Iowa attacking Bernie Sanders," Shakir wrote. After asking for donations, he added: "We have a small lead in Iowa heading into Monday's caucus. 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."

Sanders, in a video posted to Twitter later in the day, personally denounced the escalating efforts to oppose him in Iowa.

"The big money interest can run all the negative ads they want, but it's not going to work," Sanders said. "The people in Iowa, and across America are sick and tired of status quo politics, and they want a government that will work for all of us, not just wealthy campaign contributors."

J Street, the liberal "pro-Israel, pro-peace" group, did not comment on the ad itself, but argued that DMFI was not representative of most Democrats and American Jews.

"DMFI's right-leaning positions on Israel and US foreign policy are completely out of touch with the vast majority of Democrats and the vast majority of American Jews, who are both supportive of Israel and strongly critical of the far-right policies of the Netanyahu government," J Street spokesman Logan Bayroff said.

Bayroff also noted that the J Street had committed to endorse the Democratic nominee, no matter who voters choose, and pointed to the group's announcement this week that it planned to raise $1 million in a campaign fund for that candidate.

The Jewish progressive activist group IfNotNow, which has been mostly supportive of Sanders' policies but not endorsed him, slammed the ad and denounced DMFI.

"A majority of Americans and the vast majority of Democrats agree that our country should not give a blank check to Israel if the Israeli government continues with the violence of the occupation and denying the Palestinian people basic rights," co-founder Yonah Lieberman said. "The ads don't focus on any of the AIPAC-front group's foreign policy goals because they are increasingly out of touch with the Democratic Party."

This story has been updated with additional context about the Democratic Majority for Israel and a statement from Sanders.