Progressives, once enamored with South Bend mayor-turned-2020 Democratic contender, Pete Buttigieg, are turning on the second-tier candidates, including Buttigieg, amid concerns that he and the also-rans will be gunning for potential frontrunner Sen. Elizabeth Warren (D-MA) in Tuesday night’s debate.

Buttigieg has done little to dispell the notion that all guns will be turned on the Massachusetts Senator in the third Democratic debate. In an ad, released Monday evening, Buttigieg called out his far-left rivals for their commitment to “Medicare for All,” according to a report from CNN.

“The ad — titled “Makes More Sense” — highlights Buttigieg’s Medicare for All Who Want It health care plan, a middle-of-the-road approach that would not force people onto government health coverage but would offer a public option for people who choose to enroll,” CNN says. “That is a key distinction between Buttigieg’s plan and the sweeping overhaul that would be instituted under a full Medicare for All program.”

In the ad, Buttigieg specifically calls out Warren and Sen. Bernie Sanders (I-VT) for their pie-in-the-sky health plans, pointing out the shocking cost of nationalized health insurance, both in terms of tax dollars and in terms of lost private insurace plans.

It’s a shrewd calculation. CNN claims that Democrats who currently describe themselves as “undecided” view health care as a top priority. Buttigieg wants to grab that marketshare before it falls in line with one of the three frontrunners (Sanders, Warren, and former Vice President Joe Biden). If he does, he could be a real contender in a place like Iowa, where he’s running just five points behind Warren and eight points behind Biden. He’s also creeping up in fundraising, raising just a few million less than Warren in the second quarter of 2019.

That’s obviously enough to be threatening to Warren, who seems to have called out the attack dogs. The Hill reports that prominent progressive groups like the Progressive Change Campaign Committee are issuing thinly veiled threats to Buttigieg, warning him to back off attacks on far-left candidates.

“It’s sad to see the potential self destruction of @PeteButtigieg, a rising star,” Adam Green, cofounder of the Progressive Change Campaign Committee, Tweeted late Monday. “Tuesday will be a key trajectory moment. Does he attack the next President of The United States or take the high road and make positive waves by adding his unique voice to progressive issues of the day?”

Worse for Warren, Buttigieg spent Monday attacking her pledge not to accept cash from so-called “big money” donors, calling the move shortsighted in light of how difficult besting President Donald Trump is likely to be.

“My competitors can go with whatever strategy they like, but we’re going to make sure that we have the resources to compete because we are going up against the sitting president of the United States,” Buttigieg told a Snapchat forum. “He has tremendous amounts of support and allies at his back, and we’re not going to beat him with pocket change. I’m proud of the fact that we have more than half a million individuals who’ve supported my campaign. Some of it’s chipping in three bucks; some of it’s a lot more. I think you need the full spectrum of support in order to compete, especially if we want to go against someone like Donald Trump.”

That paints Buttigieg as more realistic than his progressive competition — something that could hit home with undecided Democrats in Tuesday’s debate.