A top campaign aide for former Vice President Joe Biden criticized Sen. Elizabeth Warren (D-MA) on Friday for her decades-long Republican Party affiliation.

Greg Schultz, the former vice president’s campaign manager, lambasted the senator for having been a “Republican until she was 47-years-old,” in response to comments Warren made in Iowa earlier during the day, claiming Biden was “running in the wrong presidential primary.”

“You have to be kidding me,” Schultz said. “Warren was a Republican until she was 47 years old while Joe Biden has spent his life helping elect Democrats across the country and served with honor in the Senate and with Pres. Obama.”

You have to be kidding me. Warren was a Republican until she was 47 years old while Joe Biden has spent his life helping elect Democrats across the country and served with honor in the Senate and with Pres Obama. https://t.co/E9CESedasL — Greg Schultz (@schultzohio) November 1, 2019

Schultz made the comments even though his side had technically started the altercation by criticizing Warren’s newly released proposal on how to pay for Medicare for All, arguing it was regressive.

The Biden campaign said in a statement on Friday upon the plan’s announcement:

For months, Elizabeth Warren has refused to say if her health care plan would raise taxes on the middle class, and now we know why: because it does. Senator Warren would place a new tax of nearly $9 trillion that will fall on American workers.

The flare-up between the two camps underscores the precarious position Biden finds himself in three months away from the first nominating contest in Iowa. Although the former vice president started the race as the clear frontrunner, leading the field by as much as 32 percentage points at one point, his campaign has steadily declined in recent months.

As the former vice president’s political fortunes have diminished, Warren’s have only increased. The Massachusetts Democrat has leapfrogged Biden in the early primary and caucus states. One recent poll out of Iowa showed Warren leading the field with 28 percent support among Democrats, while Biden was relegated to fourth place with only 12 percent. A similar situation has played out nationally, where most polls show Warren leading Biden, albeit with a narrow margin.

#National @YouGovUS/@TheEconomist Poll (RV, 10/13-15):

Warren 29%

Biden 25%

Sanders 13%

Buttigieg 7%

Harris 5%

Gabbard 3%

Klobuchar 2%

Yang 2%

O'Rourke 2%

Booker 1%

Bennet 1%

Delaney 1%

Steyer 1%

Castro 0%

Williamson 0%

Ryan 0%

Messam 0

Sestak 0

Bullock 0https://t.co/uSdkD01Van — Political Polls (@Politics_Polls) October 17, 2019

Warren’s rise in the polls has even led some of Biden’s former Obama administration colleagues, such as David Axelrod, to acknowledge he is no longer the frontrunner.