Warren slammed the South Bend mayor, whose rise in the polls in Iowa and New Hampshire has coincided with her own slippage, for offering his biggest donors “regular phone calls and special access.”

“When a candidate brags about how beholden he feels to a group of wealthy investors, our democracy is in serious trouble,” she argued.

Warren has made radically diminishing the influence of money on politics a key plank of her campaign, taking relatively drastic measures to underscore her anti-corruption pledge.

Warren was one of the first candidates in the Democratic primary — and remains among the few — to swear off traditional high-dollar fundraisers for her presidential campaign, instead relying on grassroots, small-dollar donations. But Warren’s stance is a novel development in her young political career: the Massachusetts senator held traditional fundraisers through her reelection last fall.

Buttigieg’s clap-back is the latest salvo in a war of words between the two candidates as the primary heats up in its final stretch before votes are cast. The escalated sniping over the past few weeks culminated this week in a series of tit-for-tat disclosures from each campaign as they attempted to scramble out from under accusations of a lack of transparency.

In response to barbs from Warren, Buttigieg has already announced he will open his private fundraisers to the press and disclose his bundlers. He also named the clients he served at the management consulting firm McKinsey a decade ago after the firm agreed to release him from a nondisclosure agreement as the mayor came under pressure from rival campaigns.

Warren, in her own concession to pressure from Buttigieg, released details about the compensation she received from corporate legal work earlier this week, though Buttigieg’s campaign is still calling for the senator to release her tax returns from before 2008.

The mayor on Friday stressed that the 2020 election is too important for Democratic White House hopefuls to argue over what he called “process purity tests” rather than debating the nuances of their various policy proposals, like his differences with Warren over health care.

“Let’s have a serious conversation about where this country is headed,” he pleaded. “Hitting somebody with a process purity test when we’re in the debate of our lifetimes about what is going to take to move this country forward” would be a mistake, he argued.