Association of Flight Attendants President Sara Nelson criticized former South Bend, Ind., Mayor Pete Buttigieg Wednesday for a tweet defending private health insurance, that appeared to characterize the employer-provided health benefits as gains won by union workers.

Buttigieg defended his proposed "Medicare for All Who Want It" plan, saying 14 million union members have "fought hard for strong employer-provided health benefits" in a tweet Wednesday morning.

Nelson, who played a key role in ending the federal government shutdown last year, called the invocation of labor rights "offensive and dangerous."

"Stop perpetuating this gross myth. Not every union member has union healthcare plans that protect them," Nelson tweeted. "Those that do have it, have to fight like hell to keep it. If you believe in Labor then you'd understand an injury to one is an injury to all."

"For profit healthcare doesn't work, @PeteButtigieg. It's killing people & putting working people in financial ruin. The whole country knows it. Collective bargaining is about problem solving," she added. "Don't use unions to promote division - that's only good for the profiteers."

Nelson has previously said "Medicare for All" has strong support among union rank-and-file and called invoking unions to argue against it "typical union busting tactic to divide people within unions and to divide union members from non-union members."

Nelson and Buttigieg's exchange comes as Nevada's powerful Culinary Union has distributed flyers arguing Medicare for All would "end Culinary Healthcare." Former Vice President Joe Biden has similarly argued that universal health care would mean the end of benefits unions fought for.