President Biden on Friday waded into a strike involving 1,400 employees at four Kellogg plants, whom the company said it planned to permanently replace after workers voted down a proposed contract this week.

“I am deeply troubled by reports of Kellogg’s plans to permanently replace striking workers,” Mr. Biden said in a statement, adding that “permanently replacing striking workers is an existential attack on the union and its members’ jobs and livelihoods.”

The strike began on Oct. 5 and has largely focused on the company’s two-tier compensation system, in which employees hired after 2015 typically receive lower wages and less generous benefits than veteran workers. Many veteran Kellogg workers, who the company says earn about $35 per hour on average, believe that adding lower-paid workers puts downward pressure on their wages.

Kellogg raised the possibility of hiring permanent replacements in November. The company and the union last week reached a tentative agreement in which the company would lift a cap on the number of workers in the lower tier, which was 30 percent under the previous contract. In exchange, the company agreed to move all workers with four or more years experience into the veteran tier, as well as an amount equivalent to 3 percent of workers at its plants in each of the five years of the contract.

On Tuesday, the Bakery, Confectionery, Tobacco Workers and Grain Millers International Union, which represents the workers, said its members had “overwhelmingly voted” against the deal. In response to the result, Kellogg said that it would “hire permanent replacement employees in positions vacated by striking workers.”

A Kellogg spokeswoman said Friday that the company had posted job listings for permanent replacement roles in each of its four locations and that its hiring process was “fully operational.” The statement added: “Interest in the roles has been strong at all four plants, as expected. We expect some of the new hires to start with the company very soon.”

Permanently replacing workers who are striking over economic issues like wages and benefits is legal, but Democrats, including Mr. Biden, have sought to outlaw the practice through the Protecting the Right to Organize Act, or PRO Act. The House approved the bill in March but it has stalled in the Senate.

“I have long opposed permanent striker replacements and I strongly support legislation that would ban that practice,” Mr. Biden said in his statement Friday. “Such action undermines the critical role collective bargaining plays in providing workers a voice and the opportunity to improve their lives.”

The statement is not the first time Mr. Biden has appeared to weigh in on a prominent labor action. The president appeared in a video during a union campaign at an Amazon warehouse in Alabama this year warning that “there should be no intimidation, no coercion, no threats, no anti-union propaganda” — an unusual interjection by a president during a union election.

Mr. Biden has made no secret of his support for unions over the years. He quickly ousted government officials disliked by unions, reversed Trump-era rules that softened worker protections and signed legislation that allocated tens of billions of dollars to stabilize union pension plans.

His nearly $2 trillion climate and social policy bill, pending in the Senate, includes numerous pro-labor measures, including incentives for employers to offer union-scale wages on wind and solar projects.