Sen. Elizabeth Warren (D-MA) is calling for the Federal Reserve and the Office of the Comptroller of the Currency to block Capital One's (COF) planned $35 billion purchase of Discover Financial (NYSE:DFS).

"Capital One’s proposed acquisition of Discover poses significant additional risks to consumers,” Democrats including Warren wrote in a letter to Federal Reserve Vice Chair Michael Barr and Acting Comptroller of the Currency Michael Hsu, according to a Bloomberg report on Thursday.

Capital One didn’t immediately respond to a Bloomberg request for comment on the letter

Warren in February, after the Discover (DFS) deal was announced, called for the planned $35 billion transaction to be blocked.

"The merger of @CapitalOne and @Discover threatens our financial stability, reduces competition, and would increase fees and credit costs for American families," Warren said in a posting on X less than 24 hours after the companies confirmed the deal. "This Wall Street deal is dangerous and will harm working people. Regulators must block it immediately."

Shares of Discover (DFS) rose 2% on Thursday, and Capital One (COF) gained 1.5%