Sen. Josh Hawley (R-Mo.) is demanding that the Biden administration block Capital One from its proposed acquisition of Discover Financial Services, according to a letter sent Wednesday evening.

Why it matters: Hawley's stance aligns him with other critics of the deal, most of whom are Democrats, such as Sen. Elizabeth Warren (D-Mass.) who also has called for regulators to block the deal.

• "This is destructive corporate consolidation at its starkest," Hawley wrote in a letter to Assistant Attorney General Jonathan Kanter. The letter was first obtained by Axios.
• "If consummated, this merger will create a new juggernaut in the credit card market, with unprecedented powers to extort American consumers. That cannot be allowed to happen," he wrote.
• "I urge you to take the first opportunity available to challenge this bad deal."

Between the lines: Hawley was the first Republican to call for the deal to be blocked, and it could signal more scrutiny from the populist wing of the GOP.

• It is a rare issue uniting him with the progressive Warren, who said Tuesday the "Wall Street deal is dangerous and will harm working people. Regulators must block it immediately."
• The opposition is also in line with legislation Hawley introduced last fall that would cap credit card interest rates.

The big picture: Capital One is the ninth-largest bank in the U.S. by assets, which already puts it under strong regulatory scrutiny.

• The $35.5 billion deal could make it the largest card issuer in the market by loan volume, as AxiosPro has reported.

Thought bubble: The Biden administration — both through the DOJ and the FTC — has been more active on antitrust than any administration in recent memory. There's a good chance they'll sue to block this one, too, Axios' Dan Primack notes.