Two U.S. senators on opposite sides of the aisle—and at the extremes of their parties—are urging federal regulators to block Capital One’s planned $35.3 billion acquisition of Discover, saying the merger would severely harm American consumers and undermine competition in the banking sector.

Sens. Elizabeth Warren, D-Massachusetts, and Josh Hawley, R-Missouri, announced their opposition this week to Capital One’s bid to acquire Discover in an all-stock transaction that would make the banking giant the most prominent U.S. credit card issuer.