The top Democrats on congressional banking committees called on the Federal Reserve to reconsider its decision to approve Capital One Financial Corp.’s purchase of Discover Financial Services, saying it would inflict “serious harm” on consumers and the banking system.

The decision sounds like the Fed “had predetermined it was going to approve the transaction and either ignored relevant facts or explained them away with baseless assertions copied and pasted from Capital One’s application,” Senator Elizabeth Warren and Representative Maxine Waters said in a letter sent to the Fed this week and seen by Bloomberg News.

Warren serves as the ranking member on the Senate Banking Committee and Waters has the same role on the House Financial Services Committee.

The Fed and the Office of the Comptroller of the Currency last month approved the deal, which would create the largest credit-card issuer by loan volume in the U.S.. Michael Shepherd, interim CEO of Discover, said at the time it would increase competition in payment networks, offer more products to customers and increase resources for innovation.

The transaction was valued at $35 billion when it was announced last year and is expected to be completed by May 18.

Warren and Waters said the Fed’s review failed to appropriately assess the competitive effects on the credit-card market and didn’t take into account the views of the Consumer Financial Protection Bureau and the Federal Deposit Insurance Corp.

“Treating the transaction as a traditional bank merger was deeply misguided,” the lawmakers wrote. “These are not two traditional banks — they are card giants.”

The Fed received the letter and plans to respond, according to a spokesperson.

Capital One and Discover didn’t immediately respond to requests for comment.

(Updates with Fed comment in 8th paragraph)