The combination of BB&T and SunTrust would create a new bank, as yet unnamed, with more than $400 billion in assets. The new bank would be based in Charlotte, which is already home to Bank of America’s headquarters and much of Wells Fargo’s business.

The combined bank’s assets would be well over the $250 billion threshold that, under the 2010 Dodd-Frank Act, mandates that lenders adhere to stricter capital requirements and closer regulatory scrutiny.

A 2018 law raised the threshold from $50 billion, which led to widespread predictions of a wave of regional bank mergers. (The country’s biggest lenders, such as JPMorgan, would still face prohibitively high hurdles to striking big acquisitions, though big regional lenders like U.S. Bank and PNC Financial might be able to buy smaller assets.)

The new regulations helped give BB&T and SunTrust certainty about the regulatory environment.

“You might expect that we wouldn’t announce something like this without feeling confident” about the response from Washington, Kelly King, BB&T’s chairman and chief executive, told analysts on Thursday.

The plan is already attracting attention from opponents of megabanks. Senator Elizabeth Warren of Massachusetts, who is running for the 2020 Democratic presidential nomination, sent a letter to the Federal Reserve on Thursday expressing concerns about the deal.

She said data that the Fed had provided her office last year showed that the central bank rarely objected to proposed deals. It approved 89 percent of bank merger applications from 2014 through 2017, and 94 percent of applications during the first half of 2018. All were far smaller than the one proposed by BB&T and SunTrust.

A spokesman for the Fed said officials had received Ms. Warren’s letter and were planning to respond.