Republicans are weighing in against the Federal Reserve's plans for subjecting U.S. megabanks to special new capital requirements, claiming that the move would be arbitrary and hurt financial markets.

In a letter sent to Fed Chairwoman Janet Yellen Monday, Republican Reps. Randy Neugebauer of Texas and Robert Pittenger of North Carolina raised concerns about Fed members' suggestion that the eight biggest U.S. banks would have to clear a higher minimum capital level in future versions of the annual stress tests.

In their letter, the two members of the House Financial Services Committee said that the Fed's arguments in favor of making the stress tests more difficult for the banks were "not persuasive," calling the central bank's specific plan "arbitrary" and redundant and potentially an infringement on Congress' authority.

The Republicans' comments are among the first signs of congressional resistance to the Fed's effort, its latest move to tighten post-crisis rules on big banks. The plan, outlined in June by Fed governors Daniel Tarullo and Jerome Powell but not yet formally proposed, would be to add the capital surcharge required of big banks to the minimum capital levels all banks must maintain during the stress tests run annually by the Fed. Those stress tests examine what would happen to the banks' balance sheets in a hypothetical financial crisis.

For banks, that would mean they would have to pass the tests with additional capital ranging from 1 percentage point to 4.5 percentage points, depending on the bank's size, making it significantly more difficult to clear the hurdle.

The rule would be toughest on JPMorgan Chase, which would face the largest additional required capital. CEO Jamie Dimon described the new rule as "not good for us" when told it was in the works in June. It would affect seven other banks: Citigroup, Bank of America, Goldman Sachs, Morgan Stanley, Wells Fargo, State Street and Bank of New York Mellon.

Including the capital surcharge in the minimum capital levels required in the stress tests would be just one of a host of measures regulators have imposed on big banks with the purpose of making them consider reducing their size. At a banking conference in June, Powell explained that while he was not necessarily in favor of breaking up banks, he favors raising "capital requirements to the point at which it becomes a question that banks have to ask themselves." Tarullo, the Fed's point man on regulatory affairs, has made similar arguments.

Referring to those comments, Neugebauer and Pittinger wrote Yellen that the Fed does not have the authority to "set industrial policy for the banking sector."

All the U.S. big banks passed their stress tests in the latest round of results announced in June, although some likely would struggle if the surcharge were included.

Asked for comment on Monday's letter, a spokeswoman for the office of Neugebauer, who is chairman of the financial institutions subcommittee, said that it would speak for itself.