The stagnant U.S. economy and Europe’s debt crisis will take their toll on U.S. banks, and the effect won’t go away quickly, says star bank analyst Mike Mayo of CLSA.

"This is going to be the worst revenue growth year since 1938,” he tells CNBC. “And in fact, this decade for U.S. banks will be the worst revenue decade since the decade of the Great Depression.”

The European turmoil already has hurt banks there, sending their stock prices reeling. And the troubles in Europe could easily spread to U.S. banks.

“What you're seeing here is when Europe sneezes, the U.S. can catch colds, so . . .  several of the largest U.S. banks have guided for lower market-sensitive revenues,” Mayo says

Our banking industry may be headed the way of Japan in the late 1990s, he says.

"Just as you saw in Japan, there's a lack of loan growth, there's margin pressure, and there's revenue pressure. . . . So it's a combination of factors that are leading to very weak revenue growth."

Bank loans have been declining since 2009, sliding 0.4 percent in the second quarter.

Another top bank analyst, Dick Bove of Rochdale Securities, says the U.S. government is hurting the entire economy by prosecuting banks for their mortgage policies.

“If the goal is to create jobs, by constantly suing and attacking the American banking industry, we know what it’s done: it’s increased unemployment,” he tells Bloomberg.