WASHINGTON  It seemed a good idea last year, when the financial crisis had turned banks into Public Enemy No. 1 and lawmakers were looking for ways to reward consumers still bitter about billion-dollar bailouts and executive bonuses.

Without much warning or debate, the Senate passed an amendment directing the Federal Reserve to reduce the hidden “swipe fees” that banks collect from retailers each time a customer makes a purchase with a debit card.

Merchants, who had complained that the $20.5 billion in annual fees were biting into their profits, were elated. Banks were stunned. Their lobbyists tried to reverse the move, but when the overhaul of the nation’s financial regulation was passed by Congress last July, the debit card cut survived.

Now, as the Fed faces a deadline in April to write the rules for the lower fees, banks and debit card companies are engaged in an all-out assault on Capitol Hill, enlisting a growing cadre of lawmakers and lobbyists to push for changes, delay or outright repeal. Banks contend the proposed cut in fees  to 12 cents per transaction from an average of 44 cents  will leave many of them unable to afford to issue debit cards to customers or will force them to raise other consumer banking charges to cover the costs. They also claim retailers will reap unfair profits.