Republicans have remained unusually silent on the tax, hoping to avoid a choice between supporting a tax increase and defending big bankers. Meanwhile, some liberal Democrats have gone further than the administration has, proposing a heavy tax on bank bonuses. Political analysts expect the bank tax to pass easily in the House but face resistance in the Senate.

There may be room for compromise. Administration officials hope to keep the proposed tax limited to major financial institutions with more than $50 billion in assets but consider that a difficult line to draw. For example, the proposed tax would not apply to large hedge funds; the mortgage finance giants Fannie Mae and Freddie Mac; or the carmakers Chrysler and General Motors.

“We believe the lines we have drawn are sound and sensible,” said Gene B. Sperling, a senior Treasury Department official. “We understand these are the type of things we will need to keep an open mind on in negotiations with Congress.”

The financial lobby has insisted that it is unfair for banks to cover the cost of losses tied to nonbank bailout recipients like the automakers and the American International Group, the giant insurer that is now majority-owned by the government. In an appearance on CNBC on Thursday, Representative Barney Frank, chairman of the House Financial Services Committee, called the argument over including the automakers legitimate.

At the lobbying group, the selection of Mr. Phillips of Sidley Austin raised eyebrows because it suggests that Wall Street may be spoiling for a fight. Davis Polk & Wardwell, another white-shoe law firm, has been advising the same lobbying group on legal matters tied to new financial regulation.

Mr. Phillips, who was an appellate lawyer in the Justice Department during the Reagan administration, brought his first case in front of the Supreme Court when he was just 29 years old. Since then, he has appeared before the court more than 60 times. Mr. Phillips declined to comment about his work for the industry, referring all questions to the lobbying group.

The group has hired him before. Last spring, it retained Mr. Phillips to examine similar legal questions after lawmakers prepared to heavily tax Wall Street bonuses in response to the public’s outrage over bonuses for A.I.G. traders. Through an extensive phone campaign and relentless lobbying on Capitol Hill, the financial lobby successfully beat back the legislation without using the courts.