Minutes after the President glanced at his watch and cut off the inquisitorial flood of questions at Wednesday's press conference, a politically astute aide commented that the press, having disposed of Bert Lance, had a new target: Jimmy Carter.

If so, it is a weekend Jimmy Carter. After violating his impossibly high ethical standards by sticking with Lance was gaining popular support. Besides managing to get the worst of both worlds in the Lance affair, the President has strengthened the widespread impression that even his most strongly held convictions are susceptible to change under pressure.

Thus, the end of the Lance affair opens the first serious time of trouble for the Carter presidency. Economic omens are grim, the Senate is ripping the President's energy programs to shreds, and foreign policy is becalmed. At such a point Carter scarcely needs the capital's remorseless corps of investigative reporters trying to prove a presidential coverup on Lance.

The last days of Lance's tenure as budget director reflect the mood of presidential vulnerability that is now at the heart of Carter's problems. The President is described as shifting back and forth last weekend on the question of whether his closet friend and adviser should go.

As late as Monday, Lance himself seemed optimistic and determined to fight it out. What worried Lance even more than continued sniping from media and congressional critics was his personal financial squeeze - especially how to pay the October interest on his huge Chicago bank loan.

But Lance is politically sensitive enough to perceive the climate at the White House. Although his opening statement to the Senate committee Sept. 15 encouraged some presidential aides to think Lance might yet be saved, the cross-examination of Sept. 16 and 17 extinguished all hope. Whatever Carter's vacillations, everybody else at the White House was convinced Lance had to go as the week of Sept. 19 began.

But the Senate hearing, while invaluable for public restoration of Lance's self-respect, made Carter's position more difficult. One presidential adviser told him during the first week of September that holding onto Lance was political suicide. Two weeks later he determined that a public backlash against the press and Congress was building rapidly.

Given tha powerful backlash, Lance's resignation was not universally applauded - especially among some businessmen who judged the affair more from the standpoint of presidential backbone than Lance's offenses. "I think we learned today," one Wall Streeter told us shortly after Wednesday's press conference, "how much Carter would protect one of us if we came down to Washington."

The fact that Lance was forced out not because the President agreed with charges of his unfitness for office, but because of political pressure, also butressed this widespread feeling: Jimmy Carter can be taken into camp by the application of pressure. The vulnerable Lance may not be the best example, but it follows presidential retreats from firm positions on the tax rebate, SALT, the Middle East and welfare reform.

Yet, the departure of Lance did not win back for Carter erstwhile admirers who felt his support for Lance repudiated lofty ethical goals (notwithstanding the President's pious language Wednesday). The press corps, with Bert Lance's scalp in hand, is determined to pin down the President on Lance's ethics.

The problem is put concisely by one intimate of Lance: "The President made the fatal error of grabbing both horns of the dilema. A professional chooses one horn. To do otherwises is the sign of the amateur."

Being branded the amateur is not what Carter needs today. Apart from the intractable deadlocks of foreign affairs, his authority is being challenged by Congress on a broad front of issues (most conspicuously the enery bill in the Senate), and the slumping economy almost surely will generate efforts to send federal spending through the roof.

The one note of optimism inside a morose White House is the feeling that the pro-Lance backlash will be directed at the press, hampering its efforts to impale the President. In fact, one Carter insider predicts the President himself will take off against the press in due course. It is not a happy formula for a President trying to work his way out of the first time of trouble.