The president's own aides were not convinced of Jimmy Carter's springtime regeneration until his answer to the last question at his April 26 press conference, when his advocacy of tax reform suddenly echoed the vigor of the 1976 campaign, but also its aberrations.

"I haven't seen this Carter since the campaign," confided one presidential aide, a sentiment widely shared at the White House. But with renewed dynamism came inaccuracy that appears not wholly accidental. What's more, it brought to light elements of his thinking still unknown outside his inner circle.

The reply to the tax-reform question was in truth the authentic Carter, warts and all. The frequently dull president reverted to the socially conscious populist from Plains, [WORD ILLEGIBLE] seldom boring but often puzzling.

Carter returned from the April 16 weekend retreat with his senior staff at Camp David in a feisty mood of self-confidence that infected his aides. "Everybody's been leaning on us, and we're going to start leaning back," said one. The president immediately upbraided Congress for opposing him on tax and energy matters, reflecting what was termed inside the White House "the new spirit of Camp David."

But his first press conference following the Camp David retreat reverted to empty prose and leaden arguments - until the last question. When asked, "Why are you so insistent on pushing tax reforms?" Carter came to life with an attack on "abuses" with this example. "One medical doctor, a surgeon, owns a yacht, and he took a $14,000 tax credit, tax exemption, for entertaining other doctors on his yacht." That would be forbidden by the Carter tax reform.

In fact, Carter was referring to neither a "credit" nor an "exemption" but a "deduction" something very different. No president has devoted so much time to educating himself in the intricacies of the Internal Revenue code. He surely knows the difference between a "credit" (a reduction in the actual tax) and a "deduction" (merely a reduction of taxable income).

What's more, Carter quickly indicated that "credit" was no slip of the tongue. "And when that doctor didn't pay his $14,000 taxes," the president declaimed, "other average working American families had to pay his taxes for him." But the doctor would be paying $14,000 less in taxes only if he took a credit, not a deduction. The fact that Carter's error was consistent suggests at least the possibility that it was intentional.

That is reminiscent of the 1976 campaign, when Carter frequently played a bit loose with facts to make a sharper point - a failing some advisers warned him about to no avail. They now admit privately that the president exaggerates a little when really impassioned about an issue.

That raises the broader question: When faced with towering problems here and abroad, why is he so passionate over tax reform in general, over entertainment expenses specifically and over deducting a $14,000 business entertainment item in particular? Additional revenue from denying all deduction for yachts, hunting lodges, country club dues and other business entertainment expenses would total $420 million this year - chicken a feed by today's budget standards.

Nor do experienced Democratic politicians, including Carter loyalists, consider it good politics. While ordinary taxpayers dislike that yacht-owning doctor, nobody really worries much about him. It is probably too late this year to build popular support for tax reform, and Carter's impasssioned if inaccurate plea of April 26 fell on deaf ears in Congress. Why, then, did the president do it?

Because one of the very few inflexible opinions held by Carter is his low esteem for doctors and lawyers. "He can't stand them," reveals one aide. Another senior staffer believes he considers those two professions "too greedy." But others see a more subtle mind-set: The President cannot tolerate professional men who do not fulfill their social responsibilities.

To Carter insiders, that is an unshakable conviction that affects his view of tax reform and national health care more than staff working papers. To outsiders, the anti-doctor, anti-lawyer bias was hinted at once: in his famous, extemporized 1974 Law Day speech at the University of Georgia. Arguing that the bar association does not care enough about clients or the medical association about patients, he called it his duty as governor of Georgia to fulfill those responsibilities, even though less qualified than the doctors and lawyers. "This bothers me," he said.

To idealistic supporters first attracted to him by that speech, here was the authentic Carter - the same Carter evoked in the answer to the tax-reform question. This suggests that the president's regeneration, while restoring his own and his staffs morale, may substitute political problems of a different nature.