Even as they struggled frantically to rescue President Reagan's tax-revision proposals, administration officials acknowledged yesterday that internal conflicts and political miscalculations threaten the president's top domestic priority of the year.

"What happened is unfathomable," a leading Republican strategist said of the scene at the White House this week when Reagan appealed to GOP House members for final passage of the tax bill, seemingly unaware that it had been sidetracked hours earlier on a procedural vote.

"It put the president in the worst possible light," said the strategist, who has frequently advised the White House.

According to officials at the Treasury Department, the White House and in Congress, seeds for this week's turmoil were planted months ago when Reagan began his campaign for overhauling the tax code.

The officials cited an uninspired attempt to build public support for the concept, crossed signals between the White House and Treasury over legislative strategy and miscalculations about what would happen this week in Congress.

Some officials also said they regret that Reagan was not more involved in the tax-revision effort, particularly once the administration joined forces with House Ways and Means Committee Chairman Dan Rostenkowski (D-Ill.).

Treasury Secretary James A. Baker III had committed the president to working with Rostenkowski earlier this year as the only chance for victory in the Democratic-controlled House, but that limited White House control over the bill.

"When you are reliant on the [committee] chairman of another party, it's not as if you can bull your way through," a senior White House official said, recalling that the administration had promised to mute comment while Rostenkowski's panel shaped its bill.

The result, he said, was legislation designed to attract 180 to 190 House Democrats but to lose 150 Republicans, unavoidably setting the stage for Reagan's trouble this week.

When the Democratic-controlled committee finished writing its tax bill Nov. 22, Baker sought a mild presidential endorsement of its product as a way to keep the effort moving forward.

Sources said that Reagan signed a memorandum favoring endorsement and that Baker and Deputy Treasury Secretary Richard G. Darman then wanted him to lobby vigorously for House passage.

But the president did not. Rostenkowski tried several times to reach Reagan, who had just returned from the Geneva summit, but his calls were not immediately allowed to go through, officials said.

The endorsement sought by Baker was pulled back by White House chief of staff Donald T. Regan and his deputy, Dennis Thomas, in response to complaints from House Republicans opposing the committee measure, the officials said. Regan and Thomas feared Baker's course would produce a Republican revolt.

If Reagan had backed the bill then, Thomas said, "the fight would have been with Ronald Reagan, not with the bill. It would have preempted the Republicans from offering a viable alternative. They would have justifiably felt excluded from the process."

However, an official familiar with Baker's thinking called the pause a mistake. It "gave off the strong odor of indecision," nurturing the House Republican revolt this week, the official said.

The uprising, which embarrassed Reagan, was also nurtured by what House Republicans describe as a sense that the White House has ignored them too often this year.

An aide to House Minority Leader Robert H. Michel (R-Ill.) said, "We're not sure they have an appreciation for the intensity of feelings over a long time where there have been serious problems of communication and cooperation with the White House."

Citing conflicts over the budget, the MX missile and aid to Nicaraguan rebels, he added, "They've lost touch with the sentiment of House Republicans and the temperature of the institution. Temporarily, we hope."

GOP deputy whip Tom Loeffler (Tex.) singled out Regan for criticism. "I think it's a good signal for the chief of staff to know this is not a corporate boardroom," he said.

Even before this week, as Reagan campaigned nationally for tax revision, senators traveling with him said they found little constituent support for Reagan's plan.