Whatever happens to President Reagan's $98.9 billion tax increase bill this week, it already has claimed as a casualty the Republican unity that has been the distinctive feature of the administration's political leadership.

"We will be fortunate if half the Republicans in the House vote for this bill," a key administration official acknowledged last week. "And it will be a lot easier for them to go against us the second time after they've deserted once."

While many think the president will score another legislative triumph on the tax increase he prefers to call "a very important building block" of his economic recovery program, his victory coalition is likely to have a completely different look than those of Reagan coalitions of the recent past.

Much of the credit for Reagan's unexpected success with Congress has been focused on the administration's ability to attract "Boll Weevil" Democrats from the South and West. But this is a phenomenon that has recurred in various forms since the Republican-"Dixiecrat" coalition that formed in Congress after World War II.

What has distinguished the Reagan presidency from predecessor Republican administrations is the near-totality of its success in keeping GOP ranks intact.

On seven key budget and tax votes in the first 18 months of the administration,Republican unity stopped just short of unanimity. Only on one vote, the budget reconciliation measure of June 26, 1981, did as many as five Republican House members desert their party. Senate votes were also near unanimous on most issues. Even on the defense authorization bill passed by the House on July 29 of this year, an issue where moderate and liberal "Gypsy Moth" Republicans have some strong reservations, 145 GOP members of the House voted with the president and only 16 against.

This week, on the tax bill, Reagan for the first time in his presidency will be counting on a Democratic majority to give him what he wants while GOP members split, bitterly and ideologically, over a measure that the White House sees as an economic necessity and some conservatives regard as a betrayal of public trust and Reagan's campaign promises.

"The president has an obligation to lead, even if it means leading with Democratic votes," a White House official said last week. "Anything that enhances his leadership status is going to maintain unity within the party."

This is an assumption of presidential omnipotence that has undermined chief executives of both parties in the past and that is now damaging Reagan among the core group of conservatives who have rallied to his banner ever since the Barry Goldwater presidential campaign of 1964.

"Maybe they can beat us by the sheer weight of the White House, but they do so at the cost of Reagan's natural base," said Rep. Newt Gingrich (R-Ga.), who has called the tax bill "the opening round of a fight for the soul and future of the Republican Party."

Gingrich, one of the emerging young Republican conservative spokesmen in the House, said he believes that threats of White House pressure, and particularly administration efforts to discredit Rep. Jack Kemp (R-N.Y.), are likely to leave an aftertaste that will make conservatives less likely to support the White House in the future.

And this view is not without private support within the administration, despite the insistence of the president and his former political assistant, Lyn Nofziger, on lock-step loyalty over the tax bill.

"Politically, we can't win on this issue," one administration official acknowledged. "We're taking on our own hard-core supporters. The conservative congressmen know that if we beat the hell out of Jack Kemp, we're likely to do it to anybody. We have put so much on the line in this one that we're going to open the campaign in total disarray, whether we win or lose on the bill."

Not every GOP member of Congress feels this way, of course. Some, such as Rep. Guy Vander Jagt (R-Mich.), call the present battle "a very special skirmish," and predict, perhaps hopefully, that the old coalition of Republicans plus Boll Weevils will merge again after the vote on the tax bill.

But even if they do, there are those in Congress and the administration who say that the trust in Reagan's leadership will never quite be the same.

At times Reagan seems to sense this and to be uncomfortable with a line of argument that suggests that he must abandon his program to save it.

Referring to the soaring deficit that the administration says makes the tax increase a necessity, Reagan said in Montana last week that, "It's like holding your nose and embracing a pig."

The president and his aides are saying, in effect, that this pig will grow to monstrous proportions unless the tax increase bill is passed.

"If you think it's going to be difficult to campaign after this tax bill is passed, consider what would happen if it's defeated," White House chief of staff James A. Baker III told a closed-door meeting of conservative congressmen on Capitol Hill last Tuesday night.

Baker's version of what would happen is that interest rates would start back up, the recovery would be thwarted and incumbent Republican members of Congress would be defeated.

The following day, at the headquarters of the Heritage Foundation, a prominent conservative think tank, White House counselor Edwin Meese III tried to calm the fears of representatives of several conservative groups.

He told them that the tax increase features of the Reagan bill had been overplayed and the additional outlay cuts underemphasized, a contention the president is expected to stress in a nationally televised speech to the nation Monday night.

But the conservatives, many of whom think of Meese as their champion in the high councils of the administration, weren't buying the argument. After the luncheon, conservative fund-raiser Richard Viguerie said, "We agreed to disagree." Meese agreed that this is an accurate summary.

Some new-right publications, especially Viguerie's Conservative Digest, have been taking on Reagan since the early months of his administration. The cover story in the magazine's July issue asks "Has Reagan Deserted the Conservatives?" The article largely answers that question in the affirmative in a piece that Reagan described in a letter to editor John D. Lofton Jr. as "one of the most dishonest and unfair" articles he'd ever read.

But other conservative publications usually in Reagan's corner also have lambasted him over the tax bill. Human Events, a Washington weekly that is a staple in the Reagan reading diet, last week devoted its lead article to the tax bill, emphasizing its inconsistency with the president's promises and saying that "the most conservative president we've ever had" had decided "to pit himself against his strongest conservative supporters."

The calmer-toned National Review, voice of the old right, also joined the chorus. Though scornfully describing the Conservative Digest broadside as "a rhetorical cluster-bomb," the magazine also leveled the tax bill, saying that "even the Carter administration was embarrassed to propose many of these changes."

All of this, including the anticipated widespread GOP desertions from the tax bill, add up to something less than a repudiation of Reagan by the political faction most consistently loyal to him.

But the growing conservative resentment over White House tactics and the tax increase bill do strike at one of Reagan's strong points, as reflected in various public opinion polls.

These surveys show that Reagan is admired or respected, even by many Americans who disagree with his policies, for philosophical consistency and for sticking to his principles. There are conservatives aplenty who think that this consistency has been undermined.

And the split in party ranks also creates a practical political problem. As long as Republicans were voting together they could at least claim some credit for whatever signs of economic recovery appeared, while trying to stick the blame for the condition of the economy on past Democratic policies.

"It wasn't an easy argument, but we at least had some internal consistency," one Republican official said last week. "We could argue that the president had a program and the party was behind him. Now we're going to have districts where the Democrat supported the tax bill and the Republican opposed it. How do we tell the voters that we need a Republican Congress to put over the president's programs?"

In the context of this division, the midterm elections on Nov. 2 become less an exercise in national politics, at which Republicans do well, than a series of district-by-district campaigns, at which Democrats excel.

Win or lose this week, Reagan will find that he and his staff have bruised some of his most loyal supporters badly.

"What happens next depends upon what lessons they draw from it," Gingrich said. "The most important question is not whether they win or lose, but whether or not they learn that they can't govern successfully against their natural allies."