The Senate revived its no-amendment approach to tax revision yesterday, voting to preserve a $100 million tax break for Phillips Petroleum Co. as the bill headed for final passage later this week.

On Friday, the Senate passed an amendment that wiped a tax benefit for Unocal Corp. from the sweeping legislation. Proponents of the bill said at the time that the amendment's passage did not signal a trend toward rewriting the bill by amendment.

Both provisions were "transition rules," designed to ease the impact of the new law on certain companies, individuals or projects. Sen. Howard M. Metzenbaum (D-Ohio) sponsored both the Unocal and Phillips amendments.

The difference in treatment of the two amendments was attributed to the fact that the Unocal provision would have opened a new loophole in the tax code, while the Phillips benefit, which was preserved by a 74 to 13 vote, simply phased in the bill's changes more slowly.

"Members did see a distinction between the Phillips transition rule and the other one," said Sen. David L. Boren (D-Okla.), who spoke in favor of the Phillips provision.

Senators will consider a far broader amendment as early as today, when Sen. George J. Mitchell (D-Maine) plans to propose adding a third, higher rate of 35 percent to the plan's two brackets of 15 percent and 27 percent. The current top individual rate is 50 percent.

Mitchell would also reduce the lowest rate to 14 percent and do away with two provisions that have the effect of raising the top tax rate.

Mitchell's amendment would give a larger average tax cut to taxpayers earning less than $75,000 a year than would the package drafted by the Senate Finance Committee, and would raise taxes on people earning more than $200,000 a year. The senator virtually predicted yesterday that he would lose, saying, "I recognize it's going to be a difficult, uphill fight."

Other potential amendments showed signs of crumbling yesterday under pressure from the bill's sponsors to pass the package intact. Sen. Lloyd Bentsen (D-Tex.) said he would not offer an amendment to make the tax-rate cuts in the package effective at the same time as the limitations on deductions. Both the House-passed tax bill and the Senate version would wipe out or limit numerous deductions six months before the rate cuts go into effect.

Sen. David H. Pryor (D-Ark.), who said last month that he would move to repeal a $500 million transition rule benefiting the steel industry, said yesterday the momentum behind the bill has made him uncertain whether to proceed.

"I really am in a dilemma," Pryor said. "I don't like the way the provision was put in there and I also think it violated what the committee had already voted on, but now I'm asking myself, 'Is it worth the fight? Is it worth it to bring the steel industry senators in to fight tax reform?' That's an awful conclusion to reach, but I really sense that the votes aren't there to defeat the provision."

Metzenbaum has decided not to offer an amendment that would phase in tax-shelter crackdowns more gradually. He also is expected to offer fewer than the 19 amendments to the transition rules that he originally proposed.

Finance Committee aides, meanwhile, denied rumors that Chairman Bob Packwood (R-Ore.) will support an amendment at the close of consideration of the bill to provide transition breaks for senators whose requests were not included in the committee bill. The Joint Committee on Taxation has estimated that those requests would cost about $7 billion over five years.

Packwood was also said to be considering appointing all 20 senators on the Finance panel to the conference committee that will meet to reconcile any Senate-passed measure with the bill the House passed last year. Such a move would be unusual, as a conference committee customarily comprises fewer than half the members of the committees that originated the legislation.

Packwood has made no decision, but the notion has been floated by ranking Finance Democrat Russell B. Long (La.), who chaired the Finance Committee from 1966 to 1980, sources said.