Members of the Senate Finance Committee said yesterday that the preliminary consensus they have achieved on tax revision could disintegrate if the committee settles for a top personal tax rate above 27 percent.

At the same time, however, they spent much of the day debating restoring deductions to the low-rate plan before them. Adding tax breaks would make it harder to cut tax rates without increasing the federal deficit.

"At 27 percent, we'll hold a good group. . . . I think 27 percent is as high as we dare go," said Finance Committee Chairman Bob Packwood (R-Ore.), who plans to take his panel into a rare weekend tax-writing session.

"I think, if we get over 26 or 27 percent, any consensus would fall apart," said Sen. John Heinz (R-Pa.).

Packwood also explained for the first time the strategy he is pursuing in the closed sessions: He wants to obtain the support of a majority of members for a comprehensive package so that, when the committee begins voting in public, amendments that would torpedo the bill or drain it of needed tax revenue can be defeated and the measure can be considered in an up-or-down vote.

"Those together on the package have to understand it has to stand as a package," Packwood said.

The version of the plan being discussed would strike a compromise on a key provision affecting thousands of federal workers near retirement, senators on the committee said. Now, federal and other retirees with similar pension plans receive the tax-exempt portion of their pensions during the first few years of retirement. The House-passed tax-revision bill would prorate those pensions so some taxable income is received immediately.

Packwood, who originally favored the House provision, now is proposing phasing in the change over five years, increasing the amount of taxable pension by 20 percent per year. Sen. John H. Chafee (R-R.I.) said he would fight to retain the current system.

Another big-ticket stumbling block appeared to be a proposal to prohibit investors from deducting losses in tax-shelter investments from any income other than that related to the investment. Although the Finance Committee staff declined to provide details of the proposal -- which would raise $50 billion in new revenue over five years -- it was clear senators were concerned that legitimate business investments could be swept up in the provision.

Reagan administration officials also are concerned about this "passive-loss" provision, as well as about Packwood's proposal to do away with low tax rates for capital gains, the profits on the sale of an asset.

Treasury Department officials are becoming more involved in the process, however. Secretary James A. Baker III encouraged committee Republicans to keep working in a meeting Wednesday, and Deputy Secretary Richard G. Darman and other Treasury officials attended the private tax-writing session for the first time yesterday.

"If Packwood can put all this together and move it, there's no question most all of the Republicans will be out front," a senior administration official said.

Assessments of the momentum the committee is building varied, however.

"When a critical mass is reached, things go very quickly," said Sen. Bill Bradley (D-N.J.).

"When a critical mass is reached, things blow up," said Sen. Daniel Patrick Moynihan (D-N.Y.).