Thirty-one senators have agreed to oppose all floor amendments to the tax-overhaul legislation the Senate will consider next week, Senate Finance Committee Chairman Bob Packwood (R-Ore.) said yesterday.

That "core" coalition, if it holds, would not be large enough to defeat amendments. But it would be coupled with a shifting group of at least 20 senators opposed to each specific proposal, Packwood said.

He told reporters there would be no "cleanup" amendment to accommodate senators' objections to such changes as curtailing the deduction for Individual Retirement Accounts for those who are covered by other pension plans. Senate Majority Leader Robert J. Dole (R-Kan.) has raised the idea of such an amendment, which would have to be paid for by offsetting tax increases in other areas.

"There is no Packwood-Dole cleanup committee amendment. There is no Dole-Packwood cleanup committee amendment. There is no Ronald Reagan amendment. We are standing firm against any amendments that do any violence to the theme of the bill . . . and I think we can hold them," Packwood said.

He also said he would leave to the full Senate any attempt to smooth out the dramatic variations in revenue in the package, which would bring in $7.4 billion more in revenue than the current system in 1986, $23 billion more in 1987, $20 billion less in 1988 and smaller variations in subsequent years. During five years, the bill would bring in the same amount of revenue as current law.

Packwood said he hopes the additional revenue raised in the early years would not be used to satisfy the deficit-cutting targets of the congressional budget resolution.

He made his remarks at a meeting of a broad-based coalition in favor of the tax bill, which reduces personal and corporate tax rates while eliminating or curtailing many deductions. In the last two weeks, membership in the "15/27/33" coalition (named for the tax rates in the legislation) has risen from 150 organizations to 600, organizers said.

Packwood said AFL-CIO President Lane Kirkland told him yesterday that, while the umbrella labor organization would push for some amendments to the bill, it would support the final package even if those amendments did not pass. A spokesman for the AFL-CIO said that the statement sounded consistent with Kirkland's views, although the union supports amendments, including ones to restore deductions for state and local sales taxes, union and professional dues, and government workers' pensions.

Packwood said he had no intention of supporting changes to the pension provisions, which would require the payment of taxes on pensions for federal workers and others immediately upon retirement rather than delaying payment of the taxable portion of a worker's pension for up to three years.

President Reagan, meanwhile, yesterday told the National Association of Manufacturers that the Senate tax package "can liberate the entrepreneurial genius of the American people and put the American spirit of enterprise into overdrive for our race into the 21st century."