A new Democratic-Republican flap concerning the GOP-backed Roth-Kemp tax cut proposal is threatening to intensify the expected floor fight over the tax bill that was just approved by the House Ways and Means Committee.

When the panel approved its bill last week, members voted to urge the House Rules Committee to allow the Republicans to bring up the Roth-Kemp measure as a floor amendment, even though it wasn't part of the committee bill.

Rep. Al Ullman (D-Ore.), chairman of the Ways and Means Committee, agreed to support the arrangement. The agreement had been part of a compromise under which Ways and Means Republicans had voted to back the committee bill.

But now, House Speaker Thomas P. O'Neill Jr. (D-Mass.) has indicated he plans to try to derail that plan in the Rules Committee to help "spare" freshman and sophomore Democrats from having to vote on the GOP measure.

The GOP proposal, which would slash taxes some 30 percent over a three-year period, is a hot political issue in some parts of the country. Republicans have said they hope to make it the focus of the November campaigns.

However, O'Neill's refusal to go along with the Ways and Means Committee agreements has angered congressional Republicans, who view it as reneging on a valid compromise the two sides struck in committee.

Rep. Jack F. Kemp (R-N.Y.), chief House sponsor of the GOP bill, warned yesterday that if O'Neill succeeds in the Rules Committee, Republicans will seek to overturn that panel's limit on floor amendments, leaving the bill wide open.

And although the Democrats still have a comfortable majority in the House, some congressional figures fear a massive GOP defection could leave the tax bill in trouble. Conservatives are charging the leadership with foul play.

Kemp said in an interview yesterday he thought it was "outrageous" for the speaker to try to block the efforts of a committee chairman to carry out a compromise. He said O'Neill simply was fearful the GOP proposal would pass.

If the Rules Committee does vote to block Kemp from offering his proposal as a floor amendment, Republicans still would be able to force a vote on the plan as part of a last-ditch motion to recommit the bill.

However, prospects for passage of the GOP plan then would be more measure on that basis could say they merely were voting to support the committee bill, not opposing Roth-Kemp.

O'Neill's office said yesterday the speaker's GOP critics were "going farther than what he has said" on the subject. One source asserted that O'Neill had not said anything on whether he plans to challenge the measure in Rules.

However, several sources confirmed that O'Neill does indeed intend to try to block the Roth-Kemp provision in the Rules panel. Ullman has remained quiet during the fray, in hopes of not exacerbating the situation.