The Reagan administration wants to reach a compromise with congressional supporters of tax simplification before sending a proposal to Congress, Rep. Jack Kemp (R-N.Y.) said yesterday.

Kemp said James A. Baker III, the newly confirmed treasury secretary-designate, has agreed to sit down with Rep. Richard A. Gephardt (D-Mo.) and Sen. Bill Bradley (D-N.J.), sponsors of one tax revision proposal, and Kemp and Sen. Robert W. Kasten Jr. (R-Wis.), who are pushing another. He said they will try to reach agreement by May.

"There is a commitment to the principles. There are some dottings of the I's and crossing of the T's that we still need to negotiate, but we're very, very close," said Kemp, adding that he had discussed the negotiations with Baker Monday evening. "I think the chances of a three-way merger with the Treasury Department proposal released in November are good."

The four legislators will emphasize the push for compromise by reintroducing their separate plans at a joint news conference today.

White House spokesman Marlin M. Fitzwater confirmed that the administration wants to reach agreement before sending a bill to Congress, perhaps by April or May, although he said officials hope for a compromise as soon as possible.

"We're not going to delay it any longer than we have to," he said.

Kemp also told reporters that he will modify his tax simplification bill to make it more like Bradley-Gephardt and the Treasury proposal and keep it from losing billions of dollars in tax revenue. He said he will propose three brackets instead of one flat rate and will raise corporate taxes slightly from his earlier version.

Gephardt sounded less convinced than Kemp when asked yesterday about the possibility of compromise.

"I'm happy to know if he's moving closer to Bradley-Gephardt, but we have not had a negotiation on the specifics of this bill," he told reporters. A compromise "is always possible, but I don't see that we are anywhere near that point."

Although the general thrust of Bradley-Gephardt and Kemp-Kasten is very similar, "there are areas of disagreement that are substantial" and would have to be negotiated, Gephardt said.

Like both those proposals, the Treasury Department plan would reduce personal income tax rates and expand personal exemptions while doing away with many deductions and credits.

Baker has not yet assumed office, but Treasury spokesman Roger Bolton said achieving bipartisan agreement on the tax plan "fits with what Secretary Donald T. Regan's intentions were."

Kemp acknowledged that Senate Republicans were considerably less eager than he to tackle tax simplification before dealing with spending cuts to reduce the deficit. Senate Finance Committee Chairman Bob Packwood (R-Ore.) and others pushed strongly for putting deficit reduction first in a meeting Monday with President Reagan.