The deficit-reduction plan approved last week by the Senate Budget Committee has been put on hold while Republican leaders meet with White House officials and key Democrats in an attempt to nail down enough support to get it, or a modified version, through the Senate.

"We're in the process of seeing where we are," Senate Majority Leader Robert J. Dole (R-Kan.) said in dodging questions about when the $964 billion budget for fiscal 1986 will come to the Senate for a vote. Republicans estimated yesterday that the plan contains $57 billion in deficit reductions.

Before the budget comes to the floor, he said, there will be talks among Senate Republicans and then consultations with White House officials and Senate Democrats -- a "three-pronged approach," Dole called it.

Senate Republican leaders met yesterday with Office of Management and Budget Director David A. Stockman and committee chairmen as the first step in the process.

The budget, approved Thursday by the committee on a party-line vote of 11 to 10, is believed to face serious troubles on the Senate floor, where it is vulnerable to being picked apart from all sides. It proposes to freeze Social Security benefits, allow Pentagon spending to grow only to cover inflation and impose many but not all of the domestic spending cuts that President Reagan has proposed.

The war between Democrats and Republicans on the House Energy and Commerce Committee over the number of seats each party will get on the panel's six subcomittees is likely to be resolved this week with the GOP getting more seats, committee sources said yesterday.

The dispute, which has been raging for weeks, has prevented the committee from organizing and has threatened to delay a host of major issues including clean water and clean air legislation.

Republicans, who make up about 42 percent of the House and about 40.5 percent of Energy and Commerce, had boycotted the committee over a plan by the Democratic majority to give them less than 38 percent of the seats on the six subcommittees.

Under a compromise plan worked out by Committee Chairman John D. Dingell (D-Mich.) and Rep. James T. Broyhill (N.C.), the committee's ranking Republican, the Republicans would be given a ratio on the subcommittees of more than 39 percent, for a net gain of two subcommittee seats, sources said.

House Democrats warned President Reagan yesterday that the GOP's plan to launch early campaign attacks on selected Democratic incumbents could scuttle any chance of bipartisan cooperation on administration programs this year.

"You should not expect bipartisan goodwill at the same time the Republican Party, with your active support, has launched an all-out assault on House Democrats," Reagan was told in a letter signed by Speaker Thomas P. (Tip) O'Neill Jr. (Mass.) and other Democratic leaders.

The warning was in response to plans announced by Rep. Guy Vander Jagt (Mich.), chairman of the House Republican Campaign Committee, to spend up to $6 million on a campaign to "soften up" some 35 Democratic incumbents before the 1986 elections. The targets are considered vulnerable because they won narrowly last November.