Washington — With an apparently last-minute addition to his State of the Union speech, President Reagan may have changed the character of the new congressional session from one filled with rhetoric over federal deficits to one seriously trying to deal with the problem.
Both parties had already begun to settle in for long, windy debates and finger-pointing exercises on the $200 billion federal budget shortfall. Democrats, who have difficulty building a case against the Reagan administration in a rebounding economy, have been sounding grave warnings for the future as the deficits mount.
Even some conservative Republicans had begun to criticize the President's indifference to the problem.
Mr. Reagan answered them all in the State of the Union by proposing to work with a bipartisan panel from Congress to try to chop $100 billion out of deficits over three years. Only hours before he had telephoned congressional leaders to sound them out on the idea.
The practical problems are legion. President Reagan has offered almost no specifics on how to make the reductions. In fact, his speech defended increases in military spending while promising not to ''raise taxes on families struggling to pay their bills.''
''Do they have any ideas as to where they can get $100 billion?'' a doubtful House speaker Thomas P. O'Neill Jr. (D) of Massachusetts told reporters Thursday. ''I've got no idea.''
Moreover, it is late in the budget process. Next week the President releases his 1985 tax and spending plan, and most of the figures, complete with enormous deficit projections, are already set. The bipartisan deficit-fighters would have little time to make their proposals.
''It was a late starter,'' said Senate Finance Committee chairman Robert Dole , who had long called for a bipartisan panel to work on deficits. The Kansas Republican said that the President has in mind only modest plans for spending cuts or tax reforms for immediate action. ''He's talking laundry lists, not big-ticket items,'' Mr. Dole said.
A beaming Dole had noted that many on Capitol Hill had been talking about deficits. ''The President's called our bluff,'' he said.
The test of the plan should come very soon if it is to make any inroads in the '85 deficit. One part of the test is whether the parties can work together even as they plan for the election campaigns.
Speaker O'Neill is clearly skeptical. ''We're not going to be hoodwinked,'' he said, and he agreed to assign only one member, majority leader Jim Wright (D) of Texas, to the panel. The speaker charged that the presidential initiative was a last-minute strategy because his advisers saw the deficit was becoming a hot political issue.
Reflecting the view of many fellow Democrats, Rep. Timothy E. Wirth of Colorado pronounced himself ''deeply suspicious'' of the Reagan proposal. And Sen. Jim Sasser (D) of Tennessee, a member of the Senate Budget Committee, called the speech ''long on rhetoric and very short on substance'' and ''geared to an election year.''
The message from Democratic leaders is that Mr. Reagan must go first in making any proposals to reduce deficits. A bipartisan appproach is ''fine with me,'' said Senate minority leader Robert C. Byrd of West Virginia. ''But there's going to have to be some real leadership by the President of the United States.''
Byrd said he had told the president, ''There's only one man who can provide the leadership on these deficits and persuade the American people that they are dangerous.''
The one certain effect of the proposal is that it has snatched away a ripe political plum from the Democrats who have been decrying the Reagan deficits. Now the onus is on Congress to act. ''I think the President is throwing out a challenge to our Democratic opponents,'' said Rep. Robert H. Michel of Illinois, GOP leader in the House.
Democrats, too, begrudgingly agreed that Mr. Reagan's speech had scored a political coup.
''He's masterful. You have to hand it to him,'' said freshman Rep. Richard J. Durbin (D) of Illinois. ''It's depressing in a way for a Democrat to face that.'' Byrd called the speech ''eloquent,'' if weak on specifics.
And House majority whip Thomas S. Foley (D) of Washington State granted that it could open the door for real action on the deficit. ''I think we could have a wonderful opportunity,'' he said, if the President will go along with ''a broad-based plan.''
At the least, the proposal has set the stage for action later. An aide to Senate Budget Committee chairman Pete V. Domenici (R) of New Mexico predicted that the deficit-cutting panel will take little action before the November elections. It will serve an ''educating'' function, said the aide, leading to decisive action in late 1984 or '85.