WASHINGTON--Most of the faces will be same, but the Senate convening next year will be feistier and more likely to challenge President Reagan on both economic and defense issues.

The new Senate will have 54 Republicans and 46 democrats, same us the current lineup.

But the numerical outcome of last week's election masks underlying factors which will partially close the Senate as the safe harbor for Reagan's program of continued cuts in the federal budget and steady increases in Pentagon spending.

Sen. Bill Bradley of New Jersey, vice chairman of the Senate Democratic Campaign Committee, said the election "augurs well for a quite difference Senate. The moderate Republicans just elected and more moderate Republicans up in 1984 will not go in lockstep" with Reagan's conservative economic policies.

Already, Senate Majority Leader Howard Baker has indicated he will support public jobs legislation and reductions in the American military build-up which Reagan is certain to oppose vigorously.

Baker aide Tom Griscom said the senator spoke after the election with Republicans who were re-elected, including conservatives who said Congress must produce jobs legislation to the 10.4 percent unemployment rate.

There will also Republican proposals to reduce defense spending, Griscom said.

"They (Republicans) are not deserting the president," he said, "But there will be some modification to his program."

The Democratic campaign committee chairman. Sen. Wendell Ford of Kentucky, said of the 33 Senate elections that some races won by Republicans were close the Democrats "came within a whisper" of retaking Senate control, which they lost in 1980.

Eighteen Democrats were re-elected to the Senate and almost all of them won big, although many opposed Reagan's legislative proposals.

Moreover, six of the Republican incumbents, mainly in the Midwest, were returned by extremely slim margins of one or two percentage points. In all six races, Democratic opponents made Reaganomics an issue in the campaign.

A good example is incumbent GOP Sen. John Chafee Rhode Island. He put as much distance between himself and Reagan as he could, but nonetheless is still awaiting the final count of absentee ballots to assure his reelection.

Those re-elected Republicans plus another eight moderate GOP senators whose terms expire in 1984, are likely to weave an independent course over the next two years, which may not parallel Reagan's agenda.

There are also at least eight senators who are certain or possible presidential candidates in 1984, among them Democrats Edward M Kennedy 54 of Massachusetts, Just re-elected; john Glenn of Ohio; Gary Hart of Colorado: Alun Cranston of California; and Republicans Baker of Tennessee and Bob Dole of Kansas.