An undecided few will be crucial factor in Bork confirmation. High court nominee's friends and foes gear up for battle in the Senate

With the start of hearings tomorrow by the Senate Judiciary Committee, the struggle over the nomination of Judge Robert Bork to the United States Supreme Court shifts into high gear. The Bork nomination has become the stage for a showdown between President Reagan and his Democratic opponents. There will be no half victories, no face-saving compromises. Either the Senate will consent to Judge Bork's appointment or it will not. This fact has aroused partisans on both sides of the aisle, and only a relatively few senators have still to decide on the vote they will cast.

Stakes are high for both parties. Conservatives see Mr. Bork's appointment as the capstone of Mr. Reagan's judicial legacy - a legacy that includes the appointment of a conservative chief justice, two conservative members of the Supreme Court, and more than half the nation's federal judges, mostly conservatives.

Bork would replace retired Justice Lewis Powell, who cast the court's deciding liberal vote on a number of controversial civil rights decisions. Conservatives expect Bork will vote to support a conservative majority in similar cases and believe his appointment will cement in place a judiciary that defers to the policymaking prerogatives of the legislature. ``It's Judge Bork's position that legislators should make laws instead of judges,'' says Sen. Orrin Hatch (R) of Utah.

At the same time, many of Reagan's allies in Congress privately admit that a good measure of the President's remaining political prestige hinges on the outcome of the Senate confirmation process.

Bork's confirmation and the consumation of an arms control agreement with the Soviet Union are the administration's top priorities as it enters its final months. The success or failure of these efforts may determine whether the President leaves office on a high note.

Democrats have similarly mixed motivations. Some of them truly worry that Bork's appointment will lead to the reversal of 20 years' progress in civil rights law. ``If Bork is confirmed,'' says Sen. Edward Kennedy (D) of Massachusetts, ``our most basic constitutional liberties [will] suddenly be vulnerable.''

Others are not so convinced, but have pledged themselves to Bork's defeat anyway - such is the spirit of the year, as both parties jockey for presidential-season advantage, and neither party is willing to freely permit the other a chance to appear victorious on any issue.

The situation has created opportunities and hazards for individuals and for the parties. At least three senators are presidential candidates, and two of them - Paul Simon (D) of Illinois and Joseph Biden Jr. (D) of Delaware - are members of the Judiciary Committee. Senate minority leader Robert Dole of Kansas, an unannounced contender for the Republican nomination, will assume some of the credit if Bork is confirmed by the Senate and share some of the blame if he is rejected. Mr. Simon, a first-term senator, has already announced his opposition to Bork's appointment, and the hearings could dramatically increase his national profile - for better or worse.

But the stakes seem greatest for Senator Biden, the committee chairman, who is running for the nomination from the back of the pack of candidates. The hearings will be covered extensively on network television - an unparalleled opportunity for a presidential candidate seeking national exposure. Nevertheless, Biden may find himself in a uniquely uncomfortable predicament.

Biden said to a newspaper reporter last year that he would support a Supreme Court nominee such as Bork. Now, he has promised to lead the opposition to Bork's nomination. Skeptics say they believe that Biden has buckled to the pressure of interest groups opposed to Bork's nomination and influential in Democratic Party politics - a charge he categorically denies.

Still, Biden's position has, says one Democratic senator, ``left him squirming like a worm in hot ash.'' Biden has left no doubt where he stands on Bork, but he must run the hearing with a degree impartiality.

In a letter to Democratic colleagues this summer, Sen. Dennis Deconcini (D) of Arizona warned that the public would view Democrats as captives of ``special interests representing the extreme left-of-center position on the political spectrum'' if they jumped to judgment on Bork. ``We want to make sure we get to the White House in 1988 so we can make our own Supreme Court appointments,'' he says. Senator Deconcini is one of only three members on the 14-man Judiciary Committee who has not taken a position on Bork's nomination.

Deconcini's call has gone largely unheeded. By one head count, the vote for Bork stands at 45 to 45, with 10 currently undecided. Other estimates put no more than 25 senators in the undecided columns, with Bork supporters and opponents nearly evenly matched.