Gen. Alexander Haig is being urged by prominent senators to take the offensive in his Senate confirmation hearings, using the planned four days of confrontation with Watergate-minded liberal Democrats to exploit Ronald Reagan's clear national security mandate.

That advice to Haig shows how much more is at stake in his confirmation hearings before a generally dovish Senate Foreign Relations Committee than allegations of wrongdoing during the national agony of Watergate.

What is really at stake is the blind horror many liberal Democrats share for President-elect Reagan's no-nonsense national security policy, following four years of unrestrained Soviet advances. Strapping Haig to the Watergate mast for 100 public lashes is destined to humiliate and weaken him (and also exact revenge for the Republican attack on Paul Warnke's nomination to head the arms control agency four years ago). Wounding Haig would undercut Reagan's ability to use him to carry out the major revival of national security he has promised.

But for Haig, it will be no easy matter to keep the hearings that start Jan. 9 glued to what he hopes to accomplish in building America's depleted strength in the world. That explains Haig's surprise coup in coming up with Joseph Califano, a prominent Democrat with recognized liberal credentials, to defend him. A better choice would be difficult to find.

Escalating political and press attacks on Haig have questioned his character, his honor and his fitness for secretary of state. The assault has deeply angered the career Army officer who won the Distinguished Service Cross in Vietnam and earned unanimous praise from America's closest allies as supreme commander of NATO.

Haig, say his friends, wants to keep himself on a tight leash. He knows he should not respond in kind to his critics, but instead should stick to his and Reagan's plans for a toughened, revitalized foreign policy.

But if his critics, led by Senate Democratic Whip Alan Cranston and Sen. Paul Tsongas, convert his confirmation hearing into a Watergate Ii, Haig will be irresistibly pressed to test the theory that whatever its allure in the capital, Watergate is an issue long since dead and buried throughout the nation.

Reagan's transition agents believe that any Democratic effort to revive the horros of Watergate would sicken voters and that the effort would boomerang. But Haig's enemies from the days of Watergate claim that if the Democrats play the Watergate card deftly, it could polarize the country all over again.

Haig stongly lines up with the Reagan transition advisers. One Haig intimate, excluding optimism over any showdown between Haig and his adversaries, says privately: "Those senators are going to be on television with Haig. If they want to take that political risk, so be it. Haig would slaughter them."

Haig would not have been Reagan's pick for secretary of state if Senate Democratic leader Robert Byrd had not publicly warned that a Haig nomination would face deep trouble in the Senate. Against such public pressure, Reagan, with Haig agreeing, felt he had to nominate Haig.

At that time, Haig did not begin to imagine what his critics had in store for him. Now that he knows, he is working full-time on his counterattack with Senate Majority Leader Howard Baker, the committee member who will lead his defense. Privately, Haig is stunned by what he has been reading and hearing about himself. Alluding to implications that it was he who arranged Richard Nixon's pardon by Gerald Ford as the price for Nixon's resignation, Haig recently confided to an ally: "Does anyone think I would ever have got out of Washington without going into the slammer if I'd tried to arange anything like that?"

Since Byrd's warning, the Haig nominination has escalated under the thickening cloud of Watergate into a struggle with major implications for Reagan's presidency. The unpublicized fact that Sen. Edward Kennedy strongly proposed Terry Lenzer, a Democratic investigator on the Senate Watergate committee to help prosecute Haig in the confirmation hearings shows the depth of liberal faith that Watergate can be used to cripple, if not defeat, Al Haig.

To Haig, that is a misreading of the national mood, a case he will be prepared to make in the first great stuggle between Reagan and the liberal Democrats.