When Sen. Paul Laxalt (R-Nev.) decided last August not to seek a third term because he wanted "more personal freedom," he wistfully expressed the view that he might continue, after leaving office, to serve his friend President Reagan as a trouble-shooter.

As it turned out, Laxalt didn't have to wait until he was out on the streets. Last October, he found himself in Manila warning then-Philippine President Ferdinand Marcos that the Reagan administration expected him to make reforms. It was not a message that Marcos wanted to hear, or heeded, but he and Laxalt became friends during a long and sometimes painfully candid discussion. When the time of decision came last week, Marcos turned to Laxalt to confirm that Reagan wanted him out.

Perhaps Laxalt has unintentionally carved out a new career for himself as diplomat to despots. He has the trust of the president, the support of movement conservatives and impeccable ideological credentials. But Reagan and Republican politicians would have preferred that Laxalt stay where he is and help the GOP keep control of the Senate.

"I realized the president was distressed with me when I said I wouldn't run again," Laxalt quipped recently. "I didn't realize how distressed he was until he told me to go see Marcos."

Laxalt's light touch fits well with Reagan, who never met a one-liner he didn't like. It has worked in the Senate, too, once Laxalt's colleagues realized that his ambition was to be a mutual emissary rather a member of the Senate leadership.

"In the last five years, I've had one foot in the White House and one foot on Capitol Hill," Laxalt said last week. "I've tried to explain executive actions to my colleagues, and the Senate to the White House."

Routinely, Laxalt has been described as Reagan's "best friend" in Congress. Conservatives and moderates alike depended on him to carry their messages to the president. During the first term, he walked a tightrope, bearing complaints from conservatives about the White House staff to Reagan while simultaneously achieving an effective working relationship with the favorite conservative target, then-chief of staff James A. Baker III.

But this delicate middleman role proved highly stressful. In the February issue of Conservative Digest, Laxalt's friend Paul M. Weyrich writes that "it was evident to all who know him well that the burden of his relationship with the president had been more than any one person could be expected to handle. For a long time, Paul Laxalt was the only person going over heads of the senior White House staff to tell the president what he needed to hear."

Weyrich is known as a conservative polemicist, but his high assessment of Laxalt's character would be seconded by many Senate liberals. As he demonstrated in leading the unsuccessful fight against ratifying the Panama Canal treaties, Laxalt has the useful quality of respecting the motives and arguments of his opponents.

"I have a lot of good friends on the opposite side of the aisle," he says. "I learned early that you never translate political differences into personal differences. Nobody has a monopoly on political wisdom."

There are conservatives who see Laxalt as a potential successor to Reagan, despite the inherent difficulties of being a senator from Nevada, and he is certainly a potential kingmaker. Tonight, he will be guest of honor at a $1,000-a-plate dinner hosted by Weyrich's Free Congress Political Action Committee to raise money for Republican candidates. Laxalt will designate where the money goes.

But I supect that Laxalt is an unlikely champion to lead the conservative charge in 1988. He harbors no known passion to be president and no illusions about the ease of the undertaking. His civilized approach and negotiating skills would lend themselves to service as White House counselor or special presidential envoy, as he was in the Philippines.

"I see a bright light at the end of the tunnel," Laxalt said last week. "There is life outside the U.S. Senate. Howard Baker found it. Maybe I can, too. If I can work on the outside and keep my hand in public service, it would be the best of both worlds."

Reaganism of the Week: Defending the policy of mass weapons procurement in his defense speech Wednesday, the president said: "The old shoppers' adage proved true -- they are cheaper by the dozen."