Paul Laxalt (R-Nev.), President Reagan's best friend in the Senate, spent last week sleeping in a tent at Marlette, a rustic campground above Lake Tahoe where his Basque father once herded sheep.

For Laxalt, 62, the trip was a picturesque pilgrimage in perfect weather to a symbolic boyhood retreat. He returned home, however, not to enjoy the scenery but to make a political decision that could hold the key to whether Republicans can hold the Senate in 1986.

Left to his own devices, Laxalt would quit the Senate next year rather than seek a virtually assured third term. He has confided to friends that he is eager to leave public life after a long career that includes 12 years in the Senate and eight years as governor of Nevada, where he built the state's once-moribund Republican Party into a potent force.

Nevada was a Democratic stronghold when Laxalt burst onto the statewide scene in the early 1960s. It was one of only two far western states that voted for John F. Kennedy for president in 1960. Four years later, despite the millstone of Republican presidential nominee Barry Goldwater, Laxalt lost the closest Senate race in U.S. history, failing by 48 votes while President Lyndon B. Johnson won more than 58 percent of the state's vote.

Laxalt met a conservative neighbor in that 1964 campaign, a Californian named Ronald Reagan. In 1966, western conservatives celebrated when Reagan and Laxalt won guberatorial races over two-term Democratic incumbents. Reagan, who was apt to treat other politicians as strangers, accepted Laxalt as an equal and a friend. Laxalt served as chairman of Reagan's presidential campaigns in 1976 and 1980 and became that most valuable of confidantes, a friend unafraid to bring the candidate bad news.

Last year, the affable Laxalt was rewarded with the sinecure of "general chairman" of the Republican Party, a signal of Reagan's intention to run again. He has continued to play the role of best friend, giving Reagan blunt political advice after his wobbly performance in the first debate against Walter F. Mondale and taking budget deficits seriously even though the president does not. When White House chief of staff Donald T. Regan clashed with Republican senators last month, it was Laxalt who arranged a truce.

Under normal circumstances, Laxalt would be deservedly thanked by his party and allowed to graze happily in the hay fields of the private sector. But 1986, even though few White House officials seem aware of it, has the potential for a Republican catastrophe that could make Reagan's last two years in office seem as unpleasant as an evening with Pieter W. Botha.

Republicans, who hold the Senate by a 53-to-47 margin, have 22 incumbents up for election next year compared with only 12 Democrats. A private Republican estimate, presented to Laxalt last week and probably no secret to him anyway, shows that seven of these Republicans are potentially vulnerable compared with only one Democrat. A net loss of four would put the Senate, and with it the entire Congress, in Democratic hands.

To draw the noose even tighter on Laxalt's personal desires, surveys taken by presidential pollster Richard B. Wirthlin confirm conventional political wisdom in Nevada that the seat is safely Republican if Laxalt runs and up for grabs otherwise. The Democrats have three potential candidates with proven vote-getting ability, none of whom is expected to run if Laxalt seeks reelection.

These are the reasons why Republicans from Reagan on down want Laxalt to run again and why they sent emissaries to Marlette last week to persuade him that he owes another term to his party and his president. They have a strong case and a view that the man who gave Nevada a Republican legacy will not want to risk turning what he calls the Reagan revolution over to the Democrats. My guess is that, when Laxalt comes down from the mountain today, he will say that he will seek a third term in the Senate.

Speakesism of the Week: Responding at a briefing in Santa Barbara last Thursday to a question on the whereabouts of Labor Secretary William E. Brock, White House spokesman Larry Speakes said: "Is Brock out of the city? Yes, Brock is out of the city. The attorney general is out of the city. The president is out of the city. The secretary of defense is out of the city. We're just out of it."