Here in the dog days of summer, when no one in his right mind is paying attention to politics, a development of true consequence is taking place: Paul Laxalt is moving step by step closer to becoming an active, avowed candidate for the 1988 Republican presidential nomination. Recently, Laxalt asked Tommy Thomas, a longtime Reagan leader in Florida and avowed supporter of Vice President George Bush for 1988, to ''hang loose.'' Others are getting the same message directly or indirectly.

Forget that the senator from Nevada, retiring this year at the end of his second term, is supported by only 2 percent of the Republican and independent voters in the latest Washington Post/ABC News poll. Forget that even his pals on Capitol Hill have a hard time recalling a memorable speech from his lips or a major piece of legislation bearing his name. If Laxalt decides next spring to run, as he is hinting he will do, he changes the entire shape of the 1988 race.

The reasons are twofold, one a demonstrable fact, the other a plausible conjecture. The fact is that Laxalt has a preemptive claim on ''the Reagan network,'' the greatest single asset in Republican nomination politics. The plausible conjecture is that he will strike Republican primary voters as the closest thing to another Ronald Reagan in the 1988 field.

Laxalt's claim on the several hundred political operatives and fund-raisers who form the core of the Reagan network rests on the rock-solid proposition that he is the president's oldest, most trusted and intimate comrade-in-arms. Bush has been Reagan's super-loyal lieutenant for the past six years; Jack Kemp embodies Reagan's expansive optimism, his free-market economics and hard-line anticommunism; Bob Dole and (before him) Howard Baker have fought Reagan's battles on Capitol Hill.

But Laxalt was in the fight before any of them, even before Reagan himself. He raised the Reagan challenge to President Ford in 1976 before Reagan announced his candidacy, and set up the structure for the 1980 and 1984 campaigns before the public knew certainly that Reagan would run.

''There's a feeling,'' one prominent Reagan loyalist told me the other day, ''that we owe him Laxalt a tremendous debt, because he was there in 1976 when no one else had the guts, and he was there in 1980 when the money guys were flirting with John Connally and the establishment Republicans were with Bush. Hardly a day goes by without someone calling and asking, 'What's Paul going to do?' If he goes, we'll all be there.''

As a staunch conservative, Laxalt seemingly poses the greatest threat to other hopefuls on the party's right, notably Kemp and the Rev. Pat Robertson. But the impact could be greatest on Bush, who has worked assiduously to redesign himself from Reagan's toughest opponent in 1980 to his legitimate heir in 1988. For Bush, running against Laxalt would mean replaying -- not erasing -- that bad tape of the 1980 nomination fight.

What's worse for Bush, Republican insiders are aware that the unstated premise of the Laxalt candidacy is the concern, as 1984 Reagan manager Edward J. Rollins put it, that Bush ''may not be able to go all the way.'' Laxalt tells reporters they are making premature judgments about Bush's stumbling. But those who talk with Laxalt privately say he expresses exactly that fear himself.

Laxalt still has important questions to answer: Nearing 62, he had decided to get out of politics. Does he now want to reverse that decision and subject himself to the searching scrutiny of a presidential campaign? In his case, that would mean not only exploring his policy ideas but his personal history, including the associations with Nevada gamblers that are part of the record of his unresolved libel suit against The Sacramento Bee. Rollins sees ''nothing on the horizon to block his candidacy.'' But another longtime intimate from the Reagan circle thinks ''Paul has not yet reconciled himself to the fact that his life would no longer be his own.''

The public reaction to Laxalt as potential president is also completely untested. My guess is that people will like what they see. He lacks Reagan's eloquence, but conveys in person and on TV the most engaging of Reagan's personal qualities -- total sincerity, strong beliefs expressed without belligerence or dogmatism, openness, friendliness, optimism and, most of all, the confidence of being relaxed and comfortable with the person he is.

Like Reagan, he presents fascinating paradoxes. As the president defends ''traditional family values'' from his background as a divorced Hollywood star with a notably unconventional family, Laxalt is the divorced Catholic father of seven, who as governor supported both legalized gambling and legalized, local-option prostitution, and yet remains a favorite with many fundamentalist leaders. He sponsored (but has since dropped) the Family Protection Act, a compendium of religious-right social-issue proposals, but said the other day that ''Nevada is a live-and-let-live state, and it's hard to be absolutist'' in its politics.

Asked if he thought he could straddle the gap between libertarian and moralist conservatives in the GOP, Laxalt replied with a smile, ''That's where I am.'' And that is exactly where the Republican Party may choose to be.