Nevada, planted more thickly with paradoxes than people, has a new one. Republicans could lose a U.S. Senate seat, and perhaps control of the Senate, because a Las Vegas lawyer, Jim Santini, is thought to have too sharp an eye for the main chance.

Paul Laxalt is retiring, and the Republican candidate to replace him is Santini, 48, who used to be a Democratic congressman. His opponent is Harry Reid, 46, the congressman from one of Nevada's two districts, the one in and around Las Vegas, where more than half of Nevada's voters live. Reid seems to be leading by about 5 percentage points, but the boys have just begun to fight, meaning: spend.

The shrinking violet is not a desert flower, and neither Santini nor Reid had to be dragged into politics. Reid was elected lieutenant governor at 29 and lost the 1974 Senate race to Laxalt by 624 votes. Santini was a congressman when Nevada had just one and has run in five statewide races. One was a 1982 primary challenge to the incumbent Democratic senator, Howard Cannon. Santini lost by 5,000 votes, but Cannon then lost the general election, and many Democrats blame Santini's challenge for the loss.

Since leaving Congress, Santini has been a lawyer in Sin City (your nation's capital, as Nevadans see it). Reid's congressional voting record recently earned him a perfect rating from the AFL-CIO, prompting Republican National Committee Chairman Frank Fahrenkopf (from Reno; friend of Reid; Santini's college fraternity brother -- Nevada is like this) to call Reid a "Tip O'Neill liberal." Fightin' words, pardner.

Reid, a Mormon, opposes abortion and the Equal Rights Amendment. Boys are raised that way in Searchlight, Nev., which had 13 brothels when Reid was growing up.

Reid's ancestors came to Nevada as gold miners before 1900, before the state found a new extraction industry: extracting dollars from tourists. The marriage, so to speak, of divorce and gambling was Nevada's entrepreneurial answer to the exhaustion of the Comstock Lode. The "Almanac of American Politics" notes that Nevada votes conservative but lives off things conservatives dislike: gambling and, worse, government. After the casinos, Nellis Air Force Base is Nevada's biggest employer, and the federal government's nuclear test facility pumps in more money than Nellis. A large and growing portion of the population consists of elderly who like the way Nevada's low taxes treat the Social Security checks mailed here by wicked Washington.

Furthermore (was an almanac ever so rude?), "Nevada is probably the least family- oriented state: 32 percent of its households do not contain families (the highest percentage in the nation), while 64 percent don't have children (a proportion exceeded only by Florida), and 44 percent aren't occupied by married couples (exceeded only by California and New York). Nevada is filled now with people who came here thinking they were sharper than others, that they had a special angle, that they could and would beat the odds . . ."

Hey, lighten up, Almanac. Some Nevadans may be nutty, but all have reasons for feeling different. The federal government owns 87 percent of Nevada. (Other western percentages: Alaska, 96; Idaho, 67; Utah, 65; Wyoming, 48; California, 45; Arizona, 44 . . . east of the Mississippi the highest percentage is New Hampshire, 12.) What with bombing ranges on Nevada's surface and nuclear tests below the surface and talk of a nuclear waste dump north of Las Vegas, Nevada is feeling, well, used.

However, between now and November, Nevadans are going to be stroked and courted mercilessly as the two parties pour in resources to win a cheap seat. The GOP has two advantages. One is money, but there are limits to its usefulness in a state with only two media markets, boh inexpensive. The race here will be less expensive than the races in two smaller states, Idaho and South Dakota. The other GOP advantage is the Gipper. Part of the Republican offer to Santini was at least two Reagan campaign visits -- talk about a test of nuclear megatonnage. Nevadans get rubbery in the knees at the mere mention of Reagan, but it remains to be seen whether even here his popularity can be transferred.

Reid and Santini have been mingling with Nevada's small electorate (300,000 registered voters) for so long, the undecided vote -- the target of all the seduction -- is small. So in late October an isolated herder of Elko may be startled out of a daydream and wonder what is spooking the sheep, and he will look up and see Marine One, the president's helicopter, descending toward him. No Nevadan will be safe from such attention until Wednesday, Nov. 5.