When President Reagan comes here July 23 to raise funds for former Texas governor Bill Clements' comeback bid, it will dramatize the national importance of the battle for control of this still-proud center of the newly humbled Sun Belt. If there is any single race that tests the long-term political effects of ''the Reagan revolution,'' it is Clements' grudge rematch with Gov. Mark White, who ousted him from office four years ago.

Clements is more than 20 points ahead in recent private polls, partly because the energy-industry crash has pushed Texas unemployment to 10.5 percent. Facing a $3 billion shortfall in the biennial state budget, White last week was forced to call the legislature into emergency session in August.

The incumbent has antagonized educators, state employees and other elements of the Democratic coalition. He has flip-flopped on enough issues that Clements' polls show White with a near-terminal 50 percent-plus disapproval rating.

Still, Republicans acknowledge White is a crafty campaigner who defeated favored opponents in the 1978 attorney general's race and the 1982 battle with Clements. At 69, Clements is hardly a fresh face, and the beaming, chuckling personality he displayed through a three-way Republican primary last spring and in an interview the other day may or may not survive White's barbs this fall. Democrats think Clements can be goaded into reminding voters of the testy, imperious fellow he sometimes was as governor, and then his own negatives will soar.

If Texas were still the basically Democratic state it was considered until Clements became the first Republican governor in 1978 by a scant 20,000 votes, White might have a chance for another upset. But there is strong evidence that Reagan has remade the fundamental political equation in the state.

Clements describes the shift this way: ''In 1978, when I first ran, the state was 35 percent Democratic, 20 percent Republican and 45 percent Independent. Today it's 35-35-30. It's shifted 15 points from ticket-splitters to acknowledged Republicans in eight years. That's Reagan.''

Richard Wirthlin, his pollster (and the president's), puts the shift in a longer time frame: ''Given the turnout patterns, Clements can get 72 percent of the votes he needs for victory this year from self-identified Republicans. When George Bush ran his last Texas race in 1970 (losing to Democratic Sen. Lloyd Bentsen), he could count on Republicans for only 36 percent of the votes he would have needed to win. That's how much it's changed.''

The change has been gradual. Clements' 1978 victory was one landmark, and Reagan's 1980 win was another. But the 1984 reelection campaign was a political earthquake, as Reagan's coattails swept out of office hundreds of seemingly safe courthouse Democratic officials in rural Texas counties as well as the fast-growth metropolitan areas.

While the Democratic presidential vote was essentially stable at around 2 million votes in 1976, 1980 and 1984, Republican totals grew from 1.95 million to 2.5 million to 3.4 million in successive elections.

Paul Burka wrote in the July issue of Texas Monthly that an even more striking phenomenon was the increase in Republican primary voting. In 1982 there was one Republican primary voter for every five Democrats. This year, it was one for every two.

''Conservative Democrats, who have been abandoning their party in fall elections in ever-increasing numbers, abandoned it in the spring primaries for the first time,'' Burka wrote. ''This is the death knell for the old order in Texas politics. . . . The Democrats' traditional advantage is gone; the only question left is whether Texas will be a swing state or a Republican state.''

On that question, there is another Reagan legacy that may tip the answer to the GOP. With an uncontested presidential nomination in 1984 and nearly unlimited funds available, the Reagan campaign and the Republican National Committee made a massive investment in identifying Reagan voters and getting them to the polls.

The heritage of that 1984 effort in Texas can be found on spools of magnetic computer tape containing names, addresses and phone numbers of 802,000 individual voters who told the Reagan-GOP canvassers they would cast their first-ever Republican vote for Reagan.

Those voters will be targeted for special mailings and phone calls from the Clements' campaign, using some of the money Reagan will raise on his Wednesday visit. This is what a popular party-minded president can do to help convert his personal success into a long-term political realignment.

And if Mark White greets Reagan by repeating his charge that administration policies are to blame for the energy and agriculture recession in Texas? ''I would encourage that,'' Clements said. ''Reagan has a 75 percent approval rating in Texas. The more my opponent talks about his failings, the more people he irritates.''