Texas Gov. Bill Clements considers himself a history buff, but he isn't against a little rewriting if it suits him.

When he announced for reelection recently, he was confronted with the fact that he had failed to deliver on two of his promises from 1978: $1 billion in tax relief and the elimination of 25,000 state jobs.

"Those weren't promises," Clements replied without blinking, "they were goals."

That kind of political agility has kept Clements' opponents so off balance that for a time it looked as if a second term was his just for the asking.

But the Republican governor has suddenly found himself with a roomful of Democratic opponents who have taken heart from President Reagan's continuing economic problems and Democrat Charles Robb's victory in the Virginia governor's race last month.

As a result, Texas is heading toward the most expensive gubernatorial campaign in its history, and one of the most rancorous.

Few states figure as prominently in the Republican Party's hopes for 1982 as Texas, and Clements is one of the reasons.

Narrowly elected in 1978 as the first Republican governor in Texas in more than a century, the tart-tongued Clements has proved himself adept at building up the party here without greatly jeopardizing the support of the conservative Democrats who helped elect him.

Clements' success in 1978 and the Reagan's landslide victory here in 1980 have Republican strategists drooling over the prospects for a breakthrough in this conservative, but longtime Democratic, stronghold.

There are many Democrats who sense the same movement, and they believe Clements cannot be beaten next November. But increasingly the governor has appeared as an attractive target, and this week the campaign began in earnest.

Two Democrats already have announced. First was state Sen. Peyton McKnight, a wealthy independent oilman. He was followed last Monday by state Land Commissioner Bob Armstrong.

Attorney General Mark White, who defeated Reagan chief of staff James A. Baker III in 1978, says he is on the brink of declaring his candidacy, and the field also is likely to include a genuine wild card in the person of Prentis Tomlinson, a young unknown Houston oilman whose only qualification seems to be an unlimited desire to spend his own money.

Several factors have turned this contest into a high-stakes shootout.

One is Reagan's policies. Clements ran in 1978 on national themes and has identified himself closely with the president. If Reagan's budget and tax programs haven't taken hold by next year and there is a feeling that the Republican agenda is geared mainly to the rich, Clements could suffer, even though Reagan remains personally popular.

There are reports that the governor's popularity dipped in the past month, and Armstrong sounded what may be a common theme among Democrats nationally next year when he promised an end to "government of the few, by the few and for the few."

Another factor in the Democratic challenge to Clements is Chuck Robb's victory in the Virginia governor's race.

Attorney General White, a conservative Democrat, was greatly impressed with the campaign Robb ran in Virginia and says he believes that the basis for a victory over Clements lies in the Democrats' ability to patch together the coalition of conservatives and progressives that kept this a one-party state for so long.

"I've always long maintained that the Democratic Party is doing better than a lot of people think," White said this week.

McKnight's campaign also may steal a line or two from Robb, because the Tyler state senator has hired as his consultant Bob Squier, who handled Robb's race this fall.

Robb will speak in Austin on Dec. 21, and there is great interest here in how his political prescription can be applied to Texas.

Finally, there is a persistent feeling among many Democrats that, despite his agile politics, Clements is more vulnerable than he appears.

Pollsters have been combing the state on behalf of the various Democratic candidates, and their conclusion was best summed up by Armstrong on the basis of a poll done for him by Jimmy Carter's adviser, Patrick Caddell. "This race is unformed and it could be won by any number of people," Armstrong said.

Lance Tarrance, Clements' pollster, has taken no recent polls specifically for the governor, but he pointedly dismisses one consistent Democratic claim, that Clements is beatable because he gets high negative ratings from the voters.

"We've done surveys in 10 congressional districts," he said, "and I have not found enough negatives in Clements' image as governor to agree at all with what's being pushed around by the Democrats."

The one prediction that is safe this far ahead of the election is that more money will be spent in this campaign than ever.

Clements spent $7.2 million in 1978 and there is speculation that he might hit $10 million in 1982.

"We have not come near to making a decision on what this campaign will cost," campaign finance director George Bayoud said in denying the accuracy of the $10 million figure.

Armstrong, whose biggest problem is raising money, is trying to turn poverty into an asset by warning that the governor's mansion should not be for sale. But he's prepared to spend $2 million to $3 million.

McKnight, who shares Clements' reputation for enjoying a tough fight, is talking about $5 million, much of it his own. He picked up about $800,000 at a fund-raiser last Monday in Fort Worth that featured singers Willie Nelson and Jerry Jeff Walker.

Tomlinson is letting it be known he's prepared to spend $6 million on the primary alone. Just what he will use as a platform is less clear. "We haven't gotten into issues yet," said one of his advisers. "We have lots of time to talk about that."