South Carolina Sen. Strom Thurmond, in an unexpected blow to Virginia Sen. John W. Warner, has decided to seek the chairmanship of the Armed Services Committee if the Republicans retain control of the Senate in this fall's elections, according to Senate staff sources.

The decision has touched off hopes in Virginia of persuading Warner, the party's senior Republican, to run for governor in 1989 in an effort to end the losses that the party has suffered in the last two statehouse elections.

A former Navy secretary who has made much of the large military presence in Virginia, Warner is hopeful that Thurmond can be persuaded to retain his chairmanship of the Senate Judiciary Committee. A source close to Thurmond said "that is not in the cards."

Thurmond, a former judge, believes he has completed his agenda on the Judiciary Committee and wants to turn to the military, one of his "greatest loves."

"It's a supersensitive subject on the committee," said one Republican staff member. "No one will talk about it on the record."

Both Republicans and Democrats say Warner has been a key player on both military and arms control issues while Thurmond has been far less involved and at 83 may be less active than Warner, 59.

Warner, third-ranking GOP member of Armed Services behind Thurmond and current chairman Barry Goldwater (R-Ariz.), has expected since 1983 to succeed Goldwater when he retires at the end of this term. Under Senate rules, Thurmond would have to give up his chairmanship of the Judiciary Committee to take over Armed Services.

Senate staffers said that the retirement of Maryland GOP Sen. Charles McC. Mathias Jr., a longtime liberal nemesis of Thurmond and second behind him on the Judiciary Committee, also played a role in the decision to switch chairmanships. Conservative Sen. Orrin G. Hatch (R-Utah) would be in line to succeed Thurmond on Judiciary.

Thurmond's decision is not only a personal blow to Warner but would be seen as a major setback for the state of Virginia, the home of numerous military installations, including extensive Navy operations in Norfolk, that would expect to benefit from Warner's leadership.

Warner declined yesterday to discuss Thurmond's decisions and said Thurmond "has not had that conversation with me in specific terms." Warner said "We normally follow the rule of seniority. That is his right."

Mark Goodin, a Thurmond spokesman, said, "He has considered taking over the chairmanship of [Armed Services] for some time and still considers that a very viable option."

However, both GOP and Democratic sources said Thurmond had firmly decided on Armed Services.

In 1981 Thurmond stepped aside from the Armed Services Committee to allow Sen. John Tower (R-Tex.) to assume the chairmanship and again in 1983, allowing Goldwater to become chairman.

Warner, a moderate conservative who was narrowly elected in 1978, has overcome his image as a dilettante known for his brief marriage to actress Elizabeth Taylor.

"He's extremely active, one of the workhorses on the committee," a Democratic staffer said of Warner. "He's been in the thick of every major issue the committee as far back as I can remember."

Warner has done nothing in recent years to play down expectations that he would succeed Goldwater, telling The Washington Post in a 1984 interview that he would try to bring more bipartisanship to the committee if he were chairman.

Asked about a possible race for governor in 1989, Warner said yesterday that he felt "an obligation to complete this term and not run for governor."

Donald W. Huffman, Virginia GOP party chairman, said yesterday it would be difficult for Warner to leave the Senate. "That is something very dear to him." But, Huffman added, "he'd be a great candidate. I'd love to see him run."