As the last of his potential challengers withdrew, former Navy secretary John W. Warner yesterday declared his candidacy to succeed the late Richard D. Obenshain as the Virginia Republican Party's nominee for the U.S. Senate.

At a crowded news conference in the State Capitol in Richmond, the 51-year-old Warner said he was "ready to hit the deck and start running hard," accompanied by his second wife, actress Elizabeth Taylor, who Warner said will campaign with him in the fall.

"I pledge every ounce of strength and wisdom I have in this race," he said.

Warner said supporters will place his name in nimination Saturday at a meeting of the Republican State Central Committee in Richmond. The committee is charged with choosing a successor to Obenshain, killed last week in a light plane crash near Richmond.

The committee's nominee will opposed Democrat Andrew P. Miller, a former state attorney general, for the seat being vacated by Republican Sen. William L. Scott.

Warner spoke as Obenshain's grief-drained supporters appeared moving slowly into his camp after a frantic search for an alternative. With the withdrawal of State Sen. Nathan Miller of Bridgewater Tuesday night, there simply was not anyone else to turn to.

Warner said it would be understandable if some dedicated Obenshain supporters "might not be ready in their bereavement to turn quickly to someone else for leadership."

Obenshain, Warner said, "was a good friend, waged a good fight and gave all he had. Fate has dictated that someone must step up and assume that responsibility."

Warner finished a close second to Obenshain June 3 at the GOP State convention, and was the logical choice from the beginning to succeed him, many party officials said.

He had been striving throughout the week to console and reassure Obenshain's conservative backers, many of whom remained skeptical of his credentials both as a Virginian and as a conservative.

Former 10th District Rep. Joel T. Broyhill, Warner's preconvention campaign manager, said yesterday, "When I called Warner in New Hampshire take right after the crash we agreed not to take any action whatsoever to seek this nomination. . . or express any desire or lust for the job. . . We had to let the initiative come from somewhere else."

It was not just a question of tactics, Broyhill said, "it was a matter of taste."

Broyhill said that neither he nor Warner had made any phone calls to central committee members or other party leaders, "but so many called us that after awhile we just had to respond."

While Warner's nomination Saturday should now be a mere formality, major organizational problems remain for the campaign. Obenshain's supporters had fielded a fiercely dedicated. highly organized effort built around intense personal and ideological loyalty to their conservative candidate.

Warner has repeatedly voiced his hope that he can keep that organization unified. while redirecting it for his own candidacy.

Obenshain supporters say he has reportedly asked Virginia's GOP National Committeewoman Judy Peachee, Obenshain's campaign director, to stay on. At least four other sources high in the state party say they and Warner hope the actual campaign will be run by William Royall, the 32-year-old tactician who directed Goc. John N. Dalton's campaign last year and President Gerald Ford's effort, which carried the state in 1976.

Broyhill declined to comment directly on the Royall-Peachee problem. "There are jealousies and factions in the leadership of any political party." he said, "and when you add the kind of emotional resistance you get tied up in a tragedy like this, hell, it's only human nature. You can't plan or orchestrate something like this . . . but I think now at this point maybe it's been put together about as well as it can be.

"Both sides are just going to have a bite the bullets in the thing as best they can . . . The main thing is that The Obenshain campaign organization not be permitted to die."

Warner said yesterday that if nominated he does not intend to inject any "significant" amount of his personal wealth into the campaign.

While he declined to define "significant", he added: "I expect my campaign to be financed largely by Virginians. I am confident I can raise the money."

Such a campaign would contrast sharply with his preconvention campaign, which was largely financed with his personal loan of more than $450,000. Warner said he recently sold 40 acres of his farm near Middleburg to settle that debt.

Democrat Miller, meanwhile, said he was not surprised at Warner's announcement of candidacy but that his campaign strategy would remain the same "whomever the Republicans choose."

Miller, who voluntarily suspended campaigning after Obenshain's death - and the death of his father a day later - was to resume his schedule to day with appearances in Newport News, Norfolk and Lynchburg.