Virginia Republican leaders were nerously seeking to block today a potential coup by a former Democratic legislator who has enrolled the Christian right in a bid to win the GOP's nomination for lieutenant governor.

The party's state convention opened here today with Gov. John N. Dalton adding his name to a growing list of GOP leaders seeking to deny the nomination to frontrunner Guy O. Farley Jr., 48, a Warrenton lawyer and born-again fundamentalist ally of television minister Jerry Falwell.

Dalton said he would cast his vote for Newport News state Sen. Herbert Bateman, 52, another ex-Democrat, who had already won backing from former Gov. Mills Godwin and his supporters in the Richmond-based conservative business community.

Bateman's campaign claims the high-level endorsements have helped push him far ahead of the third candidate in this contest, Rockingham state Sen. Nathan Miller, 37, who commands support from a cadre of GOP regulars who question the party allegiance of both Farley and Bateman.

Farley has come here with a substantial block of devoted followers, many of them previously strangers to GOP politics, although it is widely believed he will fall at least 300 votes short of the 1,692 he needs for a first ballot victory.

But his delegate strength has touched off a frantic scramble among party leaders who consider him a right-wing extremist and a potential threat to the GOP ticket and its leader, Attorney General J. Marshall Coleman, the party's certain gubernatorial nominee.

Coleman has so far managed to maintain public neutrality on the contest, although many of his chief supporters -- including Dalton -- are backing Bateman. Coleman is to be nominated tomorrow before the potentially devisive showdown between the lieutenant governor's candidates, a contest that could go long into the evening.

The bitterness of the intraparty power struggle has been increasing all week. Farley today branded his opponents as "me-too candidates" who would bring little to the ticket of Coleman and certain attorney general nominee Wyatt Durrette of Fairfax. Miller's latest campaign literature describes Bateman as a "turncoat Democrat" and "retrofitted liberal."

Miller, who contends he is ahead of Bateman, says Bateman's candidacy is solely the product of "Richmond power brokers," who have helped finance the party's decade-long streak of electoral victories.

"There's some real resentment about our party being run from Main Street in Richmond," says Fairfax Del. Vincent J. Callahan, a Miller supporter.

The suspicion and distrust among the candidates was evident today. Armed guards in brown uniforms were stationed to intercept "intruders" to Bateman's command center on the second floor of a Holiday Inn here. Two floors above, volunteers with walkie-talkies roamed the Farley headquarters, saying they had been told to "maintain security" and head off "dirty tricks" by the rival campaigns.

Bateman's security was so tight that the candidate himself was said to have been briefly turned away from his hotel room last night because he lacked the proper badge.

"That's a little like being bitten by your own dog," joked Miller, who has no guards outside his campaign center at the Sheraton. "My opponents are practicing overkill," he added.

Farley campaign manager John Alderson says the guards are necessary because "We have to maintain discipline on this floor to make sure everybody carries out their assignments." Alderson contends that discipline will pay dividends during tomorrow's balloting, which could go late into the night.

"Our delegates will be the most disciplined on the convention floor -- there will be no attrition," says Alderson, who like Farley was a major figure in Ronald Reagan's Virginia campaign last year.

Besides the fundamentalist, many former Reagan workers compose the bulk of Farley's campaign, which has support from a few party notables, including Rep. Paul Trible and GOP National Committee members William Stanhagen and Helen Obsenshain. They argue Farley can reproduce the voter support that gave Reagan a wide victory marin in Virginia last year.

But many party strategists believe the moderate-to-conservative Democratic ticket led by Lt. Gov. Charles S. Robb, nominated here last weekend, will prove far more formidable an opponent than was Jimmy Carter.

"Chuck Robb's proven to be a pretty smart boy," says former GOP Gov. Linwood Holton, a Miller supporter. "The Democrats have nominated an excellent ticket."

Miller, a young, energetic campaigner, contends he would be the most electable candidate to oppose the Democratic lieutenant governor's nominee, former Portsmouth Mayor Richard Davis. But Bateman, who has a reputation as a ponderous public speaker, says he could best attract the Byrd-oriented independents who might otherwise be tempted to vote for the conservative Robb.

The Democratic ticket came in for a blistering attack from GOP State Chairman Alfred Cramer, who opened this convention today with a speech by tagging the Democrats as "clones." Cramer accused Robb and his running mates of masquerading as conservatives, saying "the Democrats are artful imposters and the Republicans are the real things."

Other speakers stressed the party's connections to the White House, as did a convention brochure that reprinted a letter from President Reagan praising Coleman as "a rising young leader who needs your full and active support."

The White House sent Republican National Committee Chairman Richard Richards and Judy Peachee, special presidential assistant for intergovernment affairs. Besides giving today's keynote address, Richards delivered a $50,000 check for Coleman from the RNC, an amount to be added to the $100,000 that the Republican Governor's Association has already pledged to the campaign.