Former Virginia governor Mills E. Godwin, in a move that could intensify friction between conservative and moderate Republicans, endorsed conservative Richard D. Obenshain yesterday for the party's U.S. Senate nomination.

Godwin's endorsement three days before the Republican nominating session in Richmond injected another dramatic element into what is expected to be one of the most closely contested political races in recent Virginia history and possibly the largest political convention ever held in the nation.

At least 7,000 and possibly as many as 9,000 delegates are expected to jam the Richmond Coliseum Saturday to cast 3,081 votes for Obenshain, former Gov. Linwood Holton, former Navy secretary John Warner and state Sen. Nathan H. Miller in the Senate contest.

Obenshain, a former GOP national cochairman and the acknowledged front-runner, is generally expected to get more than 1,200 votes on the first ballot, but this will be about 300 short of the majority of 1,541 needed to win.

Managers for both Holton and Warner claimed again yesterday that their candidates are running a close second to Obenshain, whom they said has peaked and has the least potential of all the candidates for picking up additional delegates at the convention.Many Republican officeholders and party officials believe it will take at least three ballots to nominate a candidate.

Miller is considered far out of the running, but the 100 or more votes assigned to him by delegate counters could play an important role in the final outcome, some party leaders say.

The endorsement of Obenshain by Godwin, one of the most successful politicians in the state, was hailed by Obenshain supporters as evidence of their candidate's electability.

"It answers in rolling thunder the argument that Dick Obenshain is not electable," former state Del. Wyatt B. Durrette Jr. of Fairfax, an Obenshain supporter, said in an interview.

However, campaign officials for Holton and Warner and some experienced Republican political figures not active in this campaign said in interviews that the Godwin endorsement will have little or no effect on delegates because most delegates are already committed to a candidate.

Some said the endorsement may strengthen the resolve among party moderates supporting Holton who have been opposed to Godwin in previous elections when he was a leader of Virginia's conservative Democrats. They argued that Godwin will have his greatest influence among conservatives, especially from central Virginia, many of whom are already solid Obenshain supporters.

"We were not surprised by the endorsement," Holton campaign director Kathleen Lawrence said. "We believed he was working quietly for Obenshain but we thought as a former governor he would not take a public position. I really don't think it will affect more than eight or 10 votes."

Lawrence said she believes Godwin may have been persuaded to speak out by some of Obenshain's fund-raisers who also have raised political contributions in the past for Godwin.

Godwin is the only person to have been elected governor of Virginia twice, and he did it first as a Democrat and then as a Republican. His two-party history - he defeated Holton in his first race for governor as a Democrat - and his lingering ties to Democratic politicians still stirs resentment among many Republicans.

He angered party regulars last year by endorsing Democratic attorney general candidate Edward E. Lane, a longtime political ally, over Republican J. Marshall Coleman. Coleman won easily and now is considered the most likely Republican nominee for governor in 1981.

In 1969 Godwin as incumbent governor, endorsed moderate Democrat William Battle in a primary against populist Henry E. Howell, a step that so enraged many liberal Democrats that many stayed away from the polls that year. The result in part helped elect Holton as Virginia's first Republican governor.

Godwin and Obenshain, a party regular, always speak of political effectiveness in Virginia of the coalition of Republicans, conservative Democrats and independents.

In his endorsement statement, Godwin said, "I will support Richard D. Obenshain because he is a man of convictions and he has the best chance of winning the coalition of moderate and conservative Virginians - Republicans, Democrats and independents - have prevailed in recent elections in Virginia."

Those skeptical of Godwin's influence over Saturday's nomination pointed yesterday to the failure of the conservative coalition to elect Lane last year and also to the inability of coalition leaders to nominate their choices for attorney general and lieutenant governor at the 1977 state GOP convention.

Obenshain openly supported Durrette, who lost to Coleman, and former secretary of finance Walter W. Craigie, who lost to state Sen. A. Joe Canada in the race for the GOP nomination for lieutenant governor. Godwin was perceived as being in accord with Obenshain, but played no public role in those contests.

Reaction to the announcement among Northern Virginia Republicans was predictable - Obenshain supporters said it would have considerable impact; Warner, Holton and Miller supporters said it was neither a surprise nor of great significance.

Durrette predicted the endorsement would have "substantial impact" on undecided delegates outside of Northern Virginia, but said it would sway few votes in the 8th and 10th congressional districts.

Barbara Hildenbrand, the 10th District GOP chairperson, and an Obenshain backer, said "There still are some people who haven't put their feet in cement on the first ballot." Godwin's decision would have a positive effect on them, particularly on later ballots, she said.

Former representative Joel Broyhill, who is serving as general manager of the Warner campaign, said the Godwin announcement will not change Warner's strategy and will not change many votes.

"We'd like to have had his endorsement," Broyhill said. "But Godwin and Obenshain are good friends, and it came as no surprise."

Del. Vincent Callahan of Fairfax, a friend of Holton, also said he was not surprised, although, he added, Godwin had indicated to Holton about a month ago that he would not make a preconvention endorsement.

Callahan said the endorsement will have little effect. "Obenshain will get his maximum on the first ballot" and that won't be enough to nail down the nomination, he predicted.

State Sen. Wiley F. Mitchell Jr., who is supporting Miller as a potential compromise nominee, said he was surprised only that Godwin "decided to get into the fray at this time."

Godwin and Obenshain have been "very close for a long time, and I never had a serious thought that Godwin would do otherwise (than back Obenshain)," Mitchell said. "It will have a negligible effect. Most delegates have made up their mind for the first ballot."

Mitchell said he will stick with his legislative colleague Miller "as long as he wants me to, but I won't stay in any pond until I drown."

Edmund L. Walton Jr. of McLean who will serve as chairman of the convention's rules committee, has pledged to remain neutral until after that work is completed. But Walton agreed with Mitchell that "the lines are pretty well drawn" and that the announcement will produce "no dramatic effect."