Virginia Republicans, recovering from the defeat of their state-wide ticket last fall, today rallied behind Paul S. Trible Jr., a 35-year-old congressman from Newport News, as their leading candidate for the U.S. Senate.

Flanked by the state's ranking Republicans, including former Gov. John N. Dalton, Trible announced his long-expected candidacy at three strategic sites around the state, starting with a dawn visit to the gates of the Newport News Shipbuilding and Dry Dock Co.

Trible is the first major candidate of either party to announce for the seat being vacated by Harry F. Byrd Jr., the only Independent in the Senate and heir to Virginia's most famous political dynasty. Byrd announced last November that he will retire at the end of his term.

Republicans said their unified front will give them a head start against the Democrats who are still groping in their search for a consensus candidate. So far, at least five Democrats are considering a run for the Senate.

At a press conference here today, Trible and his wife Rosemary were greeted by Dalton, state party chairman Alfred Cramer and several dozen party officials whose presence bestowed a blessing that won't become official until the Republican Party convention in Richmond next June.

"It didn't take us long to put it back together again," said Cramer. "We're unified and coalesced and ready to go again."

In his speech, Trible dwelt heavily on themes close to the hearts of Virginia's conservatives--frugal government, a strong national defense and a pledge to continue President Reagan's budget and tax-cutting program.

"Harry Byrd and his father before him U.S. Sen. Harry F. Byrd Sr. contributed mightily to Virginia," said Trible. "They hewed to the principles of Virginia and are honored for it. Now Sen. Byrd is passing on the torch."

Trible, elected three times in a strong Democratic district, moved quickly to claim the Republican nomination soon after Byrd announced his pending retirement. Even before Byrd's announcement, Trible had been busily lining up support in the event that the GOP was unable to persuade Byrd, a staunch conservative, to run as a Republican.

By January, both Republican Rep. Stanford E. Parris of Woodbridge and Wyatt B. Durrette of Fairfax, the party's unsuccessful nominee for attorney general last year, announced they would not run for Byrd's seat.

Both had been asked to run by some conservative Republican Party officials who feared that Trible's youth and ambition made him uncomfortably similar to J. Marshall Coleman, the former Republican attorney general who led the ticket to defeat last year.

Coleman, who recently joined a Washington law firm, was at the Richmond announcement today, although he was overlooked in the initial introductions by party chairman Cramer.

Durrette, an attorney and one of only a few top Republicans absent at the kickoff ceremonies in Newport News, Rosslyn and here, said yesterday he is "certainly favorably inclined" toward Trible. "But," said Durrette, "at this very early stage, I have not yet been willing to make a firm commitment to anyone."

Another endorsement still sought by Trible is that of former Gov. Mills E. Godwin, a Democrat-turned-Republican and spiritual leader of the state's conservative establishment.

As Republicans refine strategies for the Trible campaign, the top contenders for the Democratic nomination are continuing to meet among themselves, hoping to decide on a candidate acceptable to Gov. Charles S. Robb and to the party's various wings.

Former Attorney General Andrew P. Miller, Senate Majority Leader Hunter B. Andrews (D-Hampton), state Democratic party chairman Owen Pickett and former party chairman William Thomas of Alexandria met in Richmond Saturday. State Sen. Edward Holland of Arlington, whose potential candidacy surfaced in recent weeks, was invited to attend but had other engagements. According to Miller, the group plans to meet again this week.

Many Democratic party leaders are looking to Robb to indicate his choice in the Senate contest. According to sources, Robb has asked David Doak, Robb's former campaign manager, to conduct an informal survey of party leaders to get a sense of the party's emerging choice.