One was to be a celebration; the other, a coronation.

Instead both Virginia's Democratic and Republican state conventions, which convene today, will depart from their prepared scripts.

The Democrats will gather in the Roanoke Civic Center for what they had hoped to be a celebration of their sweep of state offices last November, a gala for Gov. Charles S. Robb, the first Democrat to win the statehouse in 12 years. The party's tenuous coalition, however, has been torn apart in recent weeks by internal bickering over who should be the party's nominee for the U.S. Senate. The dissent has grown more pronounced as one by one the party's best-known figures have removed their names from contention.

The result, many party leaders say, is that the convention's 3,624 delegates may have to draft a nominee.

The 5,100 Republican delegates and alternates will gather in the Richmond Coliseum for what they had long planned as the coronation of their only Senate candidate, Rep. Paul S. Trible, the 35year-old congressman from Newport New. The party had planned the two-day event as a first show of unity for a GOP eager to reassert its dominance over Virginia.

The party's chances for unity were smashed last week when former Gov. Mills E. Godwin announced he and other members of the state's conservative coalition were launching a drive to place Sen. Harry F. Byrd Jr.'s name on the November ballot as an independent.

Byrd, the Senate's only independent, has said he will wait until next Tuesday before announcing his plans, a decision that has left both parties in confusion and uncertainty.

Here are the prospects for each convention as reported by Washington Post Staff Writers Celestine Bohlen, Fred Hiatt and Sandra G. Boodman: The Democrats

The Democrats find themselves confronted with a paradox. As of yesterday -24 hours before their convention convenes -their leading Senate condidate was a man who only two weeks ago firmly and flatly said that he would not allow his name to be placed in nomination.

Much has happened since Lt. Gov. Richard J. Davis, a former mayor of Portsmouth, self-made millionaire and the Democrats' top vote-getter in last fall's election, made that statement -one of many by leading Democrats shunning the nomination.

Yesterday there was renewed speculation that Davis, 60, will indeed seek the nomination -or, at the least, encourage other to secure it for him.

Leaders of the draft-Davis drive said yesterday they would continue their efforts until they were told by the candidate or his staff to stop. "If he wants the effort stopped, I will stop," said John McGlennon, a party leader in Davis' Tidewater home, "But so far they haven't told me to stop and I've given them plenty of opportunities."

Several Democrats said that Davis had met in Portsmouth with key financial advisors Wednesday night, seeking commitments for the "bagful of money" he had said he needed in an enigmatic interview last week. According to one Democrat, the meeting produced no decision. No comment was available yesterday from Davis' office.

The continuing uncertainty about a Davis nomination a mystery suspected by some to be an intentional ploy -has upset supporters of the three major declared candidates for the nomination, former Arlington congressman Joseph L. Fisher, 68; Fairfax County prosecutor Robert Horan, 49, and state Sen. Virgil Goode, 35, of Rocky Mount.

"I don't know how the Davis boomlet is going to sit with everybody, but I do know how it is going to sit with some people," said 10th District party chairman Mary Cahill of Reston, a strong Fisher supporter. "I think it's disruptive. It's presenting a picture of chaos that didn't need to exist. I think it is hurting the Democrats."

Northern Virginia Democrats are dividing between Fisher and Horan -a schism which some find painful. Fisher has spent the week traveling throughout the state, appearing before delegates in Fredericksburg on Tuesday, in Petersburg on Wednesday and Portsmouth on Thursday.

On Wednesday, Horan -a conservative on law and order issues who hopes to pick up support from the state's courthouse politicians -was endorsed by a number of key Fairfax Democrats, including Supervisors Martha Pennino, James Scott, Joseph Alexander and Sandy Duckworth, state Dels. Ken Plum and Dorothy McDiarmid and state Sens. Richard Saslaw of Fairfax and Charles Colgan of Prince William.

Arlington County Board Member John Milliken, a former aide to Fisher and one of his key strategists, estimated yesterday that the former congressman and a member of Robb's cabinet in Richmond, has support from at least two-thirds of the almost 700 delegates from Northern Virginia's two congressional districts. To win he will have to have the support of 1,752 of the convention delegates.

Others in the state -including 3rd District Chairman Angus McAuley of Richmond -have said that Fisher, despite his age and his perceived liberal tilt, should be considered the favorite of the declared candidates for the nomination.

There are other undeclareds besides Davis lurking in the wings, waiting for the convention--perhaps the most wide open in the party's recent history--to turn to them for rescue. Among these are former state Attorney General Andrew P. Miller of Alexandria, the subject of a unsolicited mailing by a former state aide who praised Miller as the Democrat with the most statewide name recognition and the most experience.

Party officials expect to see a number of favorite sons offered on the first round of balloting--including 7th District chairman George Gilliam of Charlottesville, Senate Majority Leader Hunter B. Andrews of Hampton and Del. Owen B. Pickett of Virginia Beach, the former state party chiarman whose forced exiit from the race May 4 set off the events that has left the party mired in confusion.

The convention begins tonight at 7 o'clock with a key note address by Sen. Samuel V. Wilson, a retired military intelligence officer who lives in Southside Virginia. Robb is also scheduled to speak at tonight's opening session. The Republicans

State and national Republican leaders went to extraordinary lengths on the eve of their Richmnd convention to convince Byrd to stick to his decision to retire and protect what they had seen as an excellent chance to increase their Senate majority.

Virginia party chairman Dr. Alfred B. Cramer III summoned reporters to say that Byrd, long a favorite of conservative Republicans despite his independent status, would face a tough campaign this fall. The Reagan administration dispatched its No. 2 political operative to Richmond to show support for the GOP's beleaguered prospective nominee. And the Trible forces unveiled a newly formed finance committee intended to demonstrate solid conservative support.

But the committee, including several longtime Byrd supportors, also illustrated that even the White House cannot overcome the loyalty Virginia's conservative establishment feels for the family that has dominated state politics for most of this century.

"I thought Harry was enough of a man to make up his mind to retire and stick to it," said Thomas C. Boushall, founder of the Bank of Virginia and a member of the Trible finance committee. "But I have to support Harry if he gets in the race."

Boushall is a member of the small coalition of bankers, lawyers and businessmen that for decades has discreetly guided Virginia politics from the corporate boardrooms that line Richmond's Main Street. Members of the coalition found themselves torn yesterday between devotion to Byrd and the fear that his entry into the race would elect a liberal Democrat.

With the desertion of some key conservative supporters, among them former Gov. Mill E. Godwin, and the recent loss of GOP strategist William Kling, Trible has begun hearing criticism that he has failed to adequately court the Old Guard and the New Right and has lagged in fund-raising.

Cramer defended Trible yesterday, saying his $330,000 warchest is a sign of a healthy campaign. He denied that the convention, originally planned as a unity rally to avenge last year's defeats and revel in the Democrats' disarray, would be spoiled by the draft-Byrd movement.

Several of Byrd's longtime supporters said they have sent unusually blunt messages to the senator urging him to stay out. "I think he'd be delivering the senatorial position to whoever the Democrats come up with," said Robert H. Spilman, chairman of Bassett Furniture Industries. "I've already committed to Congressman Trible and I'm not going to go back on any commitments."

The Trible campaign maintained a respectful distance from the draft-Byrd movement, anxious not to offend Byrd before the Tuesday night deadline for filing as an independent candidate. "Until such time as Sen. Byrd announces he is a candidate, I have nothing but admiration and praise for him ," Cramer said.

National party officials were harsher. Byrd almost always supports the Reagan administration, but he sits with the Democrats for the purpose of organizing the Senate and repeatedly has rejected GOP overtures to cross the aisle.

"We're going to the wall with Paul Trible," said one senior national party official in Washington. "This is a must situation for us."

To demonstrate their commitment, party officials plan to send Lee Atwater, deputy assistant to the president for political affairs, to read a statement from Reagan to delegates on Saturday.

Trible remained in danger, however, of losing more important supporters: those who can command contributions. Richard Brydges, a Virginia Beach lawyer and member of the powerful state highway commission, was announced as a member of the Trible finance committee yesterday. but said later in an interview that he also has a Byrd petition on his desk.

"I'm going to look at it in the morning and hope the Lord sends me some signal," Brydges said. "I might have to say to Paul Trible that when I did this, I didn't anticipate on old friend getting in the race."