At first, Northern Virginia Republicans may have assumed they had scored a coup or two at the Republican State Convention in Virginia Beach the other weekend.

It was the Northern Virginia delegation that led the last-minute stampede to Rockingham senator Nathan H. Miller, helping him win the lieutenant governor nomination.

It was the Northern Virginia delegation that held the key in dumping Miller's most formidable opponent, Herbert H. Bateman, and delivering a swift kick to Bateman's biggest backer, former governor Mills E. Godwin.

"There was no question that the convention boosted Northern Virginia's strength in the state party," says Fairfax Supervisor Thomas M. Davis III, a convention delegate."It was showing its antipathy to Main Street [the party's financial wellspring], to Mills Godwin and to the lack of services that we've had to put up with here for so long."

But as the convention euphoria fades, and it comes time to raise money for the largely unknown Miller, state Republican leaders may wonder if Northern Virginia's power play may make it tougher to raise money for the fall campaign.

Many say the Northern Virginians may have played a key role in alienating the so-called "Main Street Crowd" -- the influential state business leaders who, for almost a decade, have been the hub of the party's fundraising activities.

"I sure hope all these guys who were for Nathan and this pseudo-revolt against Main Street knew what they were doing," says one Republican fundraiser. "I hope they're prepared to make up for the possible loss of funds to the party."

The rebellion against Bateman, some fear, may have added impetus to some old-line Republicans who were threatening to bolt from the party because of the gubernatorial nomination of J. Marshall Coleman, who is preceived in some conservative circles as to liberal.

Already, Virginia Democrats are starting to brag about the number of Republican moneymen drifting into their ranks. Former statehouse appropriations committee chairman W. Roy Smith of Petersburg, a charter member of the conservative Main Street coalition, last week escorted Democratic gubernatorial candidate Charles S. (Chuck) Robb to the Emporia Pork Festival, demonstrating to the more than 12,000 revelers that the once-firm GOP ranks might be weakening.

A few days before, Robb had a received a $5,000 campaign contribution from former Virginia National Bank president W. Wright Harrison, another leading coalition member and a former campaign contributor to Mills Godwin and Gov. John N. Dalton.

Supervisor Davis and other Northern Virginian Republicans say they're not particularly worried about the political fallout created by the nomination of Miller, a Shenandoah lawyer who first won statewide attention in 1978 when he lost a challenge against the party's hierarchy over the senatorial nomination. "There are a lot of sources of money in this state besides Main Street," says Davis.

Still, some Northern Virginia Republicans admit it will be difficult to extract as much money from the Washington suburbs as the GOP has gotten from the Richmond business set in past years. For one thing, Northern Virginia's GOP base is largely a group of volunteers, long on enthusiasm but short cash.

For another, Republicans in Northern Virginia are more likely to be tied to business that are concerned with national issues -- not Virginia matters -- thus making it difficult for the party to pull in the fat corporate contributions for which the Main Street coalition is famous.

What all of this means to Coleman's campaign is anybody's guess, but party regulars are quick to point out that the only recent statewide Republican candidate who won statewide attention in 1978 when he lost a challenge against the party's hierarchy over the senatorial nomination. "There are a lot of sources of money in this state besides Main Street," says Davis.

Still, some Northern Virginia Republicans admit it will be difficult to extract as much money from the Washington suburbs as the GOP has gotten from the Richmond business set in past years. For one thing, Northern Virginia's GOP base is largely a group of volunteers, long on enthusiasm but short on cash.

For another, Republicans in Northern Virginia are more likely to be tied to businesses that are concerned with national issues -- not Virginia matters -- thus making it difficult for the party to pull in the fat corporate contributions for which the Main Street coalition is famous.

What all of this means to Coleman's campaign is anybody's guess, but party regulars are quick to point out that the only recent statewide Republican candidate who won without total Main Street support was Sen. John W. Warner -- a millionaire who tapped his own fortune to defeat moderate Democrat Andrew P. Miller. The unspoken implication seems to be that Coleman and his running mate, who have no comparable resources, are likely to turn increasingly to national conservative fundraising organizations for help.

One of those organizations, the Fund for a Conservative Majority, has pledged to spend at least $100,000 on Virginia's gubernatorial campaign this year, expecting that the election will be viewed nationally as a referendum on the Reagan administration and its policies.

That has heartened Joe Loyacono, Nathan Miller's campaign manager, who is only now beginning to shift his thoughts to fundraising after his candidate's hard-fought fight for the nomination. He says his candidate, who has still not set a campaign budget or found a chief fundraiser, expects to get a piece of whatever the conservative fund spends in Virginia.

But Paul Dietrich, the fund's executive director, says the prospects for that kind of assistance are far from bright. His organization does not normally get involved in lieutenant governor and attorney general races, he says, because they are not perceived as having much impact on the national political scene.

"I certainly understand their (the Miller campaign's) problems raising money," Dietrich says, "but our theory is if the top of the ticket is going well it will help Nathan Miller and (GOP attorney general nominee) Wyatt Durrette, too."