The certainly Ronald Reagan will head the Republican Party this fall has Northern Virginia Republicans doing something they have not done for six years: talk gleefully of recapturing the area's two congressional seats.

So confident are the Republicans of regaining the 8th and 10th district seats they lost in the so-called Watergate elections of 1974 that candidates are scrambling over each other in an effort to win the GOP nominations.

The contests, to be decided in primaries on Tuesday, are one measure of how strong is the discontent that GOP leaders say they detect with President Carter in the Washington area and in Virginia, the only southern state he failed to carry in 1976.

"There's no question that there's a feeling the people will turn to the Republican Party to solve the nation's problems," said Willaim Olsen, Fairfax County GOP party chairman.

Suburban Washington voters are angry with the Carter administration and ready to turn it out of office, Olsen said. The GOP also is counting on skillful organizing and "Reagan's coattails" to help its slate, he said.

The GOP "will be able to paint (Democratic Reps. Joseph L.) Fisher and (Herbert) Harris with the same brush" they use against Carter, Olsen said.

The recently condluded Democratic presidential primaries have added to the party's glee, said Nick Panuzio, a Reagan adviser and former head of the Fairfax GOP. "Carter is very vulnerable," he said. "When people in six out of nine states kick the hell out of him, you know even Democrats are dissatisfied."

The strongest indication that the Republicans in Northern Virginia smell blood is the upcoming primaries, which have turned into unexpectedly spirited contests.

In the 8th District, voters in Alexandria, Prince William, southern Fairfax County, the northern tip of Stafford County, will decide what is said to be a very close battle. It is between Stanford E. Parris, who is fighting to regain the seat Harris took away from him in 1974, and Robert Thoburn, an ultraconservative Christian school administrator and former state delegate.

Rep. Fisher's seat is the goal of three GOP candidates in the 10th District race, an area that includes the cities of Fairfax and Falls Church and in the counties of Arlington, Loudoun and Northern Fairfax. Lawyer Frank Wolf, who ran a close race against Fisher two years ago and never has really stopped campaigning since, is considered the front-runner over state Del. Martin Perper and Harold Miller, the outgoing mayor of Falls Church.

Despite the hotly contested primary -- Wolf has spent about $60,000 so far to win his party's nomination -- the congressional election next Tuesday is not likely to draw a large number of voters. There are no other issues on the ballot, and until recently, the races attracted little attention.

The almost certain low voter turnout has many Republicans worried more than usual because they fear the primary could be dominated by the so-called one-issue voters who have dominated such contests in the past.

Alarm about the "single-shooters," as one GOP candidate characterized them, is particularly acute these days following recent struggles over party leadership.

Panuzio, for example, was dumped from his county GOP post in a move that he and his supporters say was orchestrated by the ultraconservative followers of Olsen. Both men are Reagan enthusiasts, but Panuzio says he just wasn't "conservative enough" for the newcomers who have begun to take an active interest in a revitalized Republican Party.

"I was concerned about a growing influence of the new right in our party," Panuzio said yesterday. While stressing that longtime party activists seem to have retained control of party policy-making, he complained that issues such as abortion and the Equal Rights Amendment have intruded to become the basis for picking party leaders.

Conservative opponents of the ERA and abortion rights were said to have packed the Fairfax County GOP convention a few weeks ago to help elect Olsen as party chairman. The convention also passed a conservative-backed resolution calling on the county committee to withdraw from participating in the Fairfax County Fair Campaign Practices Commission.

"I want to win elections, and I will support Bill Olsen anywhere I can," Panuzio said. "But I will fight every time there's a move by one small group to dominate the party."

Panuzio said the party friction has helped bring out more party activists to counteract a one-issue approach to elections. He thinks newly elected party officers have gotten the GOP "switched back" to a more reasoned stance.

Olsen downplays the extent of the party sturggle, saying his election was supported by those who wanted to increase the party organization's help to candidates. His only concern about the county's fair campaign panel, he added, was that the decision to participate to be made by the county committee -- which is now studying the issue.

"There are always high emotions necessary to win contests, but I certainly hope we'll have the party unity we'll need to win elections this fall," Olsen said.

State Del. Vincent F. Callahan Jr., who supported Panuzio for party chairman says Olsen will have his and other party workers' support. Mindful of Gerald Ford's Northern Virgina victory four years ago while Callahan lost a congressional race against Fisher, the veteran delegate warned that presidential victories do not assure a sweep for the ticket.

"You can get a lot more voters in a presidential year, but that doesn't necessarily mean that a Reagan victory will help defeat incumbent Democrats in Congress," he cautioned.

Democrats, who have had their own share of intraparty battles in the past, are not taking any Northern Virginia elections this November for granted, either.

"I don't think anybody figures it will be an easy race with the national and international situation being what it is," said Dottie Schick, who chairs the Fairfax County Democratic Party. She said she believes Carter has inherited most of these problems and has not had adequate time to deal with them.

"We've not been scared off by any means," she said. "We have two good Democratic congressmen, and we intend to keep them."