Christian fundamentalists and activists on the Republican right in Fairfax County are flexing their muscles, but just how much political iron they can pump is a matter of debate.

Last week, a loose-knit coalition of pastors and right-wing Republicans known as the First Thursday Breakfast Group met for its monthly session at a Tysons Corner hotel. Over a hearty breakfast of bacon and eggs, the group, according to some who attended the meeting, wasted little time choosing two issues it sees as an attack on traditional family values and Christianity itself.

First, it cited the unsuccessful bid by Robert L. Thoburn to locate his Fairfax Christian School in a residential community just north of Fairfax City. The county Board of Supervisors rejected Thoburn's application July 8 on a 4-to-4 vote.

Supervisors who opposed the school argued it did not conform to the county master plan for that neighborhood. Thoburn and his allies in the breakfast club denounced the vote as anti-Christian.

Second, the group fired a broadside at the county-run day-care program, which does not extend to children at private and parochial schools, nor to children of parents who do not work full time. The program, according to several group members, has its roots in socialist Sweden and discourages mothers from working part-time or staying at home.

Group members also blasted a raft of state and local regulations on privately run day-care centers, which they said are designed to squeeze the facilities, including those run by churches and families, out of business. The county, they said, wants to establish a public monopoly on day care.

"It is a direct attack on traditional family life under our free enterprise system," said Neil F. Markva, a regular at the monthly breakfasts and a lawyer with the Rutherford Institute, a conservative lobbying group in Manassas.

Along with the tough talk was an implicit threat: that these conservatives will remember who has opposed them when the county supervisors are next up for election in 1987.

One supervisor who has drawn scowls from a number of supervisors is Nancy K. Falck, a Republican who represents the Dranesville District in the north of Fairfax County.

Falck was the only Republican to side with three Democrats in voting against Thoburn's school.

Thoburn, a one-term Republican state legislator who can issue a political tongue-lashing with the best of them, branded Falck a "liberal Republican" and "anti-Christian."

Other conservatives such as Larry Pratt, another former legislator and executive director of the Gun Owners of America, called Falck an "opponent of religious liberty" and spoke darkly of the "consequences" at election time.

Falck quipped that her minister would be surprised to learn that she is anti-Christian. As for suggestions that she is a liberal Republican, she replied: "You can call me a man and I still wouldn't be one."

Some county politicians tend to listen to the criticism of the far right with an almost deaf but sometimes uneasy ear.

The conventional wisdom among Republican supervisors is that county voters are more moderate than the vocal spokesmen and women of the right or of the county GOP leadership. The county Republican chairman, retired Brig. Gen. Benton K. Partin, is singled out for particular derision among some GOP board members, who consider him an ideologue with no sense of political pragmatism.

However, local politicians are mindful that the religious right in Fairfax is part of a larger national movement with wide appeal.

The three county supervisors who received invitations to the breakfast meeting last week attended. One was Elaine McConnell, the Republican supervisor from Springfield who is most closely aligned with the right. Another was board chairman John F. Herrity, a Republican who says he's always glad to appear before any group in the county.

The third was Audrey Moore, a liberal Democrat. According to breakfast group members, she was invited by mistake.