The Federal Election Commission and one of the largest conservative political action committees are racing toward the courthouse door to re-test a disputed cap on PAC spending for presidential candidates.

The only thing is, they're charging toward different courts.

The venue-shopping is the latest bit of tactical cunning in a 3-year-old legal chess game that has produced far more gambits than results.

At issue is the freedom of groups such as the Fund for a Conservative Majority and the National Conservative Political Action Committee to spend unlimited amounts on their own for presidential candidates who are receiving federal financing for their general election campaigns. In order to get federal money, a candidate must agree not to accept private contributions.

The two groups have announced plans to spend $10 million to help reelect President Reagan next year.

The complication is that Section 9012(f) of the Presidential Election Campaign Fund Act, which went into effect in 1976, bars PACs from spending more than $1,000 to further the election of a presidential candidate who accepts public financing.

The further complication is that that provision either has been invalidated by the courts, or it hasn't--depending on which lawyer you talk to.

The legal history is this: In the summer of 1980, five large conservative PACs said they were going to spend more than $50 million in Reagan's behalf. Common Cause, the self-styled citizens' lobby, and the FEC quickly filed suit against the committees in the U.S. District Court here.

On Sept. 30, 1980, a special panel of two district court judges and an appeals court judge ruled that Section 9012(f) was unconstitutional.

Relying heavily on the Supreme Court's reasoning in the landmark Buckley v. Valeo case of 1976, the panel said: "Whereas a presidential candidate by accepting public funds may choose for him or herself to do without unlimited contributions and expenditures, the candidate's public supporters have a separate, protected right to express themselves, individually or jointly." The five PACs went ahead and spent $7.7 million in the general election.

The FEC appealed to the Supreme Court. In 1982, on a 4-to-4 vote with Justice Sandra Day O'Connor disqualifying herself, it upheld the lower court.

This spring, the Fund for a Conservative Majority and NCPAC asked the FEC if it intended to enforce 9012(f) in 1984. The commission, concluding that the 4-to-4 vote did not set a precedent, said it would. The conservative groups, "astonished by the bureaucratic tyranny of the FEC," said they were going to spend their money anyway.

Now comes the venue-shopping. It started last month when the Democratic National Committee, a not disinterested bystander, filed suit in U.S. District Court for the Eastern District of Pennsylvania, seeking to uphold the FEC's right to enforce the law.

The DNC makes no bones about why it went into that court--it wanted to try a circuit where the judges might be friendlier.

This week the FEC decided that as long as the DNC had blazed a trail up there, it might as well follow. So it formally asked to join the suit.

That prompted NCPAC to call for a congressional inquiry into "improper collusion" between the FEC and the Democrats. John T. (Terry) Dolan, the head of NCPAC, said the DNC had served as "an advance team for the FEC in its search for a favorable bench."

Then yesterday it was the conservative fund's turn to strike back. It returned to U.S. District Court here and asked for a judgment and for the Pennsylvania case to be declared moot.

Since virtually everyone assumes the matter will wind up before the Supreme Court again, why all the fuss over the lower court?

Remember that 4-to-4 vote. If it were repeated, but on a lower-court ruling to uphold 9012(f), the decision would have the opposite effect this time.

But there is one last complication--O'Connor. The assumption has been that she recused herself from the last vote because her husband had contributed to a conservative PAC, Americans for Change, in the 1980 campaign. The further assumption is that she would vote this time and that her track record suggests sympathy for the proponents of unlimited independent PAC spending. PEOPLE . . . President Reagan announced this week, as expected, that he is reappointing Republican Joan D. Aikens and Democrat John W. McGarry to new six-year terms on the six-member commission.