RICHMOND, Va. — A federal appeals court yesterday revived a Republican Party legal maneuver to block Democrats from voting in their primaries in Virginia, despite laws mandating open primaries.

By reviving the case, Miller v. Brown, the 4th U.S. Circuit Court of Appeals decision could push the state a step closer to curbing “crossover voting” by members of one party in the primary of the other, potentially influencing whom their adversaries nominate.

“If you’re in the Lion’s Club and you show up at the Rotary Club and want to vote on Rotary Club officers, people would look at you as if you’re crazy, and Rotary and Lions aren’t even opponents,” said state Sen. Kenneth Cuccinelli, an attorney who argued the case for the Republican Party.

“But if you’re a Democrat, you can walk right in and help decide who the Republicans nominate,” said Cuccinelli, R-Fairfax County.

Republican conservatives have attacked the open-primary law vigorously since 1996. Angry that Sen. John W. Warner had supported an independent and spurned Iran-Contra figure Oliver L. North, his party’s nominee, in a U.S. Senate race two years before, the GOP right challenged his nomination. Warner easily won the primary, and some party leaders insisted large numbers of Democrats crossed over to help the moderate Warner.

Crossover voting, however, is more myth than reality, said University of Virginia political science professor Larry Sabato. Several studies show that the highest documented level of opposite-party primary voting is about 2%, he said.

“It’s usually far less than 2 (percent),” he said. “It exists more in the minds of the party faithful than anywhere else.”

Virginia does not require voters to register by party and allows any registered voter to participate in primaries of either party. Voters may not vote in both parties’ primaries the same day, but without party registration, enforcing closed-primary rules is nearly impossible.

Despite legislative majorities since 2000, Republicans have tried unsuccessfully for years to change the state’s open-primary law.

Cuccinelli and the GOP argued before U.S. District Judge Henry Hudson that denying the GOP the right to exclude Democrats from its primaries violates the party’s constitutional right of freedom of association.

Hudson dismissed the GOP’s claim on grounds that the plaintiffs lacked standing and that the case was not ripe because state Senate candidates can’t file to run until 2007, when all 40 Senate seats and 100 House seats are up for election.

The three-judge panel, in a unanimous and strongly worded ruling, rejected both conclusions and sent the case back to Hudson “for decision on the merits.”

“The only issue in the case is whether Virginia’s open primary law violates the plaintiffs’ First Amendment rights to freely associate,” the appeals panel wrote.

Requiring the parties to wait until the eve of the election to have the case heard risked disrupting the electoral process, the judges wrote.

“The open primary law causes immediate harm to their (GOP’s) constitutionally protected rights because they know Democrats will be participating in their primary,” according to the decision. And the GOP’s injuries “become worse each day the decision is delayed.”

State Board of Elections Secretary Jean R. Jensen, a defendant, said yesterday she had not seen the decision and could not comment.

The case arose from Republican state Sen. Steve Martin’s decision to seek a primary should he face a nomination fight next year. Virginia law gives incumbents the choice of a primary or party convention.

In 2004, the Republican Party of Virginia had amended its bylaws to exclude anyone who had participated in a Democratic primary the preceding five years from voting in a GOP primary. But it exempts those who pledge in writing to renounce affiliation with another party, to heed the Republican Party’s principles and to support its nominees.

In January 2005, the GOP notified the elections board that it intended to apply the new rules in a primary in Martin’s suburban Richmond district next June if he has a Republican challenger.

Jensen wrote the party back the next month, citing state laws that make no provision for excluding voters, regardless of party. The Republicans then asked Hudson for a summary judgment.