Republicans won an important victory Monday in their legal challenge of the election of Democratic Gov. Christine Gregoire (search) when a judge allowed them to use the "proportional analysis" method in their attempt to prove that illegal votes swayed the election.

However, Chelan County Superior Court Judge John Bridges cautioned Republicans that just because he's allowing them to use the argument does not mean it necessarily will work in the trial that is to begin May 23.

Republican Dino Rossi (search) is challenging Gregoire's November election. Rossi, a former state senator and real estate investor, won the first count and machine recount. But Gregoire, formerly the state's attorney general, won by 129 votes in a hand recount of 2.9 million ballots.

The state Republican Party intervened on Rossi's side and the state Democratic Party intervened on the state's side, defending the election. Republicans are focusing on King County, the state's most populous, where election officials have acknowledged mistakes that allowed some people to vote illegally while not counting other, legal votes.

State law includes provisions for challenging and nullifying an election, but they've never been used to try to kick a governor out of office. In allowing the use of proportional analysis (search), Bridges gave the GOP a key decision in making their case.

Republicans claim they've identified more than 1,000 illegal votes -- mostly felons, but also unverified provisional ballots and a few dead voters. Using the proportional analysis method, they want the court to subtract illegal votes from both candidates' totals according to precinct voting patterns. For example, if 10 illegal votes came from a precinct that voted 60 percent for Gregoire and 40 percent for Rossi, the Republicans would deduct six votes from Gregoire and four from Rossi. Most of the illegal votes Republicans have identified come from King County, which went 58 percent for Gregoire.

Democrats say the method amounts to statistical guessing. At the same time, they've been collecting evidence of illegal votes in Rossi-leaning counties, and plan to use the same"proportional analysis as evidence in court of illegal Rossi votes.

In arguments before Bridges at Monday's pretrial hearing, David Burman, an attorney for the Democrats, compared proportional analysis to flipping a coin.

To overturn an election, "They have to be certain," he said. "Mathematical chances are not good enough."

But GOP attorney Mark Braden said that without proportional analysis both sides would have to bring thousands of witnesses into the courtroom, to ask how they voted on a secret ballot. That, he said, "is a nonsensical interpretation of Washington law."

Both sides have acknowledged that whatever happens in Chelan County, the case will ultimately be decided by the state Supreme Court. Bridges' rulings, however, lay critical ground rules for the case.

Bridges granted a motion sought by Democrats to exclude two non-citizen votes from the list of alleged illegal votes, on the grounds those votes were challenged too late. Both the Republicans and the secretary of state's office agreed with the motion.

In a victory for Democrats, Bridges said county voter crediting files cannot be used to show who voted in the 2004 election. Democrats had argued that such files on who voted are inherently unreliable and that the GOP should be forced to find other evidence of voting, such as signatures in poll books or absentee ballot envelopes.

Other motions before Bridges concern:

— Felons: Republicans have submitted a list of felons they say voted illegally, most from the Democratic stronghold of King County. Democrats say the list has many mistakes: people whose voting rights were restored, or were juveniles when they were convicted and never had their voting rights revoked.

Bridges will have to decide what to do with the list: accept it as-is, or require more proof that the listed names belong to actual felons who actually voted illegally.

— Offsetting: The Democrats are trying to beat Republicans at their own game here, arguing that illegal Rossi votes and valid Gregoire votes that mistakenly went uncounted cancel out illegal votes for Gregoire.

Republicans want the judge to cut off that avenue. Democrats accuse Republicans of trying to "cherry-pick" errors that help their case while ignoring others.