SAN FRANCISCO — The lawyers in the landmark federal trial over the constitutionality of California's gay marriage ban may have to check their dazzling oratory at the courtroom door during this week's closing arguments.

The presiding judge wants them to answer 39 questions before he delivers his verdict.

Chief U.S. Judge Vaughn Walker has already heard 12 days of testimony in the civil rights case, which is expected to eventually reach the U.S. Supreme Court.

Walker has absorbed the opposing attorneys' written summaries and amassed a pile of competing briefs from outside interest groups on the limits of federalism and individual freedom.

Now, after a four-and-a-half-month hiatus, he is scheduled on Wednesday to wrap up the trial. Last week, he distilled his thinking about the case to a list of questions— 12 each to lawyers representing gay rights advocates and the ban's sponsors, and another 15 he wants both sides to address before he later rules.

From former U.S. Solicitor General Theodore Olson, who will be arguing on behalf of two same-sex couples, Walker wants to know what empirical proof there is that allowing gay men and lesbians to marry would reduce discrimination against them.

The judge expects to hear, too, whether he can find that withholding marriage from gays constitutes unlawful discrimination if voters "genuinely but without evidence" believed there were legitimate reasons to limit marriage to a man and a woman.

His questions for Charles Cooper, a former Justice Department lawyer representing the ban's sponsors, are equally precise.

What evidence have they produced to support their claim that same-sex marriage would have negative consequences to the institution of marriage? Conversely, how does denying marriage to gays and lesbians improve the odds that children born in California will be raised by a married mom and dad?

Sharp inquiry "is very much typical of this judge," observed Olson, who frequently was interrupted by Walker during his opening statement in January. "He has done his homework, he is very much involved in this case."

"It just shows the judge is really grappling with the issues," agreed James Campbell, another lawyer for the coalition of religious and conservative groups that put Proposition 8 on the November 2008 ballot.

Both lawyers said they found the judge's punch list to be evenhanded and do not get a sense from it how he is leaning.

The veteran jurist, a Republican appointee, has indicated he does not plan to rule from the bench. Lawyers say they are preparing for him to hand down his ruling within weeks after the closing arguments.

Walker is being asked to overturn the 2008 ballot measure that outlawed same-sex marriages in California five months after the state Supreme Court legalized it and after an estimated 18,000 couples from around the nation had tied the knot.

The plaintiffs also are seeking an injunction that would prohibit the state from enforcing the measure and immediately allow gay marriage to resume in the state. They maintain that preventing gays from getting married violates their civil rights and should be declared unconstitutional in the same way that laws against interracial marriage were.

The case has garnered national attention in part because of the unusual team that brought the lawsuit on behalf of the two California couples. Olson, a conservative who represented George Bush in the disputed 2000 presidential election, has joined with David Boies, the liberal trial lawyer who represented Vice President Al Gore.

In an unusual move, the original defendants, Attorney General Jerry Brown and Gov. Arnold Schwarzenegger, refused to support Proposition 8 in court.

That has left the work of defending the law to Protect Marriage, the group that successfully sponsored it. The group's lawyers argue gay marriage is an experiment with unknown social consequences that should be left to voters to accept or reject.

Currently, same-sex couples can only legally wed in Massachusetts, Iowa, Connecticut, Vermont, New Hampshire and Washington, D.C.

To the surprise of both sides, Walker elected to hold a trial with live testimony instead of settling the matter through written briefs. Academic experts took the witness stand to discuss topics ranging from the fitness of gay parents and religious views on homosexuality to the historical meaning of marriage and the political influence of the gay rights movement.

Olson and Boies called 17 witnesses compared to two called by Cooper's defense team. The lopsided count has led gay rights supporters to predict the judge will have an easy time striking down the gay marriage ban.

Defense attorneys, meanwhile, have openly complained that they thought Walker's rulings before and during the trial favored the plaintiffs.

"The problem with the defendant's case was not the defendant's lawyers, it was the fact that there is simply no support for the central propositions they were trying to identify," Boies said.

Andy Pugno, a lawyer who served on the Protect Marriage executive committee, acknowledged that the ban's backers faced an uphill battle in addressing one of the key questions that preoccupied Walker — whether there was any proof that sanctioning same-sex marriage harms traditional heterosexual unions.

"The difficulty with that question is it's asking the defense to speculate about a harm that is likely but would occur only if we were to experiment with redefining marriage," Pugno said. "It's hard to disprove something that hasn't happened yet."