The shocker at the Kentucky Derby this year was that a little known horse with 50 to 1 odds came in first. Lawsuits, too, have odds, though not the type that translates into numbers.

They can have a greater chance of success or failure due to which court they are filed in; which judge is randomly assigned to the case; the scope of the issue it challenges; what arguments the lawyers use to make their cases; and, of course, which lawyers have filed the lawsuit and which are lined up to oppose it.

Thus the five lawsuits filed in federal courts seeking marriage rights for same-sex couples face varying odds of success. Ultimately, it will come down to whether the lawyers pushing the cases can convince five members of the Supreme Court to side with them.

LGBT legal activists, who have been very picky about what lawsuits they enter into which courts, have long considered going the federal legal route risky. With a couple of notable exceptions, they have limited their legal action in the past 25 years to state courts, specifically, in states where the overall climate for LGBT people is considered friendly, like Massachusetts, California and Connecticut.

Their thinking, generally, has been that conditions at the U.S. Supreme Court aren't favorable for winning any pro-gay decisions. But gay legal organizations do not control the field when it comes to gay-related lawsuits. Sometimes, other lawyers mount lawsuits; and sometimes - like during last week's confirmation hearing for U.S. Supreme Court nominee Sonia Sotomayor - other interested parties try to call the race over, done, and lost.

It was only after much study and deliberation that Boston-based Gay & Lesbian Advocates & Defenders finally mounted a legal challenge this year of the 13-year-old Defense of Marriage Act (DOMA) in federal court. The lawsuit is a very narrow, "very mainstream" challenge, noted lead attorney Mary Bonauto. It tackles only part of one section of DOMA that applies to federal tax laws, Social Security and retirement laws, and other federal benefits available to married couples.

Why not challenge the entire law?

"The U.S. Supreme Court has made clear that it strongly disfavors attempts to strike a federal law in its entirety," notes GLAD on its Web site, "and prefers to evaluate cases with concrete examples of how a federal law as applied violates constitutional rights."

So GLAD found eight couples and three individuals - all of whom have applied for equal treatment under some federal benefit and been denied - and, in March, put its lawsuit on track for the U.S. Supreme Court. They chose the U.S. District Court in Boston, part of the First Circuit U.S. Court of Appeals, one of the more progressive circuits in the country.

Most LGBT legal activists who have been working on equal rights for gays believe GLAD's case, Gill v. Office of Personnel Management , which also sports a legal team of 11 attorneys, has the best chance for success.

Jenny Pizer, head of the Marriage Project for Lambda Legal Defense and Education Fund, calls the Gill lawsuit "very strong."

"The Gill case was put together meticulously," explains Pizer. She says it hones in on "specific, serious, arbitrary harms to the plaintiffs," whose claims are "ripe for court consideration because all administrative and other preparatory steps have been taken."

One thing a lawyer or team of lawyers can't control, however, is what judge will be assigned their case. By luck of the draw, the judge assigned to the Gill case is a 78-year-old Nixon appointee - which, on face value, seems tough. And it may be.

But Judge Joseph Tauro is also known as tough in another way.

"He is an activist judge, one who does not think that the law has only a passive role to play," said one defense attorney to the Boston Globe in describing Tauro in 1989. "He is not afraid to fashion a remedy which breathes life into a constitutional right."

According to the Globe, Tauro is a second-generation Italian who has said he considers himself to be "very sensitive to bigotry" and "very empathetic with those who insist that they be treated equally, in terms of ethnic or racial background."

Tauro has also been assigned the "related case" of Massachusetts v. U.S. - the latest entry into the field of federal lawsuits seeking marriage equality for same-sex couples. Massachusetts challenges the same section of DOMA as GLAD's lawsuit, and GLAD says there's a good chance these two cases will be consolidated, though there are differences.

GLAD's Bonauto said Gill "is grounded in equal protection," while the Massachusetts case focuses on a 10^th Amendment and Spending Clause claim. The 10^th Amendment says states have the power to regulate things "not delegated to the United States by the Constitution." The Spending Clause is more complicated and, argues Attorney General Martha Coakley, prevents the federal government from forcing the state to violate the constitutional rights of its citizens.

Lambda's Pizer says the Massachusetts case is both an "incredibly important case and an immensely welcome development on the national landscape."

"The case presents new legal arguments" against DOMA's provision - Section 3 - limiting the interpretation of "marriage" for any federal purpose to heterosexual couples.

As Evan Wolfson, head of the national Freedom to Marry group and one of the first attorneys to litigate a gay marriage case, puts it, Gill and Massachusetts aren't looking for a "one-shot, all-or-nothing" ruling about marriage equality nationwide. They seek to nudge the courts in the right direction - getting the courts to first say that states should be allowed to regulate marriage and treat all married couples equally, for instance, or that same-sex couples should be able to receive equal benefits. Such incremental rulings, he said, "would still hand us an enormous victory that would have positive effects nationwide."

The hobbled hazards

If ever there were a "one-shot, all-or-nothing" entry in this legal horserace, it would most certainly be Smelt v. U.S . It's also a first, of sorts.

Arthur Smelt and his spouse, Christopher Hammer, have already been to the U.S. Supreme Court. They filed a federal lawsuit in California's conservative Orange County in 2004 after a county clerk denied them a marriage license. It did not have the support of gay legal groups and activists then, and it doesn't now.

The couple's private attorney, Richard Gilbert, appealed the 2004 Smelt case all the way to the U.S. Supreme Court, which, in 2006, refused to hear it.

In 2008, the couple married during California's five-month era of marriage equality and, when voters passed Proposition 8 banning same-sex marriage last fall, Smelt and Hammer filed suit again, this time in state court, challenging both the statewide ban and both sections of DOMA. Judge David Carter, of the U.S. District Court in Los Angeles, dismissed that part of the lawsuit against the state July 16, noting that the couple is still married and thus, as a legal matter, have no standing to sue the state over its ban.

Judge Carter is a Clinton appointee and has had at least one gay case before. When Orange County schools tried to bar the organization of a gay-straight alliance in 2000, Carter issued a preliminary injunction that enabled the group to hold its meetings. (The school later settled with the group out of court.)

The remainder of the lawsuit, however, will proceed and, unlike Gill and Massachusetts , Smelt is anything but narrow. It asks the court to require "all necessary acts" be taken by "the entire nation of the United States of America, all of its territories and jurisdictions" to eliminate "any distinction in the law" that results in inequality for the plaintiff couple.

The unusually brief, eight-page complaint filed by attorney Gilbert asks the court to declare DOMA as violating the constitutional guarantees of equal protection, right to privacy, due process, as well as its full faith and credit clauses. It also asks the court to "order mandating the use of gender-neutral terms in all legislation affecting marriage." And, without explanation or context, it drops in a quote from former Vice President Dick Cheney saying, "Freedom means freedom for everyone."

A similarly hobbled lawsuit was filed in the federal district court of New Orleans by another gay male couple. Kristoffer Bonilla and John Wray, without the aid or blessing of any gay litigation group, tried to file their lawsuit in April, seeking permission to have the filing fees waived due to "pauper" status. Bonilla is a recent law school graduate. The court denied their request, so they came back a month later and filed their seven-page lawsuit.

The case, Bonilla v. Levine, has been assigned to a judge appointed by former President George W. Bush - Kurt Engelhardt. Not much is known about the judge's attitude towards gay civil rights, but the 5^th Circuit U.S. Court of Appeals is one of the country's more conservative circuits.

Next week: A look at the debate sparked by the fifth lawsuit, Perry v. Schwarzenegger, filed on behalf of two California same-sex couples.