The Senate wades into an election-year debate Friday over whether to write into the Constitution that "marriage in the United States shall consist only of the union of a man and a woman."

Its strongest proponents say a constitutional amendment is the only way to prevent federal courts from hearing cases that challenge a federal law disallowing same-sex unions. With such an amendment, they say, a court wouldn't be able to rule that gay marriage is legal.

"Some would define this as the ultimate culture battle," said Sen. Sam Brownback (search), R-Kansas.

Many Democrats say the debate is a political diversion orchestrated for the weeks running up to the presidential nominating conventions.

"It's all about politics, folks. Let's face it," said Sen. Barbara Boxer (search), D-Calif. "We're going to go on to gay marriage before the Democratic convention so some people can cast a vote that might hurt them in their election. Shame on us."

Senate Majority Leader Bill Frist (search), R-Tenn., urged senators to begin informal debate on the legislation Friday, and said debate would continue Monday and Tuesday with a goal of voting Wednesday.

Senators fighting for the constitutional amendment would have to secure a two-thirds vote -- 67 of the Senate's 100 members -- to approve it. Some supporters questioned Thursday whether they had even the 60 votes needed to overcome procedural hurdles.

"We're going to have to see how that vote comes out," said Sen. Wayne Allard, the Colorado Republican who drafted the proposed amendment.

The Senate's GOP leaders brought a prominent black conservative, Ohio Secretary of State Ken Blackwell, to discuss constitutional and cultural aspects of the issue with Republican senators.

Blackwell said he also delivered a political message: "You're not at risk of political defeat if you hold your ground."

Several Republican senators have said they're wary of amending the Constitution, the nation's two-centuries-old founding document, before exhausting all other avenues.

And some senators, Republican and Democratic, simply oppose the idea.

"Nuts," said Sen. Lincoln Chafee, R-R.I. "To be seen as the party that's coming between two people that love each other doing what they want to do ... to me that's going to be seen as a liability, politically."