Updated

Marriage between a man and woman is the "most enduring and important human institution" and one that should be protected, President Bush said Monday in a speech offering his support to a constitutional amendment that would ban gay marriage nationwide.

"For ages, in every culture, human beings have understood that marriage is critical to the well-being of families. And because families pass along values and shape character, marriage is also critical to the health of society," Bush said at the Eisenhower Executive Office Building, where religious and community leaders, legal scholars and other supporters listened to the president speak.

"Our policy should aim to strengthen families, not undermine them. And changing the definition of marriage would undermine the family structure," he said.

Debate began Monday in the Senate and a test vote is scheduled for Wednesday on a constitutional amendment defining marriage. The measure is unlikely to get the 67 Senate votes it would take to send the amendment to the House and then to the states for ratification, but backers say they want lawmakers on the record about the issue before the midterm election.

Many Republicans support the measure, they say, because traditional marriage strengthens society. Democratic opponents call the vote an intrusive ploy that demonstrates an empty Republican agenda.

"Marriage between one man and one woman does a better job protecting children — better than any other institution humankind has devised," said Senate Majority Leader Bill Frist, R-Tenn. "As such, marriage as an institution should be protected, not redefined."

"I think this just highlights the fact they have no intention — they have no plan to deal with health care. They have no plan to deal with our national security. They have no plan to deal with the energy crisis," Sen. Joe Biden, D-Del., said.

SPEAKOUT: What are your thoughts on the Marriage Protection Amendment?

A motion to allow final passage of a proposed amendment banning gay marriage got only 48 votes in the Senate in 2004; the oddsmakers are putting the vote tally at 52 this year, though polls indicate opposition to gay marriage has dropped in the last two years.

Earlier in the day, Sen. Wayne Allard, R-Colo., chief sponsor of the Marriage Protection Amendment, said if Congress doesn't act, American voters will lose their voice.

"The Constitution will be amended whether we pass this amendment or not," Allard warned. "The only question is whether it will be amended through the amendment process or by unelected activist judges."

The president said he wishes an amendment weren't needed but activist judges have overturned four state bans recently. On top of that, 19 states have approved referenda to amend state constitutions with marriage amendments by average margins of 71 percent. Forty-five of 50 states have either a state constitutional amendment or a statute defining marriage as the union of a man and a woman, Bush said.

"The people have spoken. Unfortunately, this consensus is being undermined by activist judges and local officials who have struck down state laws protecting marriage and made an aggressive attempt to redefine marriage," Bush said. "This week the Senate begins debate on the Marriage Protection Amendment, and I call on the Congress to pass the amendment."

White House press secretary Tony Snow said the president's motivation is not political, and that he's only discussing the topic now because it is on the Senate calendar. Administration officials wouldn't say whether Bush believes the issue should be decided by individual states or whether marriage should be defined all over the land as a union between a man and a woman.

But in his remarks, Bush said: "This national question requires a national solution."

Despite the president's push, parliamentary maneuvers are likely to sink the amendment. Senate procedure requires two days of debate before the 100-member Senate decides with a 60-vote margin whether to consider the amendment on an up-or-down vote.

If the amendment gets two-thirds of the House and Senate vote, it would need to be ratified by three-quarters of the states to be included in the Constitution. The House had the measure scheduled for a vote in July but if it fails in the Senate, it will be scratched since constitutional amendments must originate in the Senate chamber.

Conservative groups have expressed support of the amendment despite their acknowledgement that it will likely fail. Several groups have said they are dissatisfied with the Republican efforts on several social issues, including immigration, and want lawmakers to show their commitment to a federal marriage amendment.

Democrats have not been shy to note the political advantage Republicans are trying to gain by bringing up the vote ahead of the midterm election. In 2004, Bush won every one of the 11 states that put the gay marriage ban on the ballot. But Democrats too are appealing to their base in voting against the measure.

"There's an ugly truth: it's election season and down-in-the-polls Republicans are turning to their same old playbook — fear and division," Senate Minority Leader Harry Reid wrote in a letter for the Democratic Senatorial Campaign Committee.

Elsewhere, however, a group of black Democratic pastors from around the country are lobbying senators to support the amendment.

"We cannot sit idly by and let Democrat members we helped get elected in the past ignore the fact that the institution of marriage is suffering." said Bishop Harry Jackson, chairman of the High Impact Leadership Coalition. "The faith community will not play it safe anymore. We must act. Black Christian leaders have a voice and will make that voice heard on Election Day."

But Log Cabin Republicans, made up of gay conservatives, said a constitutional amendment banning gay marriage is "offensive" and the president's public support is "unworthy of the office of the presidency."

"Your decision to use the grounds of the White House — America's House — to advance discrimination is an insult to millions of fair minded Americans from all walks of life," said Patrick Guerriero, president of the Log Cabin Republicans.

Sen. Ben Nelson, D-Neb., has said he will vote for the measure as he did in 2004. Democratic West Virginia Sen. Robert Byrd also supported a final vote on the 2004 amendment.

About a half dozen Republican lawmakers have said they will vote against the measure, including Sen. John McCain, R-Ariz., who opposed ending the debate and going to a final vote in 2004. Other Republicans who opposed the measure in 2004 are Sens. Lincoln Chafee of Rhode Island, Susan Collins and Olympia Snowe of Maine and Sen. John Sununu of New Hampshire.

Judiciary Committee Chairman Arlen Specter, R-Pa., said he will vote against it on the floor but allowed it to survive his panel in part to give the GOP the debate party leaders believe will pay off on Election Day. Specter has chosen a different battle with the Bush administration this week — a hearing Tuesday on the ways the FBI spies on journalists who publish classified information.

Fueled by election-year politics, the gay marriage issue is the most volatile Congress will consider as it returns from a weeklong Memorial Day recess.

Other legislation has better chances for success, particularly a record-size emergency spending bill to continue U.S. military operations in Iraq and Afghanistan and provide hurricane relief along the Gulf Coast.

The Pentagon says it needs its money — about $66 billion — right away or delays could begin to affect the conduct of the war in Iraq. The Senate added new relief for farmers and other aid to the package, swelling its cost to more than $100 billion. Bush is demanding that the price tag stick within his $92.2 billion request, plus $2.3 billion to combat avian flu.

An agreement could be passed this week.

The House is expected to consider a $32 billion spending bill that would give the Homeland Security Department $1.8 billion more in 2007 than this year. It also is likely to send Bush a Senate-approved bill to raise indecency fines tenfold, to $325,000 per violation, for television and radio broadcasters.

SPEAKOUT: What are your thoughts on the Marriage Protection Amendment?

FOX News' Wendell Goler and Trish Turner and The Associated Press contributed to this report.