During his re-election campaign, President George W. Bush vowed to push a constitutional amendment defining marriage as the union of a man and a woman, and when the issue came up in Congress in June 2006, he threw it his support.

However, the effort to pass a marriage amendment, which would have effectively banned same-sex marriage, lacked the two-thirds majority necessary for passage and died in the Senate.

While there have been over 10,000 attempts throughout American history to amend the Constitution, only 27 of these attempts have been successful.

Twelve amendments were added during the 20th century, and none in the 21st. The most recent amendment was the 27th Amendment of 1992, which prohibits midterm pay raises for Congress.

Notable additions in the 20th century include the 19th Amendment of 1920, which extended voting rights to women, and the 24th Amendment of 1964, which abolished poll taxes aimed at denying blacks the right to vote.

In 1919, the 18th Amendment prohibited the sale or manufacture of alcohol. The policy was out of step with the mainstream public's wishes, and in response a booming black market was established that became the backbone of organized crime. In 1933, the 21st Amendment was ratified, repealing the 18th Amendment.

Hard-fought recent failures include the Equal Rights Amendment, which would have prohibited all forms of sex discrimination, and the Flag Protection Amendment, which would have protected the U.S. flag from abuse or desecration.

Amendments are so few and far between because the process to change the Constitution is a long and difficult one. The first step can take one of two routes. The House and Senate can pass an amendment with a two-thirds majority vote, called a super-majority. Or, two-thirds of the state legislatures can call a constitutional convention to move the amendment.

The proposed amendment is then submitted to states for consideration, usually with some kind of time limit (seven years is common). Three-fourths (38) of the 50 states must ratify the amendment for it to become part of the Constitution. This is usually done on a state-by-state basis, although the 21st Amendment repealing Prohibition was ratified through a state convention. While Congress has approved 33 proposed amendments, the requisite number of state legislatures has only ratified the current 27.

It is likely that there will be a drawn-out political and legal struggle over whether to ban gay marriage, making it improbable that the Constitution would be amended before the 2004 presidential election.

-- Compiled by Chris Nammour for the Online NewsHour