The U.S. Supreme Court will hear two cases this week related to gay marriage. And while most legal observers think it's unlikely that the court will make a broad constitutional ruling legalizing same-sex marriage across the country, even a narrow ruling could give the movement toward marriage equality a new boost.

In fact, the march toward legalizing same sex marriage in the United States sometimes seems unstoppable.

A new Washington Post poll shows that 58 percent of Americans now support gay marriage. Among those 18 to 29 years old, the number is a whopping 81 percent. 

This is a stunning reversal from the 2004 presidential election, when gay marriage was used as a wedge issue by Republicans to re-elect George W. Bush.  Karl Rove, Bush's campaign manager and the architect of using gay marriage as a wedge issue, now says it's conceivable the GOP nominee in 2016 could support it.

A new Pew Research survey finds that the rise in support for same-sex marriage over the past decade "is among the largest changes in opinion on any policy issue over this time period."

But supporters should take nothing for granted. One need only look to the Equal Rights Amendment for an example of a social movement that once seemed unstoppable but stalled on its way to becoming law.

Matthew Cooper points out that as with same-sex marriage, "the most important battleground for the ERA took place in state legislatures. The ERA seemed to have unstoppable momentum when it came out of Congress with the necessary two-thirds majority in both the House and the Senate."

But even though 30 states had passed the ERA by the end of 1973, it could never get to the 38 states necessary to amend the Constitution. Efforts to resuscitate the ERA since then have all failed.

Supporters of same-sex marriage must take heed of Yogi Berra's famous saying, "It ain't over, until it's over."