Get ready for what could be one of the all-time great backfires in American political life, a colossal demonstration of the law of unintended consequences.

If the U.S. Supreme Court strikes down California's ban on same-sex marriage, known as Proposition 8 — two days of hearings begin Tuesday on this and on the federal Defence of Marriage Act — it could mean that same-sex marriage will be legal not just in California, but everywhere in the country.

How's that for irony? And how devastating would that result be for the architects of Prop 8 who set out deliberately to force this issue?

Their campaign in 2008 depended heavily on whipping up public hysteria against gays and lesbians with wild warnings that same-sex marriage would lead to the legalization of prostitution, polygamy and incest. It had already done so in the Netherlands, they lied.

Prop 8 passed with 52 per cent of the vote, and the right to same-sex marriage that had been recognized by the California Supreme Court just a few months earlier was extinguished.

Now, five years of legal battles later, Prop 8 may turn out to be the ramp that put same-sex marriage onto the fast track toward becoming a constitutional right.

Views changing

Maybe it won't turn out that way. Maybe the court will decide that striking down Prop 8 has effect only in California.

Or maybe it will decide to uphold Prop 8 and continue the ban on same-sex marriage, though, given what a pathetically weak case was made in defence of Prop 8 at the lower courts, that seems unlikely.

No matter. Whatever the justices decide won't change the fact that Prop 8 has already played an important role in encouraging people to think about same-sex marriage, and to think about it in a way that has led to a "changing of minds."

More important, that changing of minds has been mainly, probably exclusively, in only one direction: many who opposed it now support it.

That includes some who have deeply held religious convictions.

They've come to understand that marriage as a civil institution does not threaten marriage as a religious institution; that churches will remain free to treat gays and lesbians differently, just as they treat men and women differently.

Interactive Map: Same-sex marriage rights by U.S. state 

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Pro-gay marriage supporters celebrate the overturning of Proposition 8 by a California appeals court in February 2012, a ruling that set up this week's hearing in the U.S. Supreme Court. (Jonathan Alcorn / Reuters)

The changing of minds has been especially true in the birthplace of Proposition 8.

Today you wouldn't see 52 per cent of Californians voting to ban same-sex marriage.

Now they've thought it over and, were they to vote today, 61 per cent would favour legalizing same-sex unions, according to a February poll by Field Research Corp.

Generational change

The same poll also offers a glimpse into the future. Seventy-eight per cent of voters aged 18 to 39 support same-sex marriage.

That may be partly because the generation that grew up with the AIDS crisis of the 1980s looked beyond the terrible cruelty of that disease and saw the many loving and brave relationships that had been in the shadows.

Whatever the reason, the evidence points to an overwhelming recognition of the equality of same-sex couples that continues to spread across the country.

The only groups in the Field poll that continue to cling to the belief that marriage is reserved for straight people are conservatives and Republicans.

But they, too, are changing, if slowly. Parts of the Republican Party are trying to freshen its brand and shake off a perception among younger Americans that it is "rigid", "not progressive", "old fashioned" and "religious", terms offered up by a focus group and reported in the New York Times last month.

That may be why, as the Prop 8 case has moved closer to its day in court, even some Republicans have become new converts to the cause and have raised their voices.

A group of a hundred or so high-profile Republicans filed a brief to the Supreme Court on Prop 8,  urging it to recognize a constitutional right to same-sex unions.

Among them are former presidential candidate John Huntsman, who opposed same-sex marriage during the Republican primaries last year; and the former Republican candidate for governor of California, Meg Whitman, who, likewise, opposed it during her 2010 campaign.

Clint Eastwood's name is there, too, though he's not a recent convert. He's been a same-sex marriage supporter for more than a decade, a libertarian from the "what's it to me if punks wanna' marry" wing of the GOP.

Obama's conversion

The most important reconsideration has happened in the mind of the president of the United States.

Barack Obama first won the presidency on the same day Prop 8 won approval in California.

At that time, he opposed same-sex marriage. Over the next few years he would bob and weave his way through questions about it, and sometimes allow that his thinking was evolving.

By the time of his re-election campaign last year, Obama had finally decided where he stood.

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One of the most recent gay-marriage converts, President Barack Obama, shown here following his 2013 inaugural address with the man who gave him a push, Vice-President Joe Biden. (Paul Sancya / Associated Press)

Now, his administration has asked the court not to allow the Prop 8 ban on same-sex unions and it has said it will no longer support the prohibition in the federal Defence of Marriage Act, the other same-sex issue on the court's agenda this week.

In his inaugural address in January, Obama gave pride of place to his support for same-sex marriage. From the Capitol steps, he said "if we are truly created equal, then surely the love we commit to one another must be equal, as well."

The more important line was this: "The most evident truth" is that we are all created equal, "that is the star that guides us still, just as it guided our forebears through Seneca Falls and Selma and Stonewall."

Interactive Map: Same-sex marriage laws around the world

Stonewall is the Greenwich Village Inn where, in 1969, persistent police persecution and brutality against gays finally provoked riots. It was "the last straw" that became the catalyst for the gay-rights movement.

For Obama to situate Stonewall rhetorically alongside Seneca Falls and Selma is to put equality for gays and lesbians in the company of the two great social and political struggles of American history: rights for women and for African Americans.

It is to say that equality for gays and lesbians is a civil right; that equal marriage is a civil right; and that, if the court disagrees, then to continue the struggle is to be on the right side of history.