Last week President Obama announced that his evolution on gay marriage had culminated in his decision to support it.  This statement was a milestone for the gay and lesbian rights movement and for America, especially as it came when same sex marriage is prohibited in most of the United States.

Pundits–and indeed most Americans–thought that Obama’s shift was politically motivated.  After all, public opinion on same sex marriage has shifted dramatically in the past few years.  More Americans support same sex marriage than oppose it now (see Pew’s poll series), and the difference is dramatic among younger people–who might vote.  Should we be disturbed if Obama, like most politicians, weighed the political payoffs of the policies he advocates?  That’s supposed to happen in a democracy; it’s called accountability.  And one way movements exercise influence is by giving politicians good political reasons for embracing the policies activists advocate.

President Obama’s statement will surely galvanize opponents of same sex marriage, heightening the threat they see and provoking more anti-marriage activism.  It gives more visibility to the issue, and provides a ready focus for mobilization–Obama.

Endorsing gay marriage is also likely to increase the enthusiasm and participation (including financial contributions) of some of the people who already support President Obama.  It’s not clear whether President Obama will lose the support of anyone who otherwise would have voted for him.

It is likely to promote more acceptance for gay marriage.  The President, even one you don’t like, is a powerful symbol in America.  It’s not a trivial endorsement.  And when we vote for someone who disagrees with us on some issues (as we always do), we make a decision that the antagonistic position just isn’t significant enough to throw out everything else.  Often, we find ways to justify the dissonant position, sometimes by rethinking our positions.

It’s also worth thinking about what President Obama didn’t do.  Immediately after North Carolina amended its constitution at the ballot box (61 percent!) to prohibit same sex marriage (AND civil unions), Obama offered an alternative view in an informal interview on television, but proposed no legislation.  Marriage remains the concern of state governments.  Obama did not issue an Executive Order prohibiting government contractors from discriminating against gay people, nor did he suggest that the IRS might offer to treat gay couples the same way it treats couples who are legally married.  The words aren’t unimportant, but there’s more that could be done.

For an idea of what a president might do, think about Lyndon Johnson’s evolving stance on civil rights.  In 1965, a week after police had violently prevented non-violent civil rights marchers from crossing the Edmund Pettus Bridge in Selma Alabama, President Johnson seized the moment to promote the Voting Rights Act.  And, by all accounts, he used every resource at his disposal to beg and bully Congress into passing it.

President Johnson didn’t reveal his evolution on civil rights in an informal interview.  Instead, he made a televised speech to the entire Congress announcing his intent.  It’s an eloquent and ambitious text; every American should read it.

In it, President Johnson explicitly described the Voting Rights Act as one significant step in a much longer and larger struggle for justice:

But even if we pass this bill the battle will not be over. What happened in Selma is part of a far larger movement which reaches into every section and state of America. It is the effort of American Negroes to secure for themselves the full blessings of American life. Their cause must be our cause too. Because it’s not just Negroes, but really it’s all of us, who must overcome the crippling legacy of bigotry and injustice.

And we shall overcome.

Unlike President Obama, President Johnson did not enjoy a reputation for eloquence, but read the speech.  Johnson wanted to do more than share his evolution; he wanted to promote America’s progress.

And think about President Obama’s discussion as one small step in a much longer, uh, evolution.