Barack Obama’s evolution reached its conclusion Wednesday when the 44th president of the United States said he supported same-sex marriage.

It was a historic change for the politician who had tried, like many others, to find a middle ground on this either/or issue in the form of civil unions — reserving the word “marriage” for couples made up of a man and a woman. But like others who have tried to make that argument, the president found that it can’t be reconciled with a view that all Americans are entitled to equal protection under the law, and that separate is not equal.

Since he first started running for president, Obama tried to hold the shaky middle ground, while hinting to gay supporters that he might come around because “attitudes evolve, even mine.”

This final evolution is a watershed moment in the back-and-forth struggle for equality fought by and on behalf of families led by same-sex couples. Gains measured by opinion polls, court rulings and legislative action in some states have been offset by losses at the ballot box in others.

Obama’s announcement comes after voters in North Carolina enshrined their state’s anti-same-sex marriage statute in the state constitution. The vote was as troubling a setback as supporters of marriage equality have seen.

But never before has a person of such power and influence as the president of the United States said he supported same-sex marriage, making Wednesday look more like a turning point than a defeat, and it could influence events in states like Maine, where same-sex marriage and Obama will be on the same ballot in November.

Maine rejected a same-sex marriage law in 2009, and the question will be back before Maine voters this fall. Obama’s announcement is not likely to directly change minds — people aren’t looking for authorities to tell them how they feel about an emotional issue — but it will provide an illustration of the process that many voters have gone through in the relatively short period since Massachusetts became the first state to issue marriage licenses to same-sex couples in 2004.

Many people who are not themselves gay or lesbian have, like Obama, seen their position on this issue change over time.

Some who voted to veto the same-sex marriage law in 2009 said they had wished there had been an alternative which gave all families similar legal protections, but kept the word “marriage” for families that looked like their own.

Like Obama, many of them will find that position is a hard one to hold. Like him, they will find that this issue is really about families — moms, dads and kids bound together by love and in need of the same rights and protections other families already enjoy.

Obama’s change of heart could provide a real-life example how even a deeply held position can change.

And the willingness to go through that process is the kind of leadership that could make a difference.