WASHINGTON -

In an exclusive interview with ABC News anchor Robin Roberts, President Barack Obama announced he believed same sex couples should be allowed to get married.

The president's position on same sex marriage, which his administration has described as "evolving," has shifted over the course of his political career.

He did favor defining marriage as between a man and a woman, with same sex couples receiving the same rights as married couples, but Mr. Obama has indicated lately that he was possibly changing that position.

"Today is a proud day for all Americans," said Theodore B. Olson, one of the attorneys in a federal lawsuit seeking to overturn Proposition 8, California's ban on same-sex marriage. "The bedrock American principles of freedom and human dignity are central to the political and legal convictions of Republicans, Democrats, liberals and conservatives alike.

"President Obama's words remind us that marriage and equality are  universal values that unite us all. They remind us that we are all -- as a  people and a nation -- striving to form a more perfect union," he said.

Republican presidential hopeful Mitt Romney says he believes marriage should be restricted to one man and one woman and that he's held that view "since running for office."

Romney on Wednesday called same sex-marriage "a very tender and sensitive topic" as he contrasted his position with President Barack Obama's unequivocal declaration of support Wednesday for allowing same-sex couples to marry.

Obama is the first president in history to support gay marriage.

Polls show the country is evenly divided on the issue.

Romney did not go so far as to accuse Obama of changing his position on the issue. Questioned by reporters, the presumptive Republican nominee for president said news reports indicate Obama has shifted his stance.

Romney says states should be able to decide whether to grant certain legal benefits to same-sex couples.