WASHINGTON —  Republican Gov. Mitt Romney, who is weighing a White House bid, dismissed criticism that he has flip-flopped on the issues of gay marriage and abortion and reaffirmed his opposition to both.

"Like the vast majority of Americans, I've opposed same-sex marriage, but I've also opposed unjust discrimination against anyone, for racial or religious reasons, or for sexual preference," Romney said in an interview with the National Review magazine published online Thursday.

Regarding abortion, Romney said — as he has said previously — that although he campaigned for governor as an abortion-rights supporter, he changed his position several years ago after being briefed on embryonic stem-cell research.

"I'm committed to promoting the culture of life," the Massachusetts governor told the conservative magazine. "Like Ronald Reagan and Henry Hyde and others who became pro-life, I had this issue wrong in the past."

The comments were Romney's first public explanation of his stance on the two key social issues since the publication last week of a 1994 letter — sent in the final weeks of his failed campaign against Sen. Edward M. Kennedy — in which he cited his sensitivity to the concerns of Log Cabin Republicans, the party's gay group.

The governor returned earlier this week from a trip to Asia.

"As a result of our discussions and other interactions with gay and lesbian voters across the state, I am more convinced than ever before that as we seek to establish full equality for America's gays and lesbian citizens, I will provide more effective leadership than my opponent," Romney wrote in the letter.

During that same campaign, Romney also stated his personal opposition to abortion, but said he would not seek to change state abortion laws. As proof, he cited his mother's own 1970 candidacy for the U.S. Senate as an abortion rights supporter.

The letter and his history on abortion have prompted conservative religious activists — a key constituency in the Republican party — to question whether Romney is truly committed to a conservative social agenda and to demand he explain his positions.