Romney, a former Massachusetts governor, also called on his top rival Mike Huckabee to apologize to President Bush. In an article in the journal Foreign Affairs, Huckabee criticized Bush's foreign policy as an "arrogant bunker mentality."
Huckabee said no apology is necessary and that Romney should read the article.
Appearing on NBC's "Meet the Press," a somewhat defensive Romney acknowledged that he has shifted positions on some issues and explained that he did so after learning from experience. He said it would be a mistake if a candidate "stubbornly takes a position on a particular act and says, 'Well, I'm never changing my view based on what I've learned."'
"If you're looking for someone who's never changed any positions on any policies, then I'm not your guy," Romney said.
At the same time, Romney insisted that as governor he kept all of his campaign promises despite changing some views, and said he would stick to his promises if elected president.
"Bottom line: All the positions you laid out today as a presidential candidate, can you assure the voters you won't flip back to some of the positions you had when you were governor of Massachusetts?" asked NBC moderator Tim Russert.
"Of course," Romney responded.
Among the issues:
Abortion. Romney acknowledged changing his views in 2004 from supporting abortion rights to opposing abortion. He said he did not entirely betray abortion-rights voters, either, because he did not seek to change Massachusetts abortion laws.
Taxes. Romney said he promised not to raise taxes as governor and did not go back on his word by raising fees by about $240 million to help balance the budget. The fees were on services such as gun licenses and training to combat domestic violence. He explained that because the fees were not on broad-based services, such as driver's licenses, they did not "have a sense, a feeling like a tax." "I ran as an individual who would not raise taxes, and I didn't," he said.
Same-sex rights. Romney acknowledged that he initially supported federal efforts to ban discrimination based on sexual orientation, but now only supports such laws at the state level. In an unsuccessful Senate run in 1994, Romney promised to be "more effective on gay rights in the Senate than Ted Kennedy."
Romney's talk-show appearance came as he is trying hard to overtake Huckabee in Iowa and reclaim the lead he enjoyed for much of the year. Huckabee recently moved to the front of the Republican presidential field in Iowa, with the state's caucuses less than three weeks away on Jan. 3.
Romney has defended Bush against Huckabee's criticisms of his foreign policy. On Sunday, he went a bit further, labeling the charge "an insult" and calling on Huckabee to tell the president he was sorry. Huckabee leveled his criticism in an article in the journal Foreign Affairs.
"Mike Huckabee should apologize to the president," Romney said in response to a question.
Huckabee said Romney should read the article.
"It would really help if he would do that. Because if he did, he would see that there's no apology necessary to the president," Huckabee said on CNN's "Late Edition." Huckabee said he stood with Bush on tax cuts and sending more U.S. troops to Iraq when Romney did not.
"So, you know, I don't have anything to apologize for. But I'm running for president of the United States. I've got to show that I do have my own mind when it comes to how this country ought to lead, not only within its own borders but across the world," he said.