(File image) Mitt Romney

Washington - Republican presidential nominee-in-waiting Mitt Romney apologised on Thursday for high school pranks that may have hurt others, after a report that he and other students at a Michigan school bullied a student who was presumed to be gay.

“I did some dumb things and if anybody was hurt by that or offended... obviously, I apologise,” Romney said in response to a Washington Post story that detailed a 1965 incident in which Romney pinned down a fellow student and cut his hair.

The apology came as Romney sought to contrast his opposition to same-sex marriage with President Barack Obama, who voiced his support this week in an ABC-News interview.

Obama's political gamble to shift gears and now support gay marriage reverberated on the campaign trail. While it is unclear how big an impact the issue will have in the November 6 presidential election, battle lines were drawn.

Senior Romney campaign adviser Ed Gillespie said on Thursday that the Romney campaign would use Obama's gay marriage support to illustrate many differences between the Republican challenger and the Democratic incumbent.

“It's an important issue for people and it engenders strong feelings on both sides,” Gillespie told MSNBC. “I think it's important to be respectful in how we talk about our differences, but the fact is that's a significant difference in November.”

Obama pivoted to support gay marriage after two years in which he said his position was “evolving” on an issue crucial to his liberal base. His move may help energise liberals who have not shown the passion for him that they did in electing him America's first African-American president in 2008.

On the right, Romney has struggled to convince conservatives that he is truly one of them, and the gay marriage issue may play to his benefit. By stating his belief that marriage should be between a man and a woman, he may trigger some enthusiasm for his candidacy among social conservatives.

How the issue plays among the broader electorate is unclear. Polls show Americans fairly evenly divided, and North Carolina on Tuesday became the 30th state to pass a constitutional amendment banning gay marriage.

The White House made clear Obama believes Romney is vulnerable on the same-sex marriage issue, noting that Romney supports a constitutional amendment defining marriage as between a man and a woman.

“Governor Romney is for an amendment to the US Constitution that would enshrine discrimination into our founding legal document,” White House spokesperson Jay Carney told reporters.

A story published by The Washington Post provided a complication for Romney that put him on the defensive. Recounting an incident at the prestigious Cranbrook School in Michigan, it said Romney orchestrated an incident in which a presumed gay student was bullied.

The student, John Lauber, was tackled and pinned to the ground by a group of students led by Romney, who wanted to cut the student's bleached-blond hair.

“As Lauber, his eyes filling with tears, screamed for help, Romney repeatedly clipped his hair with a pair of scissors,” the Washington Post reported.

Responding promptly to the newspaper report, Romney went on a conservative talk radio show hosted by Fox News' Brian Kilmeade to explain his side of the story.

“You just say to yourself that, back in high school, I did some dumb things... But overall, high school years were a long time ago and I'm glad I've got some good friends from those years,” Romney said.

Asked about the specific incident involving Lauber, Romney said: “I don't remember that incident and I'll tell you I certainly don't believe that I - I can't speak for other people, of course - thought the fellow was homosexual. That was the furthest thing from my mind back in the 1960s, so that was not the case.”

Democratic National Committee spokesperson Brad Woodhouse quickly tweeted a link to the story to his followers and peppered Romney with criticism.

“One of the biggest conversations we're having in this country is about the bullying of kids in school. Romney was a bully as an 18-year-old,” Woodhouse tweeted. - Reuters