The nationally televised forum in New Hampshire will include most of the top-tier contenders in the battle for the right to challenge President Barack Obama, who still leads the Republican hopefuls in most opinion polls.

"This marks the start of a new phase for the campaign as more people pay attention and the candidates begin to engage," said Fergus Cullen, a former chairman of the New Hampshire Republican Party.

Romney leads the Republican pack in most polls but is an uneasy front-runner in a group that has drawn complaints from some in the party for being a weak field.

A CNN/Opinion Research Corporation poll released on June 13 showed Romney ahead of former Alaska Governor Sarah Palin at the head of potential Republican candidates.

Twenty-four percent of Republicans polled said they would support Romney for the nomination, and 20 percent chose Palin, who has not said whether she will enter the race

The biggest competition at the debate could be among candidates vying to become the prime alternative to Romney.

Romney, Michele Bachmann and Newt Gingrich skipped a lightly attended debate last month, but will appear on June 13 with four contenders who participated in the first one -- former Senator Rick Santorum, former Minnesota Governor Tim Pawlenty, former pizza executive Herman Cain and Representative Ron Paul.