8 December 2011 Last updated at 22:56 GMT
Newt Gingrich now leads opinion polls by a wide margin in Iowa, South Carolina and FloridaKey backers of Republican presidential hopeful Mitt Romney have slammed the leadership and conservative record of chief rival New Gingrich.
Missouri Senator Jim Talent and New Hampshire Governor John Sununu said the former House speaker was not a “reliable and trusted” leader.
On Thursday Mr Romney launched a series of TV commercials in Iowa, whose early nominating contest is four weeks away.
Polls in several early voting states show Mr Gingrich significantly ahead.
Already ahead in Iowa, the first state to vote, Mr Gingrich now has a clear lead over Mr Romney in South Carolina and Florida.
Correspondents say the former Massachusetts governor is rallying his supporters in an effort to derail Mr Gingrich’s surging popularity.
Polls from CNN/Time released on Wednesday indicated Mr Gingrich has a lead of 23 points in both South Carolina and Florida.
‘Irrational behaviour’
In a conference call with reporters, a series of senior Republicans said Mr Gingrich had undermined the conservative agenda – citing his description earlier this year of Paul Ryan’s plan to reform Medicare as “right-wing social engineering”.
They also said his “off-the-cuff” style of decision-making was not best suited to the job of president.
Mr Talent said that Mr Gingrich could say “outrageous things that come from nowhere”.
“And he has a tendency to say them at exactly the time when they most undermine the conservative agenda.”
Mr Sununu said that as speaker of the House in the 1990s Mr Gingrich had shown a pattern of “anti-principled actions that really irritated his own leadership”, adding that his “irrational behaviour” would not be desirable in the White House.
Mr Gingrich, who is currently campaigning in South Carolina, made light of the comments, saying he was trying to stay “positive”.
Correspondents say the head-on attack on a fellow candidate marks a shift in Mr Romney’s campaign strategy, indicating that he now sees Mr Gingrich as his principal rival.
Mr Talent emphasised that he was reluctant to criticise a fellow Republican but said he had felt compelled to make the comments.
“I just think the stakes are so high, not just for the Republican party but for the country as a whole that we have to do everything that we can” to choose the right candidate.