Newt Gingrich, the former house speaker who insisted he would never "go negative" to win the Republican presidential nomination, has begun doing so in a scramble to save his campaign in Iowa.
With his poll numbers in sharp decline, Mr Gingrich said he could not vote for Ron Paul if he were the Republican candidate, while a group backing him described Mitt Romney as "dangerous" for America.
After surging into the lead in polls last month, Mr Gingrich promised to abide by Ronald Reagan's so-called eleventh commandment: "Thou shalt not speak ill of any fellow Republican."
"It is critical the Republican nominee emerge from this primary campaign unbloodied, so that he or she can make the case against President Obama from a position of strength," he told aides.
Mr Gingrich (68) even pledged to "disown" staff members or groups backing him if they deviated from his "relentlessly positive" campaign based on ideas to improve the US.
But having fallen behind Mr Paul, a Texas congressman, and Mr Romney, a former Massachusetts governor, before next Tuesday's Iowa caucuses, he has gone on the attack.
He told a television interview that Dr Paul's views were "totally outside the mainstream of virtually every decent American" and said he could not bring himself to vote for him against Mr Obama.
Newsletters released by Dr Paul in the 1990s contained racist, anti-semitic and homophobic material that he has since disavowed.
Dr Paul now heads the field in Iowa with a 1.7pc lead over Mr Romney, after Mr Gingrich dropped almost 14 points in a fortnight, according to an aggregate of polls by the polling data organisation RealClearPolitics.
Mr Gingrich, who is on a 22-stop Iowa bus tour, also hit out at Mr Romney, ridiculing him for questioning the former speaker's "conservative credentials" given his own record as "moderate Mitt".
His campaign, which has been vastly outspent by Mr Romney and groups backing him, has driven itself further into debt by spending hundreds of thousands of dollars on television advertising.
At the same time two Gingrich-backing "super PACs", -- political action committees -- which can use unlimited secret donations from companies and individuals, have released material attacking Mr Romney. (© Daily Telegraph, London)