Republican White House hopeful Mitt Romney, a Mormon facing doubts about his conservative credentials, said Saturday he had the endorsement of five former US ambassadors to the Vatican.

Romney's campaign unveiled a letter from the ex-envoys hours before a debate in which he was to face off against his rivals, including Catholics Rick Santorum and Newt Gingrich, who have cast him as a weak-kneed moderate.

The former ambassadors said they were "united" behind Romney "because of his commitment to and support of the values that we feel are critical in a national leader" and "experience, vision and commitment to the common good."

The group included Thomas Melady, Raymond Flynn, James Nicholson, Francis Rooney, and Mary Ann Glendon, who served under both Democratic and Republican presidents as envoys to the Holy See.

They cited Romney's opposition to abortion and his "outstanding record in defense of marriage and the family."

Santorum, a former US senator and devout Christian conservative, and Gingrich, a former House speaker, have been hammering frontrunner Romney for weeks on grounds that he is not a true conservative.

Earlier this week Republicans kicked off a months-long nomination battle to determine who will face off against Democratic President Barack Obama in November elections.