WASHINGTON: A smaller group of Republican presidential candidates returns to the main debate stage Thursday night, one of the last high-profile opportunities to sway voters before the Feb. 1 Iowa caucus.

After months of civility toward rival Ted Cruz, front-runner Donald Trump is aggressively targeting the Texan, asserting that the Canadian-born senator may be ineligible to be president.

Cruz dismisses that claim and accuses the businessman of having “New York values” and questioning his foreign policy credentials.

Four other candidates are fighting to become the more mainstream Republican alternative to the pair of anti-Washington candidates leading the field. Florida Sen. Marco Rubio is seen by some as having a slim edge, opening him up to a torrent of criticism from his rivals about his voting record in the Senate and his immigration policy, as well as the 44-year-old’s youth and relative inexperience.

Former Florida Gov. Jeb Bush has been among the few establishment Republicans jabbing Trump in recent weeks. He sharply criticized Trump on Wednesday for holding positions on taxes, guns and health care that he says are out of step with conservatives.

Host Fox Business News tightened the qualifying rules for Thursday’s debate, resulting in the smallest group of candidates in the headline event to date.

Also on the main stage will be New Jersey Gov. Chris Christie and Ohio Gov. John Kasich, who are battling Rubio and Bush for the establishment vote, as well as retired neurosurgeon Ben Carson, whose standing in the race has steadily fallen.

Christie is enjoying a burst of momentum in New Hampshire, another early-voting state, where he’s devoted significant time to courting its quirky blend of moderate and libertarian voters.

The debate rules resulted in businesswoman Carly Fiorina being bumped to the undercard event. Kentucky Sen. Rand Paul was also demoted but is choosing not to participate in the early-evening contest.

Republicans have one more debate scheduled before voting begins in Iowa, a Jan. 28 event in Des Moines.