WASHINGTON, D.C. -- The latest CBS News national poll finds Donald Trump leading his closest rival, Ted Cruz, nearly two to one. Marco Rubio is the only other candidate in double digits.

Cruz, deadlocked with Trump for first in Iowa, returned to New Hampshire for the first time in two months. He continued to try to dispel Trump-planted questions about his birth in Canada and legal status for the White House.

"The legal question is quite straightforward, which is that the children of U.S. citizens born abroad are natural-born citizens, are citizens by birth," Cruz said.

But one of Cruz's law professors at Harvard, Democrat Lawrence Tribe, wrote Tuesday that "the constitutional definition of a 'natural-born citizen' is completely unsettled."

John McCain confronted similar questions as the GOP nominee in 2008. McCain was born on a U.S. military base in Panama.

Senator John McCain CBS News

"I am very confident that Ted Cruz is legitimately qualified to run for president, but it needs to be looked into," McCain told CBS News.

McCain also said he thinks candidates like Donald Trump -- and to a lesser extent, Ted Cruz -- have tapped into the anger and frustration within the Republican party.

"I disagree with him on a number of the statements and positions that he has taken obviously. But if that's the verdict of the Republican party and the majority of Americans, then I will do everything I can to help that president."

McCain went on to say he will endorse the party's nominee, no matter who it is, because party loyalty will not allow him to walk away.