John McCain

"I obviously have seen no evidence of illegal voting," McCain told CNN's Anderson Cooper. "My focus has been on national security, getting Gen. Mattis in as secretary of defense ... doing those issues."

McCain added that he had learned to disregard at least some of the President's newsmaking remarks and focus on the business of governing.

Mr McCain said Mr Trump did not have an "unfettered licence to defame those who are the best among us".

Democratic lawmakers and the soldier's father have called on Republicans to disavow Mr Trump.

US Army Capt Humayun Khan was killed by a car bomb in 2004 in Iraq at the age of 27.

His father, Pakistani-born Khizr Khan, told the BBC on Monday that Mr Trump could not insult women, judges and even members of his own party and not expect to face criticism.

"We all have same equal rights," Mr Khan said.

McCain, a Republican, stood next to an enlarged, close-up photo of Aylan Kurdi, who drowned along with his 5-year-old brother and mother when their small rubber boat capsized as it headed for Greece. 

McCain said the photo has "opened the world's eye to this devastating crisis."

McCain said it should haunt Americans that "the United States will continue to do nothing meaningful" about conflicts like that in Syria that have led to a surge in refugees.