US Senator John McCain, the Republican presidential nominee in 2008, joined 2012 nominee Mitt Romney on Thursday in criticising 2016 Republican front-runner Donald Trump, especially on foreign policy.
"I share the concerns about Donald Trump that my friend and former Republican nominee, Mitt Romney, described in his speech today," McCain said in a letter published online.
"I would also echo the many concerns about Mr. Trump's uninformed and indeed dangerous statements on national security issues that have been raised by 65 Republican defence and foreign policy leaders," he added.
McCain, chairman of the Senate Armed Services Committee, said at a time of global turmoil, "I want Republican voters to pay close attention to what our party's most respected and knowledgeable leaders and national security experts are saying about Mr. Trump."
Romney gave a blistering rebuke of Trump on Thursday, leading an attempt by the party establishment to stop the rise of Trump.
Romney called on Republicans in states that have not yet held nominating contests to back Trump's opponents to stop his march to the nomination for the Nov. 8 election to succeed President Barack Obama.
"Here's what I know. Donald Trump is a phony, a fraud," said Romney.
"He's playing the members of the American public for suckers. He gets a free ride to the White House and all we get is a lousy hat," he added.
In a speech he wrote himself, Romney said Trump's economic policy would sink America "into prolonged recession" and slammed his business acumen and temperament.
Romney’s speech came hours before Trump and his rivals share a stage for a debate in Detroit at 9 pm local time (0200 GMT Friday).