Frustrated with Donald Trump’s ability to blunt attacks on his domestic policies, his rivals for the Republican presidential nomination increasingly are taking aim at his approach to foreign policy, arguing that the flamboyant business leader has disqualified himself from consideration as commander in chief because of his “isolationist” views.
Fresh off Mr. Trump’s latest stumble, when he reversed himself on whether invading Afghanistan in 2001 was a mistake, Sen. Marco Rubio and former Gov. Jeb Bush of Florida say the real estate magnate has shown himself to be in over his head when it comes to facing down the world’s biggest threats.
Mr. Trump has channeled former President Ronald Reagan’s “peace through strength” mantra, vowing to make the military so strong that no country would want to mess with the United States.
But in the past few weeks, he has faced scrutiny for saying that the U.S. has no role in fighting the Islamic State or the ruling regime in Syria, and that it is OK for Russia to strike targets, including U.S.-backed rebels, inside Syria.
That has spurred feverish reaction from rivals.
“Donald Trump has no idea about this war,” said Sen. Lindsey Graham of South Carolina, a member of the Senate Armed Services Committee.
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He told CNN that the billionaire businessman is an isolationist and lumped him with Sen. Rand Paul of Kentucky, the libertarian whose criticism of forward-leaning American power has sparked a deep divide within the Republican Party.
The foreign policy attacks have intensified as world events have heated up — and as attacks on Mr. Trump’s domestic policies have done little to dent the Republican front-runner’s standing in polls. Indeed, two Republican rivals — former Texas Gov. Rick Perry and Wisconsin Gov. Scott Walker — dropped out of the race shortly after getting into dust-ups with Mr. Trump over immigration and budget policy.
Republican officials, though, say it could be hard for Mr. Trump to defend his foreign policy views at a time when concerns about a nuclear-armed Iran, the rise of Islamic militants in the Middle East and Russia’s meddling in Eastern Europe and Syria are unifying broad swaths of the party base.
“Up until this point in the campaign, he has traded on his personal attributes more than any detailed command of foreign policy,” said Brian Murphy, chairman of the Rockingham County Republican Party in New Hampshire.
“People are drawn to his decisiveness, and there is a sense of confidence that he will lead given the opportunity,” Mr. Murphy said. “But the next president needs to be job-ready Day One. And Donald Trump will need to make his case on foreign policy.”
Rep. Raul R. Labrador, Idaho Republican, told The Washington Times last week on Capitol Hill that the Trump hype is all “media-driven” and suggested that Mr. Trump could lack substance on national security issues.
“Eventually, people are going to start looking at his policy,” Mr. Labrador said.
A Quinnipiac University poll released last week also had warning signs for Mr. Trump, which showed the overall Republican front-runner trailing Ben Carson in Iowa.
What’s more, the survey found that Mr. Trump is favored by 29 percent of likely Republican caucus-goers in Iowa on the economy, 28 percent on illegal immigration and 19 percent on taxes. But Mr. Rubio edged out Mr. Trump on foreign policy, 18 percent to 17 percent.
The poll coincided with another spat between Mr. Trump and Mr. Bush — this time over how much blame his brother, former President George W. Bush, deserves for having been in office during the Sept. 11, 2001, terrorist attacks. Mr. Trump said he didn’t blame the former president outright, but he asserted that no such attack would have happened on his watch.
Jeb Bush shot back in a National Review op-ed by saying Mr. Trump’s remarks sounded like “the attacks of Michael Moore and the fringe left.”
“Let’s be clear: Donald Trump simply doesn’t know what he’s talking about,” Mr. Bush said. “And his bluster overcompensates for a shocking lack of knowledge on the complex national-security challenges that will confront the next president of the United States.”
Still, the foreign policy attacks could fall as flat as the domestic policy attacks.
Andrew Smith, a pollster at the University of New Hampshire, said voters don’t get caught up in policy proposals.
“Human traits, such as personality, leadership, toughness, humor, etc., are far more important,” Mr. Smith said. “And in a period in which Republicans have not been happy with their elected political leadership, a guy like Trump can simply run on that rather than providing detailed plans for what he plans to do.”
Rep. Morgan H. Griffith, Virginia Republican, said people should not underestimate how well Mr. Trump’s message on foreign policy might resonate with voters and that Mr. Trump is savvy at using the media to broadcast his views.
Mr. Griffith said he listened to Mr. Trump’s recent radio interview on the “John Fredericks Show” in Virginia, where the presidential candidate panned the Iran nuclear deal and said Russia’s air campaign in Syria is fine with him because it could leave Russian President Vladimir Putin mired in the Middle East for decades.
“If Russia wants to go in and bomb the hell out of ISIS, I am OK with it. I can live with it,” he said. “So I’d like to know what the alternative is, because the alternative could be very much worse, and in the meantime we have to build our own country.”
Mr. Griffith said many Virginians who tuned into the broadcast likely could say, “Trump sounded reasonable this morning.”
“It doesn’t have to be a complicated message,” he said, adding that Mr. Trump is outworking his rivals on the airwaves. “He has got money, and maybe he is outworking them because instead of him making fundraising breakfast calls, he is making John Fredericks calls.”