Republican presidential candidate Donald Trump on Wednesday questioned the feasibility of giving preference to Christian Syrian refugees trying to enter the country, as some of his rivals have suggested in the wake of the Paris attacks.

Speaking to reporters before a rally in Worcester, Massachusetts, Trump suggested it would be difficult to prove refugees' faiths.

"Well I don't know if you can prove that they're Christian," said Trump, who opposes allowing any migrant refugees into the country. "How are they proving it?"

Both former Florida Gov. Jeb Bush and Texas Sen. Ted Cruz have argued the U.S. refugee program should give preference to Syrian Christians over Muslims fleeing their war-torn country — a suggestion President Barack Obama condemned as contrary to American values.

Trump spoke later Wednesday in front of a crowd of thousands, arguing that his business success and temperament made him far better suited for the role of president than his rivals.

Days after going after Ben Carson during a 95-minute speech in Iowa, Trump once again focused on the retired neurosurgeon, who has been tied with Trump in some polls.

This time, Trump focused on recent reports that Carson has been struggling to grasp the complexities of foreign policy.

"We can't do this," Trump told the crowd. "We need somebody that's sharp and tough and smart and can figure it out fast."

Trump's event was interrupted several times by protesters who were escorted out by police. While Trump usually brushes off interruptions, he responded to one man with an insult.

"It's amazing. I mentioned food stamps and that guy who's seriously overweight went crazy," Trump said in response.

Before the rally, Trump, who has said he believes the U.S. will have to close some mosques because of radical activity, also predicted he'll win with Muslim voters.

"I'm going to do great with the Muslims," said Trump. "They understand what I'm saying... We're talking about safety. We're talking about security. We need it."

Trump also continued to criticize President Barack Obama's response to the attacks in Paris, blaming American actions for destabilizing the Middle East and fueling the Islamic State group's rise.

He has repeatedly criticized Democrats, including the president, for avoiding the term "radical Islamic terrorism" — a phrase they say unfairly criticizes all Muslims.

"Unless you're going to use the term, you're not going to solve the problem," he said. "I don't even know if he wants to solve the problem, frankly."