The Republican presidential candidates turned their fire on Rep. Ron Paul at Thursday’s debate, showcasing what’s become the Texas Republican’s Achilles’ heel in the Republican primary: his foreign policy views.

Paul’s positions on national security remain his biggest obstacle to a realistic chance at the GOP nomination, even as he’s shot up in the polls in Iowa on his steadfast libertarian economic views.

Paul called the war in Iraq “useless” Thursday and said Iran shouldn’t be bombed to stop it from getting nuclear weapons — positions loathed by many Republicans.

Pundits give Paul a legitimate shot at winning the wide-open Iowa caucuses on his economic message, but say he’s unlikely to go much further.

In Paul’s 1988 and 2008 presidential campaigns, he was mostly viewed as a fringe candidate trying to publicize his views. But the libertarian’s economic message has been tailored to this year’s anti-government sentiment among GOP voters, and he’s slowly risen in the Iowa polls behind a fervent base.

The disparity between Paul and the rest of the Republican field was on full display at Thursday’s Fox News debate, where the candidates who trail Paul escalated their attacks on him.

Rep. Michele Bachmann (R-Minn.) called his positions on Iran “dangerous” as she and Paul had a terse exchange over whether the country is getting close to obtaining nuclear weapons.

Paul said there wasn’t evidence the country was producing weapons, and that “the greatest danger is that we will have a president that will overreact, that we will soon bomb Iran.”

Bachmann responded that she’d “never heard a more dangerous answer for American security than the one that we just heard from Ron Paul.”

“The problem would be the greatest under-reaction in world history if we have an avowed madman who uses that nuclear weapon to wipe nations off the face of the Earth,” Bachmann said.

READ MORE: THE HILL