Obama the campaigner is back.

The president has been itching to hit the campaign trail since at least December and, given his popularity, it looks like he will be a key ally for Hillary Clinton—the first time a sitting president will have had an active role in a presidential campaign since Ronald Reagan helped George H.W. Bush get elected in 1988. With Donald Trump running as the anti-Obama, the president has a great deal at stake, which may help explain why he’s begun to lash out at the presumptive Republican nominee. (The fact that Trump was the country’s leading birther and has suggested that Obama is a terrorist conspirator is certainly also a motivating factor.)

On Tuesday, Obama delivered an off-the-cuff assault on Trump for demanding that he should resign for not using the phrase “radical Islam.” “The main contribution of some of my friends on the other side of the aisle have made in the fight against ISIL is to criticize this administration and me for not using the phrase ‘radical Islam.’ That’s the key, they tell us. We can’t beat ISIL unless we call them radical Islamists,” he said. “What exactly would using this label accomplish? What exactly would it change? Would it make ISIL less committed to trying to kill Americans? Would it bring in more allies? Is there a military strategy that is served by this? The answer is none of the above. Calling a threat by a different name does not make it go away. This is a political distraction.”