By being accusatory. Instead of warmly welcoming Democrat sedition as "honest disagreement", he should say that the Democrats are siding with the terrorists, because it's the truth. He needs to stir up a hornets nest of indignation. Then when the Democrats cry, "How dare the President question our patriotism?!", he should reply, "Patriots don't stab American troops in the back.", and so forth. Bring the battle to the enemy.
He has to say that. He's not some Senator or Congress Critter that can just go off on a Kennedy or Byrd style rant.
The President is supposed to appear above the fray.
The ones that NEED to be expressing the Presidents sentiment are those Congress Critters that call themselves "Republicans" in the House and Senate.
They are the ones that are supposed to say what a President with any kind of class can not.