Nationalism is a dangerous thing. It is elevation of "nation" above all else.
We have a constitutional republic. That constitutional system is elevated above all the flag waving, patriotic music, speeches, and pro-America emotionalism that a demagogue preaching nationalism can dish out.
After all, national socialism was extremely nationalistic about pre-war Germany.
I think that's what the president meant, and I agree with him.
Do you happen to have Bush's specific quote from today?
By the way, just because Hitler and the rest of his lousy gang did what they did in Germany does not make nationalism a bad thing. That was just a deplorable usage of it.