BEWARE of an aroused citizenry. It's an admonition that should be ingrained in the brain of any run-of-the-mill politician, let alone someone who has ascended to the United States Senate. But from the Olympian heights of the world's greatest deliberative body, it is often forgotten. So senators got a reminder in the humiliating defeat of a "comprehensive" immigration bill that had the support of the president of the United States, a bipartisan group of senators with the blessing of the leaders of their caucuses and the support of the editorial boards of the country's most important newspapers.