Impeachable offenses
Sorry, but the idea that a president "might" be doing something is not an impeachable offense,
You're misstating the issue, of course. It's the fact that the President is deliberately circumventing the safeguards that are meant to provide assurance that he and his government are obeying the law and not pursuing a political agenda. It's the very fact that Bush "might" be doing almost anything and we'd never know about it that is the problem. Representative democracy requires transparency and openness, not the furtive secretiveness of the Bushies.
You do understand that in a republic the chosen leaders are obliged to obey the laws that form the employment contract between the person and the nation, don't you? Rogue leaders like Bush engage in, at a minimum, breach of contract with the people. We didn't elect a dictator, we thought we were electing a President, and that's what we have every right to expect.
You'd like it to be narrowly about the spying, because you can then argue technicalities. But for the rest of us it's about breach of faith and trust and an attack on representative democracy itself. The infraction could be about anything, not just spying; it's the arrogant disdain for us and our nation's rule of law that's the sticking point.