Clinton's situation is exactly like Nixon's, except she doesn't have anything to resign from.
Think about it. Nixon was raked over the coals by the press and the Dems. Nixon was never convicted. A president let Nixon off "for the good of the country."
Clinton is being raked over the coals by the press and the GOP. Clinton won't be convicted as long as the Dems hold the White House. The current president is making excuses for Clinton.
Clinton might actually get convicted if a Republican wins the White House, although I doubt it. The Clintons have a lot of influence, even across the aisle. It'd also set a precedent, and no politician wants to face the chance of being held responsible for their actions. Honestly, I was shocked when Scooter Libby was convicted (although note who was convicted, what he was actually convicted of, and that jail was considered "too harsh" for what could be argued was a national security breach).
As far as Obama's situation - what situation are you talking about? What has Obama done that a president would fear impeachment? Any Democratic president would have the nuttier part of the Republican party screaming for impeachment, just as the nuttier part of the Democratic party was screaming for Bush's.
The entire point of political parties is to create a group of people who believe in similar things so that they can pull favors from each other and give each other a lift when they need it. There's nothing democratic about them at all - they consist entirely of back room deals and power plays. They're a necessary evil - our system of government couldn't exist without them - but don't expect justice or personal responsibility.