Anyone over the age of about 35 has lived through some of the most intensely political acts in our nation's history. We can honestly debate the merits of impeaching George W. Bush because of his actions. Less than ten years ago we saw President Clinton impeached and ultimately acquitted by the Senate. In 1974 Richard Nixon was facing impeachment but chose to resign before he could be impeached.
There was one other time in our history where a president was impeached.
President Andrew Johnson was impeached in 1868. Like Clinton, it was a politically-motivated move by Congress after they passed a law to prevent the President from firing his executive appointees.
The reason this was done is that Johnson, the vice president and a Southern Democrat, became president after Abraham Lincoln was assassinated. Lincoln was a Republican (but not like the GOP of today!), so Johnson inherited an administration filled with appointees by the opposing party. Republicans in Congress tried to shield these appointees by passing the Tenure of Office Act in 1867 which prevented the President from firing any appointee without Senate approval. Congress overrode Johnson's veto to enact it as law.
Johnson chose to test the constitutionality of this law by firing the Secretary of War, Edwin M. Stanton. The Supreme Court was bullied into not ruling on the law by Republicans in Congress (gosh, another cowardly court, imagine that), and this cleared the way for Congress to impeach Johnson for violating the Tenure of Office Act. Johnson survived impeachment by a single vote in the Senate.
As our history shows, the only two times impeachment has actually gone forward against a president it was for purely political purposes. The legitimate case for impeachment, President Nixon's crimes, actually came out of investigations and hearings. Because Congress started with investigations and came to impeachment instead of starting with impeachment, the case was so strong against Nixon that he resigned instead of facing the inevitable.
By the way, the Supreme Court finally ruled the Tenure of Office Act unconstitutional in 1926.
Update: Elwood Dowd succinctly states my point in the comments:
The Johnson and Clinton impeachments were brazen political acts, and failed. The Nixon impeachment was fully justified legally and Constitutionally, and succeeded.