Richard Nixon was the most hated man in the 20th Century by the Left. He was accused of having defeated Senator Helen Douglas (I knew her daughter) by lying about Douglas's 'Red' connections and slandering her. (Nixon's charges against her were entirely accurate.) Nixon was defeated in the 1960 presidential race because of ballot stuffing in Illinois and Texas but bided his time carefully and was elected President in 1968 against Hubert Humphrey after Bobby Kennedy, the logical candidate, was murdered by an anti-Israeli Palestinian.
The Democrats controlled the House and Senate but did not have 60 votes in the Senate, until Nixon's second term (which he won by victory in 49 states). In the 1974 off-year election the Democrats gained 60 seats in the Senate, enough to impeach Nixon, which they proceeded to do, first by forcing Nixon's VP (Spiro Agnew) out of office and replacing the VP with a fellow senator. We know the affair as Watergate which was run by leaks from Mark Felt the acting head of the FBI who was a Democrat who hated Nixon.
Congressional Republicans never forgot.
By 1995 the Republican party gained control of both houses of congress for the first time in 44 years. They began investigating the Democratic President Bill Clinton. By 1998, when they saw that they had two more years of Congressional control, the Republicans brought impeachment proceedings against Clinton. With only 55 Republicans in the Senate they couldn't succeed with the impeachment but they got their revenge.
The Democrats gained control of both the House, Senate and Presidency in 2008 and have begun their on-going revenge. So far they have attacked the previous president, Bush, by attempting to close Guantanamo Prison, renaming the Bush War on Terror, trying five Bush indicted terrorists as criminals, re-opening a Justice Department probe of the CIA interrogation procedures and attempting to punish Bush's ally Karl Rove (failed effort). The Democrats don't have a solid 60 Senate votes so their power is limited.
Legislatures have memories and they can be vindictive.