The Dems lost in 1968 due to the Vietnam War. They lost in 1972 because the war was ending, and they ran George McGovern, who was the most liberal Senator in the country.
The Republicans lost in 1976 due to Watergate. The Dems lost in 1980 due to runaway inflation and the Iran hostages. They lost in 1984 because they ran Mondale, one of the most liberal Senators in the country, and people liked Reagan. The Dems lost in 1988 because they ran Michael Dukakis, a Governor from the most liberal state in the country.
The Republicans lost in 1992 because of Ross Perot, and the fact that Bill Clinton was a southerner, and considered a moderate. The Republicans lost in 1996 because they ran Bob Dole. The Republicans won 2000 because Al Gore was too liberal to carry his own state.
The Dems lost 2004 because the Dems ran John Kerry, one of the most liberal Senators from the most liberal state. You probably remember the last 8 years.
I did this all from memory and my perspective, not anybody else's view of the history of the elections.