Okay, this probably requires alien space bats, but I just want to make sure. Basically, not a single conservative may be elected for president from the year 1978 to 2011. Rockafeller Republicans are okay, but no one like Reagen, no one like George W Bush, and basically no one advocating laize-fair and/or things like anti-gay.
Now, I'm guessing this is completely impossible without the help of alien space bats. You would have to have historical events that undid the entire conservative revolution of the 80s and 90s.