2004: Frist beats Daschle, just barely
2000: Daschle beats Lott, again close
1996: Daschle beats Dole (assuming Dole stays as leader)
1992: Mitchell vs. Dole - unsure who would win
1988: Dole beats Byrd
1984: Baker beats Byrd (again)
1980: Baker beats Byrd
1976: Mansfield beats Scott (again)
1972: Mansfield beats Scott
1968: Dirksen beats Mansfield (again)
1964: Dirksen beats Mansfield
1960: Johnson beats Dirksen
Of course, it all depends on the circumstances. This is just my opinion. If you don't mind, TommyC, I might want to change make this into an alternate history timeline once my current one runs out of steam, if that's ok with you.
Pretty much spot on except I think a reverse in 1960.
