It seems that most people that I know are drinking diet drinks these days. This feeling isn't born out by the latest (2010) market share analysis, as diet drinks fall behind their non-diet equivalents (see Beverage-Digest). This fact makes my metaphor even better. In full disclosure, I don't like diet colas.
The Republicans repeat a couple of mistakes. First, unlike the Democratic party, the Republican party tends to select a presidential nominee according to "who's turn" it is. Call it the old-boys-network or whatever that you want to call it, but it never works. Sen. Dole, Pres. Ford, and Sen. McCain are all recent examples of this phenomena. It was their turn, I guess, to lose. The Democrats on the other hand, don't really seem to have that particular issue. One could argue that Sens Kerry and Gore were both examples of this, and yes, it apparently was their turn to lose too.
The second, and related possibly, mistake that Republicans make is to run a Liberal-Lite. A Liberal-Lite candidate is a big government Republican who is socially "progressive" and eschews conservatism. These mistakes may be related in some way as each of the above listed Republican presidential candidates were all Liberal-Lite too. When Republicans run these against the Democratic Full-bore Liberal, they lose every time. Just like Coke beats Diet Coke; Pepsi beats Diet Pepsi; Liberal beats Liberal Lite.