The most influental U.S. presidential loser of the 20th Century?

[FEMA Camp Administrator]

Goldwater.

He shifted the GOP back to Cosnervatism, became a Cosnervative icon, and, don't forget this, ended up inspiring Reagan to get into politics, which led to something called the "Reagan Era", which shifted America back to the right, more to pre-FDR alignment. Reagan caused a more moderate candidate like Clinton to win the Democratic nomination instead of a genuine, bonafide Liberal, and thus, this all leads back to Goldwater's devastating loss in 1964.

[FEMA Camp Administrator]

Despite all the ardent arguing for Goldwater, there may be another...

According to wikipedia, Al Smith helped for the "New Deal Coalition" of workers and immigrants that helped FDR get elected. I this is accepeted as true, then Al Smith may be the most influential loser, having casued subsequent success for the Democrats for five straight terms, as well as helping America move more to the Left between 1932 and 1980. However, that's only if what Wikipedia said is true about the "New Deal Coalition".