There was a threat about this a few months ago:
https://uselectionatlas.org/FORUM/index.php?topic=188084.0
Republicans in 2020:
Tom Cotton
Marco Rubio (Assuming he doesn't get the nomination in 16')
Rand Paul (Assuming he doesn't get the nomination in 16')
Scott Walker (Assuming he doesn't get the nomination in 16')
Ted Cruz (Assuming he doesn't get the nomination in 16')
Kelly Ayotte
Paul Ryan
Brian Sandoval
Going back 60 years, there have been 5 Republican non-incumbents who lost the general election: Nixon 60, Goldwater 64, Dole 96, McCain 08, Romney 12. Of those, Goldwater lost by over 20 points and 400 electoral votes. Dole and McCain, had they won next time around, would have turned 80 in their first term. So none were viable for the next cycle.
Nixon though was elected president next time he ran and Romney was leading most 2016 GOP polls and was a viable candidate. If Scott Walker (or Marco Rubio) is the 2016 nominee, they're well-positioned to be a or the frontrunner again in 2020, even with a general election loss up to 5 points. Cruz or Paul would be less likely because party leaders and FOX would gleefully pile on blaming the nominee of blowing it in a way they wouldn't with Walker (or Rubio). (If Bush loses the general election by the much, making it 3 moderate Republican losers in a row, the conservative wing is well-poisitioned for 2020.)
In other words, Scott Walker is in a great position to be the GOP's 2020 nominee if he is a strong 2016 runner-up but even if he is nominated and loses to Hillary. Unless he turns in a humiliating performance which feels like a distinct possibility.
