History shows that when an incumbent President faces a serious challenge for the nomination from within his own party, he loses. Being forced to run a campaign to be re-nominated drains resources from those that would otherwise be spent running for re-election in the general campaign. In many ways it neutralizes many of the inherent advantages that incumbency offers. The fact that Buchanan challenged Bush for the Republican nomination contributed to Bush's defeat in 1992.
Same could be said of Reagan and Ford in 76.