Interesting historical fact. Make of it what you will.
Since 1952, there have been eight elected presidents who ran for re-election: Eisenhower, Nixon, Carter, Reagan, G. H. W. Bush, Clinton, G. W. Bush, and Obama. Six of them (all but Carter and the first Bush) were re-elected. That suggests that the odds are three to one (66.6%) that Donald Trump (elected in 2016) will be re-elected in 2020.
Breaking things down by party, Democrats are 2-1 in re-election and Republicans 4-1. That suggests that the odds are four to one (80%) that Donald Trump, a Republican, will be re-elected in 2020.
Note: Gerald Ford served as president, but he was not elected. What I'm searching for are cases in which the American people said "yes" to a person the first time and "no" the second time. The American people never said "yes" to Gerald Ford.
