But still, here goes: If we go as far back as Richard Nixon, with two exceptions, when Republicans run for re-election to president, they get more votes than when they were first elected and the reverse is true for Democrats.
President, election votes, re-election votes in millions of votes. (Note: I collected the data quickly and may not have rounded perfectly, but should be within 50,000 votes)
Nixon, 31.7, 47.1
Carter, 40.8, 35.4
Reagan, 43.9, 54.4
GHWB, 48.8, 39.1 (exception to rule)
Clinton, 44.9, 47.4 (exception to rule)
GWB, 50.4, 62.0
Obama, 69.5, 65.9
Trump, 62.9, 74.2
First, my theory, then my explanation of the exceptions: My theory is that the mostly leftist press has a lot more power to define a candidate before they become president compared to after they’ve served a term. Before being elected, we take the press’ word for the character of the candidate, after being elected, we judge by their actions. Republicans tend to be better than advertised and Democrats worse.
There are two parts here: 1. The press will always be more critical of Republicans than Democrats. And 2. The power of the media is greater when there isn’t much to go on. Before a man becomes president, the national public doesn’t follow their day-to-day decision making and leadership style, but once they’re president, even if the press criticizes or praises every action, depending on the party the president belongs to, the public judges the actions by the actions in context and are not so moved by apologetics or criticism.
The exceptions: GHWB: Bush was a Republican elected after two terms served by Reagan. IMHO, he was a fine president but the public was tired of 12 years of Republican governance. Clinton: Clinton was an unusually skilled politician and his increase in votes was less than the gain seen by any Republican. Further, there was an unusually strong 3rd party run in Clinton’s original win, (Ross Perot) who got almost 20 million votes.