The Economist, usually a fairly conservative, knee-jerk pro-business paper, endorsed its first D - Obama - in 2008. They have already trashed Trump and will probably endorse Clinton in 2016.
The Economist endorsed Kerry in 2004 and Clinton in 1992. It also declined to endorse Reagan in 1984 and Bush in 1988. None of that detracts from your point—which is interesting—but let's not be inaccurate. Also interesting is that The Economist has shown a tendency toward anti-incumbency and Obama is the only president they endorsed for a second term (out of only four opportunities, but still), largely due to Romney's pandering to the far right. (And that doesn't sound familiar at all...)
I was thinking this morning about how this will be the first election to have a sitting lame duck president campaign on behalf of the party since 1988. I wonder if that will make much difference, or if the increased partisan divide will negate any effect.