When George H.W. Bush ran for the presidency in 1988, he was running as the second-in-command to a still-popular Ronald Reagan. He beat Democrat Michael Dukakis handily. When his son ran in 2000, he was running against the second-in-command to a still-popular (more popular than Reagan, even) Bill Clinton. But he won.

Which raises the question: Next year, when a third Bush (or some other Republican) looks to succeed a two-term Barack Obama, how much does Obama's popularity matter?

The short answer, as the University of Virginia Center for Politics' Alan Ambramowitz wrote earlier this year, is that, historically, higher approval ratings for the outgoing president have correlated with a higher percentage of the popular vote for the member of his party looking to replace him. We'll note only now that George W. Bush won the presidency in 2000, but not the popular vote.

Ambramowitz figures that each 10 percentage points of approval for a president is worth about 1.8 percent of the popular vote for the candidate from his party. If Obama's at 50 percent approval, the eventual Democratic nominee, whoever she might be, might be expected to earn just more than half of the popular vote. If Obama's at 45 percent, Ambramowitz figures that Democrat is looking at just more than 49 percent of the vote.

What's particularly interesting is how much more strongly the president's approval rating in presidential years correlates to his party's success in those years than his approval in non-presidential years links to results in the House. There are a lot of reasons for this, including that we included the president's reelections in the data below, which naturally has a robust correlation. The difference is still dramatic, though.

Right at the center, where the two 50 percent lines overlap, is the election of 1976.

In other words, just because Obama was at 44 percent approval right before last year's midterms isn't necessarily why his party got clobbered. (It certainly didn't help.) In 1990, George H. W. Bush was much more popular, but Republicans got a lower percentage of the vote than Democratic House candidates did in 2014. (There are a lot of reasons for this, too, including that there were far more Democratic incumbents in 1990.)

A number of other large factors (like the strength of the economy) will weigh for which ever Democrat wins the party's hotly contested nomination. And some small ones will, too, but ... to a smaller extent. How Obama is viewed by this time next year is not a dealbreaker. But if he's at, say, 25 percent approval, the Post's post-Election-Day front page can probably be prepared in advance.