How popular is Joe Biden?

If it’s possible to sum up a presidency in a single number, that number would be the president’s approval rating — or the share of Americans who approve of the job he’s doing. Arguably, that simple percentage can determine the fate of an entire presidency.

For instance, a high approval rating can marshal support for a president’s agenda and minimize his party’s losses in the midterm elections — not to mention help the president himself win reelection. But a low approval rating can be electoral poison and imply that a...