WASHINGTON — As the election approaches, President Joe Biden is making regular calls to former President Barack Obama to catch up on the race or to talk about family. But Obama is making calls of his own to Jeffrey Zients, the White House chief of staff, and to top aides at the Biden campaign to strategize and relay advice. This level of engagement illustrates Obama’s support for Biden, but also what one of his senior aides characterized as Obama’s grave concern that Biden could lose to former P