Barack Obama has entered the fray of the 2020 presidential campaign with a series of thinly veiled criticisms of his successor as he prepares to help his former vice president, Joe Biden, win back the White House for the Democrats in November.

President Trump has responded by stepping up his own criticism of Obama, which has been plentiful over the past four years. Biden has, by contrast, hugged Obama tightly since entering the race, making his service under the 44th president a key part of his argument to Democratic primary voters — a strategy that paid dividends as the 77-year-old revived his once-moribund campaign and became his party’s presumptive nominee.