Former President Barack Obama—who vowed not to endorse any candidate during the primary—congratulated his former vice president, Joe Biden, over his solid win in South Carolina on Saturday, which injected life into his campaign and ended Mayor Pete Buttiegieg's run.

Bloomberg News, citing two people familiar to the call, reported that the former president plans to let the primaries play out, and then work to unify the party and defeat President Trump.

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Biden suffered poor results in Iowa's caucuses and New Hampshire’s primary, but he captured 48 percent of Democratic presidential primary voters in South Carolina, crushing Sen. Bernie Sanders by nearly 30 percentage points in the process.

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Biden, who appeared emboldened, took a swipe at one of Sanders' signature lines during an appearance on "Fox New Sunday": "The people aren’t looking for revolution. They’re looking for results."

Obama’s decision not to endorse a candidate during the primary is a deliberate one. The Bloomberg report said the phone call on Saturday night, although falling short of an endorsement, shows that the two are still close.

Biden has been known to talk about his former boss while on the campaign trail. But if their relationship is still close, it appears feelings with David Axelrod, Obama's former top strategist, has cooled.

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Axelrod has in the past described Biden as “confused” and suggested he looked like “part of the past rather than the future,” after a heated exchange during a June debate between Biden and Sen. Kamala Harris, D-Calif.

Fox News' Paul Steinhauser, Ronn Blitzer and the Associated Press contributed to this report