Mr Obama said Democrats risked alienating voters if they lurched too far to the left politically.
The former president, speaking at a fundraising event, said most voters didn't want to "tear down the system".
Mr Obama is yet to publicly back a Democratic candidate.
The field is crowded, with 18 Democrats vying for the nomination to take on Republican President Donald Trump in the 2020 election.
The frontrunners are former Vice-President Joe Biden, senators Elizabeth Warren and Bernie Sanders, and Pete Buttigieg, the mayor of South Bend, Indiana.
At the event held in Washington on Friday, Mr Obama did ...
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