Former US President Barack Obama has issued a warning to Democratic presidential candidates, cautioning them against policies that are not "rooted in reality".
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 not mention any candidate by name nor criticise any specific policy proposal.
Instead, he used the appearance to urge Democrats to "pay some attention" to voters on issues such as health care and immigration.
These voters, Mr Obama said, did not necessarily have the same views as what he called "certain left-leaning Twitter feeds" or "the activist wing of our party".
The comments, which come less than four months before the Democratic primaries, represent one of Mr Obama's most pointed interventions in the race so far.
They may be seen as a critique of senators Sanders and Warren – widely seen as two of the most left-wing candidates in the field.
Both candidates have called for far-reaching political and economic change, including policies that would end private health insurance and decriminalise illegal border crossings.
But Mr Obama, who occupied the White House from 2009 to 2017, said the country was "less revolutionary than it is interested in improvement".
"Even as we push the envelope and we are bold in our vision, we also have to be rooted in reality," Mr Obama said at the meeting, reportedly attended by wealthy liberal donors.
The Democratic race is still largely up in the air even as the first of the state-by-state votes that will decide which of the contenders challenges Mr Trump for the White House looms in Iowa in February.
Some Democrats are concerned that Mr Biden, a moderate, will struggle to beat Mr Trump, prompting a flurry of latecomers to join the race.
In recent days Deval Patrick, the two-time former governor of Massachusetts, entered the field amid speculation that former New York Mayor Michael Bloomberg may follow suit.