Though Obama did not mention anyone by name, the message delivered before a room of Democratic donors in Washington on Friday was a clear word of caution about the candidacies of senators Elizabeth Warren and Bernie Sanders.

The two have called for massive structural changes - and in Sanders' case "revolution" - that would dramatically alter the role of government in people's lives.

The centrist wing of the party has warned for months that a far-left nominee could alienate moderate Republicans and independent voters needed to oust President Donald Trump.

"The average American doesn't think we have to completely tear down the system and remake it. And I think it's important for us not to lose sight of that," Obama said.

"There are a lot of persuadable voters and there are a lot of Democrats out there who just want to see things make sense. They just don't want to see crazy stuff. They want to see things a little more fair, they want to see things a little more just. And how we approach that I think will be important."

Obama has largely refrained from publicly opining on the Democratic primary, which has exposed a growing rift between an ascendant progressive wing of the party and old-guard centrists like his former vice president, Joe Biden.

But on Friday he said he felt compelled to weigh in because some of the loudest and most strident voices, particularly on social media, aren't representative of where most in the party are at.

Immigration and health care are two issues he cited as cases where Democratic candidates are out of sync with public sentiment.

"Even as we push the envelope and we are bold in our vision, we also have to be rooted in reality and the fact that voters, including the Democratic voters and certainly persuadable independents or even moderate Republicans, are not driven by the same views that are reflected on certain, you know, left-leaning Twitter feeds," Obama said.

Obama delivered his remarks at a gathering of the Democracy Alliance, a group of wealthy Democrats who raise large sums for the party.

He was interviewed by Stacey Abrams, a rising star in the party who narrowly lost the Georgia governor's race last year.

He also sought also to ease jittery Democrats who have been wringing their hands over the size of the sprawling field, which some worry will lead to a prolonged contest that will leave the eventual nominee with limited time to prepare for the general election.

"I just have to remind you that I had a very robust primary," Obama said. "Not only did I win ultimately a remarkably tough and lengthy primary process with Hillary Clinton, but people forget that even before that we had a big field of really serious, accomplished people."

Australian Associated Press