Vermont Sen. Bernie Sanders, making his second Democratic presidential nomination bid, has spent most of his nearly three decades in Congress as an independent. And that's not endearing him to some Democrats.

Some party activists have long memories about Sanders, recalling his criticism of President Barack Obama for not pursuing policies further to the left, as advocated by Sanders, a socialist first elected to the Senate in 2006 after 16 years in the House. Sanders was an independent throughout those years, and he only became a Democrat to pursue the party's nomination in 2016, which he lost to Hillary Clinton.

Democratic skeptics of Sanders cite a July 2011 radio interview with liberal host Thom Hartmann. During that appearance, Sanders blasted Obama over a deal with House Republicans on the debt ceiling. Sanders then suggested another Democrat should primary the president in his upcoming reelection fight.

"So my suggestion is, I think one of the reasons the president has been able to move so far to the right is that there is no primary opposition to him and I think it would do this country a good deal of service if people started thinking about candidates out there to begin contrasting what is a progressive agenda as opposed to what Obama is doing," Sanders said. "I think it would be a good idea if President Obama faced some primary opposition."

The comments came as a shock to the Obama campaign, who already saw the upcoming race as far from an assured victory, given the country's bumpy economic recovery.

[ Previous coverage: DNC to ask Bernie Sanders to affirm he's a Democrat]

"Obama world is still reeling from Sanders' call for a primary. You know, we've actually won elections," a former senior Obama campaign official told the Washington Examiner. "A lot of those who worked on that campaign and the White House felt that Bernie's been spending most of his time attacking Obama. Obama was a Reagan-like figure for the Democratic Party, so virtue signaling isn't going to win you the primary."

Sanders would go on to face questions about his comments during the 2016 Democratic presidential primary. In November 2015, he dismissed the idea that he had any hostility against Obama as "media stuff."

"The idea that I've worked against Barack Obama is categorically false," Sanders said during an interview with ABC.

While the dynamics of the 2020 race differ greatly from 2016, Sanders is in some ways more vulnerable to attacks that he isn't a loyal Democrat.

It was only in March of last year that Sanders officially signed a pledge, imposed by the Democratic National Committee, to become a member of the Democratic Party if he decided to run again. In his filings to run for reelection in the Senate in 2024, he's still registered as an independent.

In the 2020 race, Sanders is trying to take down Democratic front-runner Joe Biden, Obama's vice president. Sanders has hardly been playing the Democratic insider game, which requires requisite praise for Obama.

"Sanders' team played a little bit with the Democratic establishment early on in the primary when Sanders was leading the polls, but they don't really see the return on investment anymore in trying to court those kinds of people," said Tim Lim, who worked on the fundraising team for Obama's 2012 reelection campaign. "They've been crying foul during this primary season, just like last primary season. It's getting old."

Another Obama campaign official, who declined to be named, agreed with Lim's assessment.

"People in politics have long memories," the individual said, referring to Sanders' attacks on the Obama administration over the years. "There are still voters and operations who blame Bernie for Hillary Clinton's loss in 2016. Speaking to people involved in [Sanders'] campaign, though, I don't think they really care."

Sanders' polling, aside from a few outliers, has plateaued. A RealClearPolitics average of polls has Sanders nearly tied with Elizabeth Warren in second place at 16.7% support.

Many of his supporters in 2016 have said it's time to move on.

"We need to approach things differently this time around," Ron Abramson, a 2016 New Hampshire delegate for Sanders, who has now thrown his support behind Warren and hosted a campaign event at his house for her in July, said. "In 2015, we had very limited choices. This time, we have a plethora of choices. Warren is offering a far more comprehensive and newer campaign."

Abramson said, "Maybe Bernie hasn't changed, but the times have."