Democrats are worried that the fight between former presidential nominee Hillary Clinton Hillary Diane Rodham ClintonJeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Momentum growing among Republicans for Supreme Court vote before Election Day Warning signs flash for Lindsey Graham in South Carolina MORE and Sen. Bernie Sanders Bernie SandersNYT editorial board remembers Ginsburg: She 'will forever have two legacies' Two GOP governors urge Republicans to hold off on Supreme Court nominee Sanders knocks McConnell: He's going against Ginsburg's 'dying wishes' MORE (I-Vt.) is doing lasting damage to their party.

As the bickering between the former rivals grows more intense, Democrats say it needs to end.

“It reiterates that our party is still led by people with fatal flaws,” said one former senior Obama administration official.

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Another former Obama official added, “It’s like watching two children bickering.”

The only winner from the infighting, they say, is the Republican president.

“Donald Trump Donald John TrumpUS reimposes UN sanctions on Iran amid increasing tensions Jeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Trump supporters chant 'Fill that seat' at North Carolina rally MORE must love the backbiting between Sanders and Clinton,” said Democratic strategist Brad Bannon.

“The two of them and their supporters have lined up in a circular firing squad at a time when our guns should be trained on Trump,” Bannon added. “The infighting indicates that we have not learned the lesson from 2016 that should apply to 2020.”

The fight began last week with the release of Clinton’s book, “What Happened.”

In the memoir, Clinton said Sanders “certainly shared my horror at the thought of Donald Trump becoming president, and I appreciated that he campaigned for me in the general election.

“But he isn’t a Democrat — that’s not a smear, that’s what he says,” she said. “He didn’t get into the race to make sure a Democrat won the White House, he got in to disrupt the Democratic Party.”

Clinton has piled on during her book tour, which served as a reminder of the bitterness between the Clinton camp and her rival during the 2016 primary.

“The political sin he committed was the failure to move quickly to unify the party and his supporters,” Clinton told CNN’s Anderson Cooper last week, contrasting her rival’s behavior with how she said she helped unite the party on the heels of her contentious primary fight with Barack Obama Barack Hussein ObamaMomentum growing among Republicans for Supreme Court vote before Election Day Warning signs flash for Lindsey Graham in South Carolina Majority of voters say Trump should not nominate a Supreme Court justice: poll MORE in 2008.

Sanders fired back on Sunday with a simple “wow” in an interview on NBC’s “Meet the Press.”

“I worked as hard as I could after endorsing Hillary Clinton,” he said. “I went all over this country.”

In the final week of the campaign, for example, Sanders appeared at 17 events in 11 states for Clinton.

On Monday, Sanders's allies defended him.

“I’ve admired Bernie’s restraint and I think his remarks were right on,” said one person close to Sanders. “He personally did everything he could. It was nonstop for him. It’s not Bernie’s fault she lost to Donald Trump. Bernie didn’t tell her to stay out of Wisconsin.”

The Sanders ally pointed to the senator’s recent book, “Bernie Sanders Guide to Political Revolution” and how it refrained from attacking Clinton.

“It was a guidebook on where Democrats go from here,” the ally said.

Still, the ally acknowledged the recent bickering isn't helpful: “The last thing the Democratic Party needs right now is to be refighting the primary battle of 2016. It’s a tremendous disservice to the party.”

A Sanders spokesman did not want to comment. A spokesperson for Clinton did not reply to a request for comment for this story.

But a Clinton ally said the former Democratic nominee has “every right to air her grievances.”

“He did in fact contribute to her loss,” the ally said. “I think her comments are fair game.”

In the meantime, other Democrats are cringing at the ongoing spat.

“In truth, virtually all of the most respected Democrats in America would rather have root canal work done by a sadistic dentist than watch Hillary Clinton attack Bernie Sanders at this moment in American history,” said Brent Budowsky, a former Democratic aide and a columnist for The Hill.

“Democrats need a convergence of unity from those who love Bernie, those who love Hillary, and those who love both of them.

“Fortunately for Democrats, after a few more Trump tweets — and while voters worry about potential war in Korea, and the future of the Dreamers, health care and the tax code are on the table — the unfortunate interlude of Hillary criticizing Bernie will soon be forgotten,” he added.

Still, some political observers say the Clinton-Sanders infighting could be a blessing in disguise.

“When Democrats are struggling to really define their agenda and put forth a compelling argument for the next set of elections, it is better that they hash this out now rather [than] in a couple of years,” said Julian Zelizer, a professor of history and public affairs at Princeton University.

“Some of the debates are good ones and reflect unsettled questions that the Democrats have to make some decisions on, like health care. Republicans have had many bitter fights about key issues, they still do, and they have managed to do well. So divisions are difficult for a party but they can be harnessed in positive ways.”

But that needs to happen sooner rather than later, said Bannon.

“The internal divisions cost Democrats in 2016 and we’ll pay the price in 2020 if we don’t get over the past and move forward,” the strategist said.