WASHINGTON — Senator Elizabeth Warren carefully avoided picking sides in the 2016 Democratic presidential primary, winning a place on Hillary Clinton’s vice presidential short list for her restraint.

But now, a full 17 months after Clinton secured the Democratic presidential nomination, Warren has weighed in on a controversy that hung over the Democratic primary: She says the nominating contest was “rigged’’ in favor of Clinton after all. Just like the progressives who backed Vermont Senator Bernie Sanders said it was.

Warren made the surprising allegation in three of the five national TV interviews she granted Thursday, responding to charges in a new book by former acting Democratic National Committee chairwoman Donna Brazile that the party’s supposedly neutral apparatus was actually controlled by Clinton’s Brooklyn headquarters.

“We recognize the process was rigged, and now it is up to Democrats to build a new process, a process that really works, and works for everyone,’’ Warren said to PBS’s Judy Woodruff.

It’s an unexpected foray into an intraparty dispute that the Massachusetts Democrat had previously ignored, at least publicly. Observers said it shows both the power that the progressive wing of the party holds at the moment and the intense interest that Warren has in shoring up good will from the Sanders voters.

Warren, if she decides to run for president in 2020, is seen by many in the Democratic coalition as the rightful heir to the Sanders army of support, if he opts not to seek the nomination again. But she would have to overcome some lingering resentment from his backers, known as Bernie-crats and Bernie bros, because she sat on her hands in 2016 instead of cheerleading for his insurgent candidacy.

“There is a Bernie donor base that’s very important if you want to be in national political office. This is a play for that,’’ said one former senior Sanders campaign strategist, who requested anonymity to avoid offending Warren. “It’s a calculation. Repairing that relationship is more important than poisoning the well with the Clintonites.’’

Warren also appeared in an unusual joint video with Sanders Thursday to discuss their shared opposition to the Republican tax plan. “I’m going to call you Larry David,’’ Warren quips to Sanders at the top, referring to the “Seinfeld’’ creator and “Curb Your Enthusiasm’’ star who has played Sanders on “Saturday Night Live.’’

Throughout the video, they nod and listen to each other level criticisms at the Republican Party for, as they put it, squeezing the middle class in favor of the wealthy.

“So I’m in this fight, how about you, Bernie?’’ Warren asks.

“We’re there,’’ Sanders replies.

Warren declined to answer questions from the Globe about when she came to believe the Democratic primary was rigged, and why she is talking about it now.

A Warren aide, speaking on condition of anonymity, said Warren respects Brazile, who leveled the charge in a Politico op-ed Thursday previewing her new book, “Hacks: The Inside Story of the Break-Ins and Breakdowns That Put Donald Trump in the White House.’’

The Warren aide said the senator was particularly concerned by Brazile’s characterization of a joint fund-raising agreement with the Democratic Party, dating to 2015, that allowed Clinton to control cash that was coming into the party from her donors. The agreement allowed her campaign and the DNC to coordinate large-scale fund-raising through the national and state parties. Under the agreement, Clinton’s campaign controlled that money.

Sanders signed a similar agreement, but didn’t use it to raise money, in part because his fund-raising strategy involved small, Internet contributors. He did not have the support of the sort of donors who would give large sums through state Democratic parties. Had Sanders won the Democratic nomination, he would have had control of the money Clinton raised for the election.

Sanders backers also complained during the primary about other ways the party establishment had its thumb on the scale for Clinton, including the outsized role of so-called super delegates, or party elders, who have automatic delegate votes toward the nomination.

The allegation about Clinton’s fund-raising arrangement isn’t new. Politico had reported on it in May 2016.

When MSNBC’s Chuck Todd asked Thursday for her reaction to Brazile’s piece, Warren also sided with Sanders: “Donna Brazile tells about something that’s really terrible. . . . Bernie and the representatives of Bernie Sanders have to be satisfied that the rules going forward are rules that are fair for everyone, and that the people, that the Democrats, the Democratic folks across this country, will be the ones who elect our next nominee.’’

Warren assiduously struck a more neutral tone during the nomination fight. In her April 2017 book, “This Fight Is Our Fight,’’ she wrote, “People put some pressure on me — actually, a lot of pressure — to take sides,’’ Warren wrote. “But I kept to my path and just kept doing my job.’’

At least some in Clinton’s world were not upset to see Warren take Sanders’ side now, since Clinton lost and it is advantageous for Warren to avoid ire from the left wing of the party.

“Without getting into the details of whether it’s right or wrong, siding with Bernie yesterday was clearly the path of least resistance,’’ said one former Clinton campaign staffer.

Stu Loeser, a Democratic strategist, said Warren was acting as if she doesn’t need the support of the Beltway-based Democratic National Committee bureaucracy.

“In an era in which you don’t need a grocery store to get groceries, or a car to drive around, she’s betting you don’t need the party to win the party’s nomination,’’ said Loeser. “She might be right.’’

Hank Sheinkopf, a New York-based Democratic strategist, said the revival of the Clinton-Sanders primary dispute shows the Democratic Party is in dire straits, just like the GOP, breaking down into factions warring over power, not principle.

“What Donna Brazile did is not productive for democracy,’’ he said. “It reduces faith in parties, makes people believe that everything’s fixed, creates division among Democrats that was unneeded and gives the opposition a reason to smile.’’

Indeed, President Trump took to Twitter on Friday keep the national conversation focused on Democratic dysfunction in 2016.

“The real story on Collusion is in Donna B’s new book. Crooked Hillary bought the DNC & then stole the Democratic Primary from Crazy Bernie!’’ Trump wrote.

He also called out Warren, referring to her by a nickname mocking her claims to Native American heritage.

“Pocahontas just stated that the Democrats, lead by the legendary Crooked Hillary Clinton, rigged the Primaries! Lets go FBI & Justice Dept,’’ Trump wrote Friday morning.

Warren responded to his comments on social media: “I understand your desperation to change the subject, @realDonaldTrump. Your campaign mgr was just indicted for conspiracy against the US,’’ she wrote. “The DNC shouldn’t play favorites. But that’s a whole lot different from illegally conspiring with Russia. The FBI knows the difference.’’

Annie Linskey can be reached at annie.linskey@globe.com. Follow her on Twitter @annielinskey. Victoria McGrane can be reached at victoria.mcgrane. Follow her on Twitter at @vgmac.