New York (CNN) More than two years since President Donald Trump took office, Democrats are still searching for ways to explain what went wrong in 2016.

Now, as the party's 2020 primary field takes shape, a long-simmering debate over the last election has turned into a political minefield for the candidates hoping to win the next one.

Hillary Clinton's defeat sent shockwaves around the world. But even as Democrats rallied early on to resist the Trump agenda, often forging alliances across ideological lines to defend programs like the Affordable Care Act, internal disputes over how and why Clinton fell short on Election Day -- despite winning the popular vote -- have persisted, most pointedly in Democratic circles on social media, where a consensus remains elusive.

The party's political message in 2019 sounds a lot more like the one offered by Sanders during the 2016 primary, but those victories haven't dampened some progressives' taste for online battle with Clinton aides and loyalists -- many of whom are equally eager to engage. It's a dynamic many on both sides will concede, at least in private, descended long ago into a self-defeating war of attrition.

Pete Buttigieg, the mayor of South Bend, Indiana, is the latest Democrat with designs on higher office to step boldly -- or stumble, depending on your point of view -- into the fight.

In an interview with the Washington Post published shortly before he launched his presidential exploratory committee, Buttigieg offered his abbreviated 2016 autopsy report, saying, "Donald Trump got elected because, in his twisted way, he pointed out the huge troubles in our economy and our democracy. At least he didn't go around saying that America was already great, like Hillary did."

Two-and-half months later, with his name ticking up the primary charts, Buttigieg felt the backlash.

In late March, Clinton adviser and spokesman Nick Merrill tweeted out the quote in a pique . "This is indefensible. @HillaryClinton ran on a belief in this country & the most progressive platform in modern political history," Merrill wrote. "Trump ran on pessimism, racism, false promises, & vitriol. Interpret that how you want, but there are 66,000,000 people who disagree. Good luck."

This is indefensible. @HillaryClinton ran on a belief in this country & the most progressive platform in modern political history. Trump ran on pessimism, racism, false promises, & vitriol. Interpret that how you want, but there are 66,000,000 people who disagree. Good luck. https://t.co/N3yRgHPVDL — Nick Merrill (@NickMerrill) March 30, 2019

Confronted with the criticism from Merrill and other former Clinton staffers, Buttigieg said he has "enormous respect" for the former Secretary of State, but thinks she was "ill-served by a strategy and a media environment" that made the election "much more about all the problems with... Donald Trump and much less about the concerns of voters."

It is a familiar criticism to most Democrats, but time has not taken off the sting -- nor offered a clear explanation for how the candidates running to take on Trump in 2020 can frame, or even discuss, what happened three years ago in a way that doesn't risk setting off another round of fiery recriminations.

"What was disappointing about Buttigieg is he, more than most of the other candidates, has gotten so much right in his ability to talk philosophically about the party and the country, articulate what people are feeling, where we are in the arc of history, and what challenges we need to be responsive to," Merrill told CNN this week.

The worry, he added, is that the Democratic candidates get mired in navel-gazing punditry while Trump continues his straight-to-the-gut pitch to voters.

"Shorthanding it by scapegoating Hillary Clinton is most alarming," Merrill said, "because it is a failure to recognize what's ahead, and if candidates don't understand those challenges and come up with ways to deal with them, then we are all in trouble."

Another reason Clinton's legacy continues to loom over the 2020 race is that the former Secretary of State remains engaged -- albeit, behind the scenes -- in the primary process. To date, Clinton aides tell CNN, the 2016 nominee has met with former Vice President Joe Biden, Sens. Amy Klobuchar, Kamala Harris, Cory Booker, Elizabeth Warren, former Colorado Gov. John Hickenlooper and Los Angeles Mayor Eric Garcetti about the campaign. Garcetti has since declined to run. Biden is expected to announce his decision about running later this month.

Most of the frontrunners have carefully steered clear of any hard analysis, or even implicit criticism, of Clinton's campaign. Others, though, have been more willing to engage. In February, Klobuchar followed her campaign kickoff speech by telling reporters, "We're starting in Wisconsin because, as you remember, there wasn't a lot of campaigning in Wisconsin in 2016."

"With me," she said, "that changes."

Sen. Amy Klobuchar announces her presidential bid in front of a crowd gathered at Boom Island Park on February 10, 2019 in Minneapolis, Minnesota. Klobuchar joins a crowded field of Democrats vying for the 2020 nomination.

Sanders, without naming Clinton, frequently discusses 2016 on the 2020 trail. But his remarks are often couched in a broader argument about the increasing mainstream acceptance of leftist policy priorities. Sanders allies' successful campaign to change Democratic National Committee rules they believe hamstrung his first presidential campaign are often wrapped, as evidence of his movement's power, into the same riff. At a rally in Iowa last month, Sanders was interrupted by applause from supporters as he touted a DNC reform that will diminish "the power of superdelegates at the Democratic National Convention" by stripping them of votes on the first nominating ballot.

The charged online environment can also provide opportunities for lesser known candidates. Hickenlooper won praise from Clinton devotees when he described her in an interview as "one of the smartest policy experts on almost anything" and slammed the FBI for "(making) allegations ten days before the election that turned out to be baseless."

After a Twitter user thanked Hickenlooper for the comment, the governor responded , "There's no need to thank me, I'm just speaking the plain and honest truth... Clinton faced historic interference and obstacles in her campaign, and still won the popular vote."

There's no need to thank me, I'm just speaking the plain and honest truth. @HillaryClinton faced historic interference and obstacles in her campaign, and still won the popular vote. She's a brilliant mind, a caring individual and a shining example to everyone in our party. https://t.co/j4gjgVERw3 — John Hickenlooper (@Hickenlooper) March 28, 2019

But the social media discussion is typically less polite and, at its most divisive, often driven as much by by personal animus as any substantive policy debate. For some, the bickering threatens to obscure more deeply rooted concerns.

"Every campaign does some things wrong. The bigger issue here is that Democrats keep winning the popular vote and keep losing the presidential," Rebecca Katz, founder of the progressive New Deal Strategies, told CNN. "We have to get serious about our broken democracy, and we can't do that if we're pointing fingers."

An increasing number of top-tier Democratic campaigns have begun pivoting in that direction -- potentially laying the groundwork for a less contentious way of talking about 2016. Candidates like Harris, Warren, Booker and Sanders have all pitched or floated fundamental reforms to the current election system.

Republicans have won the popular vote in only one of the last five presidential elections -- George W. Bush in 2004 -- but claimed the White House three times in that span. Like Buttigieg, Warren, speaking at a CNN town hall last month, called for the abolition of the Electoral College. Harris and Booker have signaled their potential support for the idea. Sanders has said it was time to "change" the system.

"Trump received 2.5 million fewer votes than Clinton, yet he'll soon be president," Sanders tweeted in December 2016. "Clearly, in a democratic society, this shouldn't happen."

Still, like other progressives who have found themselves caught up in testy Trump era skirmishes, Katz said she is tired of the potshots and worried that, as the primary progresses, a dynamic reminiscent of 2016 could emerge.

"I wonder, if it's going to be Bernie versus somebody else, if the entire establishment is going to rally around somebody else just because they're not Bernie -- and what that will do to the party," she said. "I think about it a lot."