Former President Barack Obama isn't the only past Democratic leader who refuses to ride off into the sunset. Hillary Clinton reemerges Tuesday with a new book explaining yet again her loss in the 2016 presidential election — without much insight into how they can win next time.

Some Democrats worry that the continued prominence of both Obama and Clinton will overshadow a new generation of their party's leaders and rising stars. Both have already contributed to the thinning of the Democratic bench: Obama presided over massive down-ballot losses while Clinton's defeat last November left Republicans with unified control of the federal government.

"She should be behaving as an elder statesman, not a political critic," a Democratic strategist told me during a previous wave of Clinton recriminations earlier this year. "It's very selfish for Democrats who want to move the party forward and into the future."

On that front, Clinton is in a league of her own. Obama's efforts with former Attorney General Eric Holder at the National Democratic Redistricting Committee are forward-looking and designed to alleviate structural problems that have plagued Democrats in recent midterm elections, and could perhaps dog them again in 2018.

Clinton keeps focusing on the past, last year's campaign, which ended with a result that was very traumatic for much of the Democratic base: the election of President Trump. While many Democrats view the former first lady, secretary of state and two-time presidential candidate as a leader in good standing of the Resistance.

As popular as Clinton remains among many liberals, she has also become a divisive figure inside her own party. She recovered from the bitterly divisive primary fight against Obama in 2008, in no small part because Obama brought her into the fold as his first secretary of state, but the wounds from her surprisingly competitive battle against Sen. Bernie Sanders, I-Vt., are still fresh.

In fact, Clinton devotes space in What Happened to pouring salt in those wounds. "His attacks caused lasting damage, making it harder to unify progressives in the general election and paving the way for Trump's 'Crooked Hillary' campaign," she writes of Sanders.

While Clinton certainly took considerable friendly fire from Sanders supporters, the septuagenarian senator turned unlikely hero to liberal millennials was actually slow to attack her and said he was sick of hearing about her " damn emails." This reticence didn't last throughout the primaries but Sanders wasn't as tough on Clinton as Clinton was on Obama.

Moreover, there is a case to be made that Sanders was a warning to Clinton that she wasn't populist enough, that a number of Democrats remained steamed about her vote for the Iraq war, that 1990s were no longer necessarily seen as a golden age, and that she could effectively be tied to her husband's more centrist brand of Democratic politics even when she had little direction connection to the policies in question.

All these things, as much as "Crooked Hillary," hurt her against the populist Trump in the general election.

Most of the Democrats flirting with 2020 presidential campaigns are running not in the mold of Clinton, but emulating Sanders. This includes not only unabashed liberals like Sens. Elizabeth Warren, D-Mass., Jeff Merkley, D-Ore., and Sherrod Brown, D-Ohio, but also officials whose records may draw scrutiny from progressives, like Sens. Cory Booker, D-N.J., and Kamala Harris, D-Calif.

Even New York City Mayor Bill de Blasio is the subject of presidential speculation. An aging socialist with a loose affiliation with the Democratic Party running better than expected in 2016 has created a new sense of the possible.

To the extent at Clinton is seen as retarding this leftward trend, or even chasing behind it, bad blood between her supporters and Sanders' persists. "We get lost in a debate between the Clinton and Sanders voters," Clinton ally Neera Tanden complained in Politico. "I hope sometime in my lifetime the 2016 Democratic primary will be over."

Asked about the Clinton book, Tanden is later quoted as saying, "I think Hillary has a right to discuss what happened in the campaign, but I actually hope that we can quickly move on to the fights in front of us."

Obama, by contrast, is still a unifying figure inside the Democratic Party who won twice. He can paper over differences between the Clinton and Sanders factions, which have been continued in a handful of primaries this year and the race for chairman of the Democratic National Committee.

Like Clinton, Democrats are angry about "what happened" in 2016. The party's future, however, likely belongs to someone more interested in what happens next.