This commentary originally appeared on The Hill.
Democrats grappling with the shock of Hillary Clinton's loss to Donald Trump are also beginning to turn their attention to 2020, and pondering who could defeat Trump as he vies for reelection.
Here are The Hill's initial rankings of where the potential candidates stand.
1. Sen. Elizabeth Warren (Mass.)
How would the 2016 election have panned out had Warren challenged Clinton in the primary? That's one of the great unknowables of Democratic politics. But now, there is little doubt that the Massachusetts senator is the leading contender for the 2020 nomination.
Warren, a former Harvard Law School professor, has been beloved by the left throughout her late-blooming political career, largely because of her no-punches-pulled attacks on banks and the financial industry. She got under Trump's skin via Twitter during the 2016 campaign too.
The recent news that Warren will join the Senate Armed Services Committee in January has stoked speculation that she is looking to bolster her foreign policy and national security credentials in advance of a presidential run. Warren would be 71 by the time of the next election, but she is three years younger than Trump.
2. Sen. Bernie Sanders (I-Vt.)
Sanders came from semi-obscurity in the Senate to give Clinton a serious run for her money in the battle for the Democratic nomination this year.
He won 23 contests and amassed more than 13 million votes. He also fired the enthusiasm of young voters and progressives, two pillars of the Democratic base that Clinton struggled to charm.
The Vermonter's focus on income inequality and his broader point that the system is rigged against working Americans resonated. Sanders's main problem when it comes to a 2020 run could be his age. He will be 79 next Election Day. Still, Sanders might well be tempted to try one more time — especially if Warren stood aside.
3. Sen. Cory Booker (N.J.)
Booker raised eyebrows earlier this month when it emerged that he would join the Senate Foreign Relations Committee when the new Congress convenes. As with Warren and the Armed Services panel, his decision was interpreted as an effort to burnish his resume for a potential presidential run.
Booker is just 47, and he is one of only two African-Americans in the Senate for now. (That number will rise to three in January when California's Kamala Harris will be sworn in.)
He is also one of the most media-savvy members in the upper chamber — a trait that has been apparent since the start of his career, when his first, failed bid to become mayor of Newark was captured in a sympathetic documentary, "Street Fight."
Booker is far from the most liberal member of the caucus. During the 2012 presidential campaign, he criticized an Obama campaign ad that hit Mitt Romney's business record, insisting on NBC's "Meet the Press", "I'm not about to sit here and indict private equity."
An optimistic view is that he could bridge the gap between the progressive and center-left strands of the party. Skeptics will question whether he is a little too corporate-friendly for the tastes of Democratic primary voters.
4. Sen. Amy Klobuchar (Minn.)
Klobuchar has already appeared on several shortlists of likely contenders for the nomination, and it's not hard to see why.
The New Yorker called her, "popular, practical, appealing [and] progressive." She is from a state where the currents of labor and progressivism run strong. But the no-nonsense, affable Klobuchar could also plausibly appeal to Rust Belt voters whom her party needs to win over.
One issue for Klobuchar right now is that she does not have a high profile outside of her native state and the Beltway. There is plenty of time to change that if she wants to run and win in 2020. But she could be eclipsed by higher-wattage candidates.
5. Sen. Kirsten Gillibrand (N.Y)
Gillibrand followed in Clinton's footsteps when she replaced her as a New York senator in 2009. Could she do the same at the presidential level — but actually win the White House?
It's certainly possible. Gillibrand's profile has risen in tandem with her making the prevention of sexual assaults in the military a signature issue. Representing New York, she has easy access to the national media and to powerful Democratic fundraising networks.
But Gillibrand's similarities with Clinton, superficial though they may be, could go against her. It's just not clear Democrats would roll the dice again, as soon as 2020, on another prominent female nominee from New York.
Critics also charge that Gillibrand emphasized more centrist positions as a congresswoman from a somewhat conservative district than she does as a senator from a liberal state.
6. First lady Michelle Obama
If the first lady exhibited even a slight inclination to run, she would be ranked near the top of this list.
There is no figure in public life, with the possible exception of her husband, who has so strong a hold on liberal hearts and minds.
Obama has become more comfortable with her public role over the years. Her two major speeches during the 2016 campaign — one at the Democratic convention, another excoriating Trump for "hurtful, hateful language about women" — were among the most powerful delivered during the cycle.
The first lady insists that she won't run, citing the effect such an effort would have on her two daughters among other factors. But Malia and Sasha Obama will be 22 and 19, respectively, by the time of the next election. When it comes to the first lady's future plans, many Democrats still cling to the audacity of hope.
7. Gov. John Hickenlooper (Colo.)
Hickenlooper presides over a state that is considered a key battleground, even though it has become more solidly Democratic in recent years. Colorado has gone for the Democratic nominee in the past three presidential elections and Clinton won the state by five points.
Hickenlooper, who has a politically effective down-to-earth persona, could potentially boost the party's appeal in the heartlands. He has enjoyed solid approval ratings during his time in office.
One problem? While his chances are talked up among Beltway pundits, he is almost unknown in the nation at large.
8. Sen. Chris Murphy (Conn.)
Murphy has come to the fore on the issue of gun control. He can speak with moral authority on the issue: In his state, a gunman killed 20 young children, as well as six adults, at Sandy Hook Elementary School in December 2012. President Obama has called that moment the worst day of his presidency.
Politically speaking, Murphy would need to display more policy breadth and heighten his national profile if he is to be a genuine contender. For the moment, he's one to watch.
9. Vice President Joe Biden
The vice president could have definitively ruled himself out of the running, but hasn't. He joked with reporters about the possibility earlier this month, and then sought to clarify by saying he had "no intention" of running.
Biden would clearly have loved to run in 2016, were it not for the fact that he was still grieving the loss of his son, Beau. Biden's age is a real issue, however. He would be 77 by next Election Day. If he won, he would turn 78 before being inaugurated.
For all his political skills, his two previous runs for the presidency, in 1988 and 2008, ended in failure.
10. Gov. Andrew Cuomo (N.Y.)
On paper, Cuomo looks like a strong candidate. He is the governor of a huge, liberal state and hails from a well-established political family. Cuomo's late father, Mario, served as governor of the Empire State for three terms.
No one doubts the younger Cuomo's ambition, but whether he is the right fit for the times is a tougher question. In a party where the left is ascendant, he has positioned himself as a centrist foil to New York City's liberal mayor, Bill de Blasio. It's not clear what Cuomo's power base would be for a primary fight.
11. Sen.-elect Kamala Harris (Calif.)
Harris is one of the bright spots for Democrats who are dismayed by their failure to retake the Senate. She will succeed the retiring Sen. Barbara Boxer in January.
Harris has been seen as a rising star in the party for some time, her fans including President Obama, who once praised her in imprudent terms.
Harris, a leading lawyer before shifting into politics, is the daughter of an Indian-American mother and a Jamaican-American father. It's not clear she has any presidential ambitions and, if she ran in 2020, she would face criticism about her relative lack of political experience. But she would be as experienced as then-Sen. Obama was when he began his 2008 White House run.
12. Former Secretary of State Hillary Clinton
Could she run again? It's possible. Many people thought Clinton's electoral ambitions had ended in 2008, with her devastating loss to Obama in the Democratic primary. That turned out not to be the case.
There is still a large, wealthy circle of Clinton loyalists, who would back any future run. But, even if she had the appetite for a 2020 bid, she would have enormous hurdles to overcome.
One of the biggest would be the question of how she lost the presidency to Donald Trump. Beyond the hardline Clintonistas, there aren't many Democratic insiders who were wowed by her campaign. In a USA Today/Suffolk University poll released earlier this month, 62 percent of Democrats and independents said Clinton should not run again.
13. Former Gov. Deval Patrick (Mass.)
Patrick has considerable political skills and was once talked up as a potential inheritor of President Obama's mantle. David Axelrod, one of the aides closest to Obama, worked with Patrick as well, and both Patrick and Obama adopted "Yes We Can!" as a campaign slogan.
But Patrick left office in 2015, and it's just not clear whether he could — or would want to — come off the sidelines for 2020. He also joined Bain Capital, which is hardly the ideal launching pad for a quest to win over liberal activists.
14. Sen. Tim Kaine (Va.)
Kaine achieved a new national prominence when Clinton named him as her 2016 running mate. But his performance was a mixed bag.
The Virginia senator gave some energetic speeches on the campaign trail, defying his reputation for dullness. On the other hand, his showing in his sole debate with his counterpart, Indiana Gov. Mike Pence, was uneven at best.
15. Oprah Winfrey
Trump proved how powerful a currency celebrity can be — and there may be no more trusted celebrity in America than Oprah. Having steered largely clear of partisan politics for most of her career, Winfrey became an enthusiastic backer of Obama when he looked a long shot to beat Hillary Clinton to the 2008 nomination.
Winfrey has said she "couldn't breathe" after Trump won in November. She softened her stance later, but could she be tempted into a race to defeat the president-elect?