Presenting the 2020 Democratic bracket

With Vice President Biden's entry this week, there are more than 20 Democratic presidential candidates. Only a handful will be serious contenders when Iowa and New Hampshire voters go to the polls early next year.

To sort out how it might evolve, a good model is the  college basketball tournament — divide aspirants into the following brackets or lanes with the underlying rationale for each:

The governing lane. As well as defeating Donald TrumpDonald John Trump Former US ambassador: 'Denmark is not a big fan of Donald Trump and his politics' Senate Democrats push for arms control language in defense policy bill Detroit county sheriff endorses Booker for president MORE, it's also critical to elect someone with executive experience who can get things done.

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The Vice president, with eight years serving with Barack ObamaBarack Hussein Obama'Forever war' slogans short-circuit the scrutiny required of national security choices Which Democrat can beat Trump? Middle East scholars blame Trump for an Iran policy 40 years in the making MORE and an extensive background as a Senator, is the best credentialed and the heavy favorite in this bracket. If he falters, as the 76-year-old political veteran did in two previous presidential runs, possible beneficiaries might be Governors: former Colorado chief executive John HickenlooperJohn HickenlooperPoll: Trump trails three Democrats by 10 points in Colorado The Hill's Campaign Report: Battle for Senate begins to take shape The Hill's Morning Report — Trump and the new Israel-'squad' controversy MORE or Washington state's Jay InsleeJay Robert InsleeInslee unveils climate plan to support rural communities Harris to appear in CNN climate town hall after backlash Castro qualifies for next Democratic primary debates MORE.

Others who could fill this void, if they got in, would be two successful mayors: Mitch Landrieu of New Orleans or New York's Michael Bloomberg. The hour probably is too late.

The left lane. Much of the energy in the party is on the left, their champion is Bernie SandersBernie SandersSenate Democrats push for arms control language in defense policy bill Top adviser on Sanders: 'He's always been underestimated' 'The Simpsons' pokes fun at Trump's feud with 'the squad' MORE. The Vermont Senator runs strongly in the polls, along with Biden, well ahead of the pack at this early stage, and in grass roots fund-raising.

The prospect of the 77-year-old Socialist as the nominee, or even setting the agenda, terrifies mainstream Democrats.

The only alternative in a bracket that will make the Democrats' Final Four is Massachusetts Sen. Elizabeth WarrenElizabeth Ann WarrenSenate Democrats push for arms control language in defense policy bill Top adviser on Sanders: 'He's always been underestimated' 'The Simpsons' pokes fun at Trump's feud with 'the squad' MORE, a decidedly liberal capitalist. She has offered some thoughtful policy proposals, as well as a lurch or two to the fringe left, but so far just hasn't caught on. There’s still time; when it gets down to choosing time, there's not room for both her and Sanders.

The generational lane. With pervasive cynicism about the ways of Washington, it's time to move beyond the baby boomers, and there are three attractive alternatives: 46-year-old Beto O'RourkeBeto O'RourkeThe Hill's Morning Report - Trump hews to NRA on guns and eyes lower taxes Harris to appear in CNN climate town hall after backlash Castro qualifies for next Democratic primary debates MORE who dazzled Democrats around the country when he came close to right wing Texas Sen. Ted CruzRafael (Ted) Edward CruzGOP strategist predicts Biden will win nomination, cites fundraising strength 3 real problems Republicans need to address to win in 2020 The Hill's Morning Report - Trump on defense over economic jitters MORE last fall; Pete ButtigiegPeter (Pete) Paul ButtigiegTop adviser on Sanders: 'He's always been underestimated' Biden, Sanders lead Trump in hypothetical match-ups: poll The Hill's Morning Report - Trump hews to NRA on guns and eyes lower taxes MORE, the 37-year-old Mayor of South Bend, Indiana, a former Rhodes scholar who was deployed as a Naval Intelligence officer in Afghanistan; and Massachusetts Congressman Seth MoultonSeth Wilbur MoultonMoulton, Stewart pen op-ed backing three-digit suicide prevention hotline 2020 Democrats react to NYPD firing of officer in Garner case: 'Finally' Biden, Buttigieg bypassing Democratic delegate meeting: report MORE, a Harvard graduate and Marine who served four tours in Iraq.

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A year ago most of the chatter would have focused on Moulton, who made a big splash when he arrived in Congress and enlisted some of the young veterans who unseated Republican congressmen last November, but he took a hit with his clumsy effort in opposing Nancy PelosiNancy PelosiGOP group calls on Republican senators to stand up to McConnell on election security in new ads The Hill's Morning Report - Trump hews to NRA on guns and eyes lower taxes Hobbled NRA shows strength with Trump MORE as Speaker, supporting instead an undistinguished Ohio Congresswoman, Marsha Fudge.

O'Rourke, only a month ago, generated much excitement among the rank and file, particularly young voters. That has given way to Buttigieg, seen as the “authentic Beto.” We'll soon see if the young mayor of a town of 102,000 can take the big league heat or if either of other two can restart.

The diversity lane. Barack Obama, still the most popular political figure in the country, broke the mold becoming the first African-American President; people of color are the party's base. There are three substantive candidates: California Sen. Kamala HarrisKamala Devi HarrisSenate Democrats push for arms control language in defense policy bill Top adviser on Sanders: 'He's always been underestimated' 'The Simpsons' pokes fun at Trump's feud with 'the squad' MORE, New Jersey Sen. Corey Booker, and former Housing Secretary Julian CastroJulian CastroHarris to appear in CNN climate town hall after backlash Castro qualifies for next Democratic primary debates Castro releases animal welfare plan as part of presidential campaign MORE of Texas.

Booker so far hasn't lived up to his anticipated potential and is seeking to reignite his campaign. Harris has the most natural path to the nomination if she improves as a candidate with a more coherent message and avoids the sort of mistakes she has made.

It's exceedingly difficult to see a path for most others. Perhaps someone like Minnesota Sen. Amy Klobuchar will have a moment as Howard Dean did in 2004. But it's hard to see how that's sustainable.

In the snows of January, as voting starts, there probably will be one, perhaps two (though that's remote), from these brackets. There won't be enough oxygen for the others.

This is a scenario with a caveat: Four years ago, on the Republican side, Donald Trump didn't fit into any conventional political lane.

He still doesn't.

Albert R. Hunt is the former executive editor of Bloomberg News. He previously served as reporter, bureau chief and Washington editor for the Wall Street Journal. For almost a quarter century he wrote a column on politics for The Wall Street Journal, then the International New York Times and Bloomberg View. Follow him on Twitter @alhuntdc.