What is now effectively a two-man race for the Democratic presidential nomination gets under way on Tuesday a week after Joe Biden benefited from a rapid consolidation of moderate and establishment voters, while Bernie Sanders outlasted Elizabeth Warren as the progressive left’s last hope.
Here are the big questions heading into six primaries that put a combined 352 delegates at stake between Mr Biden and Mr Sanders:
Fresh off Mr Biden’s Super Tuesday surge, when he won 10 out of 14 state contests, the former vice president has 670 delegates to Mr Sanders’ 574, with some still to be allocated.
Mr Sanders’ deficit is not insurmountable, considering 1,991 delegates are needed for the nomination.
But the Vermont senator’s hill is steeper than it might look given Democrats use a proportional system to award delegates, not a winner-take-all model like Republicans.
Michigan and Washington, the states with the two largest delegate counts on Tuesday, are critical.
Mr Sanders beat Hillary Clinton in both states four years ago, with his Michigan primary win giving him the momentum to carry on through the end of the nominating calendar in June. Even then, he never could catch Mrs Clinton.
– If there is no real path will a Sanders protest campaign remain?
Top Sanders aides look back at 2016 and admit that the democratic socialist was running to make a point and did not expect his bid to blossom the way it did.
This time, everyone in Mr Sanders’ circle said he is running to win.
– What does that mean if any reasonable path is closed?
If Mr Sanders remains, and his passionate base along with him, will they act more as advocates for his progressive preferences as an aggressive insurgency bent on attacking Biden?
Mr Sanders and Mr Biden have stuck mostly to differences on policy and vision.
But Mr Biden has made clear he is aware of Mr Sanders’ more outspoken supporters on social media.
“I know I’m going to get a lot of suggestions on how to respond to what I suspect will be an increasingly negative campaign that the ‘Bernie Brothers’ will run,” Mr Biden told a group of donors over the weekend.
“But we can’t tear this party apart and reelect Trump. We have to keep our eyes on the ball.”
– Where do Elizabeth Warren’s backers go?
Massachusetts Senator Elizabeth Warren ran as a progressive in Mr Sanders’ ideological lane, for months even avoiding her own single-payer health insurance proposal with a simple line: “I’m with Bernie.”
But Mr Sanders and Mr Biden’s aides have always quietly argued that Ms Warren had more overlap at the ballot box with candidates like Pete Buttigieg, Amy Klobuchar and Kamala Harris, each more moderate and establishment than Mr Sanders.
In fact, there was evidence that some of her supporters chose Mr Biden even before Ms Warren dropped out, since Mr Biden won her home state of Massachusetts.
– Turnout: Will a November anti-Trump coalition continue to take shape?
More surprising than Mr Biden’s win total on Super Tuesday was the coalition behind it.
In Virginia, about 500,000 more voters cast ballots in 2020 than in the 2016 primary, and it did not help Mr Sanders, who watched his vote share drop from 35% in 2016 to under 25% this year.
Mr Biden, alternately, drew about 200,000 more votes than Mrs Clinton did four years ago, fuelled by his expected base of African Americans but also by white voters in the northern Virginia suburbs of Washington DC.
Those trends in two November battleground states suggest an expanding Democratic electorate deeply dissatisfied with President Donald Trump.
The diversity in Michigan — with heavily African American Detroit, the racially and ethnically mixed Detroit suburbs and wide swathes of working-class voters — will offer another general election battleground measure for that coalition’s enthusiasm.
– Will non-southern black voters deviate from southern trends?
African Americans showed up for Mr Biden in a huge way on Super Tuesday, helping him win a competitive Texas primary and by wide margins in Alabama, North Carolina and Virginia.
Whether that translates beyond the South could determine the outcome in Michigan and Missouri.
In Detroit, Michigan’s largest city, more than 78% of residents are African American.
Mr Sanders likely will be hoping for a younger black electorate: AP VoteCast data from Super Tuesday showed the senator about even with Mr Biden among black voters younger than 45.
But older African Americans usually vote in higher proportions.
Mr Biden expects a big victory in Mississippi, where a solid majority of Democratic ballots will be cast by African Americans.
In neighbouring Alabama last week, more than seven out of 10 black voters chose Mr Biden.
– Who consolidates the unions’ rank and file?
Michigan has more than enough union members to sway a presidential primary (about 600,000, according to federal statistics).
Mr Biden and Mr Sanders both make a claim on being labour’s candidate, Mr Biden more through relationships with union leaders and Mr Sanders with direct appeals to the rank and file.
The Culinary Union there nodded to Mr Biden by criticising Mr Sanders’ single-payer “Medicare for All” health insurance idea, but the union did not endorse, and caucus sites up and down the Las Vegas Strip showed Mr Sanders got rank-and-file support.
Mr Sanders will likely need a repeat in Michigan, where the most high-profile labour group, the United Auto Workers, has not taken a side.