The scale of former Vice President Joe Biden's achievement on Super Tuesday was astonishing, but it should not have been surprising. The sprawling Democratic debate had been strangely disconnected from the core question of 2020: Will President Trump serve four more years or not?


This was Biden's question, and it finally landed on the day that mattered most.


Of course, what Biden did was unprecedented. With little time or money, a weak "ground game" and virtually no ads, he marched through the South, won a surprise victory in Texas, and took Minnesota and Massachusetts.


The latter is the home of Sen. Elizabeth Warren, but she was pushed into third place by voters who decided that the race was now a binary choice. Former New York City Mayor Michael Bloomberg acknowledged this on Wednesday, ending his campaign and endorsing Biden.


There have been hints of this from the beginning. In all the early-voting states, a substantial majority of Democratic voters told pollsters that what mattered most to them were not the issues consuming so much debate time — single-payer health care above all — but ousting Trump.


With the 77-year-old Biden looking less in command than he did when he served with former President Barack Obama, these voters searched desperately for an alternative. In Iowa, many gravitated to former South Bend, Indiana, Mayor Pete Buttigieg. In New Hampshire, Sen. Amy Klobuchar, D-Minn., surged into third place after a commanding debate performance.


Then states that are not overwhelmingly white started voting. Biden pushed himself into a weak but important second place in the Nevada caucuses, setting the stage for his South Carolina landslide, thanks to African American voters urged on by Rep. James Clyburn, the powerful veteran congressman from the state's capitol city.


That broke the dam. The searching was over.


Yes, Biden won timely endorsements that got him free media time he desperately needed. But he was powered to victory not by "the Establishment" but by a pragmatic coalition of rank-and-file Democrats that came to life to stop Sanders. They feared that the proud democratic socialist would destroy the party's chances of defeating Trump, maintaining control of the House of Representatives and winning the United States Senate.


Three groups of Democrats were key to this alliance, African Americans above all. On Tuesday, Biden showed South Carolina was no fluke by winning roughly 60% to 70% of black voters across the states of Virginia, North Carolina, Tennessee, Alabama and Texas.


The suburban middle class that helped the Democrats win one House seat after another in 2018 also massed behind Biden, evident, for example, in his victories in Fairfax and Loudoun counties in Virginia.


And the outcome was a reminder that the Democratic party is a coalition of the center and the left — and that the left, while important and growing, is still a minority.


In state after state Sanders appealed to voters who told the Edison Media Research exit poll that they were "very liberal." But these voters accounted for only about a quarter of Tuesday's turnout overall. Biden won those calling themselves "somewhat liberal" in all but three states, and carried moderate and conservative Democrats in every state but Sanders' Vermont.


Sanders, technically an Independent, seemed suddenly to grasp that he needs a broader constituency, and that he is running in Democratic primaries. In his Tuesday night speech, he replaced attacks on "the Democratic Establishment" with a rebuke of a vaguer "political establishment." He also aired a remarkable ad overlaid with praise from Obama, whom Sanders has criticized in the past.


For Biden, the challenge is to avoid playing into Trump's game of dividing Democrats from the outside. Biden's core weakness, and it's a big one, is with younger voters who backed Sanders in droves. Trump renewed his faux sympathy for Sanders Wednesday, tweeting that he was "crushed" by "The Democrat Establishment." More efforts to demobilize the young and the left are coming.


Biden hasn't won yet, and he needs to sharpen his debating skills for the Sanders onslaught to come. But the last moderate standing must also pivot quickly to becoming a party unifier who would work with those to his left.


Sanders, who regularly calls out Trump's "greed, corruption and lies" must decide far earlier than he did in 2016 if his main goal is to defeat the president, or to build an opposition movement that has the Democratic Party itself in its sights.


He needs to remember that the party's voters — as they made clear in an impressive turnout wave on Tuesday — see ending the Trump presidency as a matter of national urgency.


E.J. Dionne (ejdionne@washpost.com) is a columnist for The Washington Post.