Joe Biden's surprise victories on Super Tuesday were powered by Democratic voters who broke his way just days before casting their ballots - a wave of late momentum that scrambled the race in a matter of hours.

The late deciders helped Biden win Massachusetts, Minnesota, North Carolina, Texas and Virginia, among several other states. In some states, they made up roughly half of all voters, according to VoteCast surveys of voters in eight state primaries conducted by The Associated Press news agency. And the surveys show they lined up behind the former vice president. In Virginia, where nearly half of the voters waited to make up their minds, Biden won two-thirds of the late deciders.

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The surveys show the power of a well-timed surge in a race that has been defined by a crowd of candidates and confused voters agonizing over the best challenger to President Donald Trump in November. Biden's big win in South Carolina on Saturday revived his struggling campaign and within 72 hours pushed three of his rivals towards the exit and swung the opinions of voters in distant states.

Yet Vermont Senator Bernie Sanders found a way to blunt the impact. By organising aggressively and banking early votes from his liberal, young and Latino coalition, Sanders won Colorado and California. About 80 percent of Sanders voters in California said they picked their candidate before the final stretch.

Here's a snapshot of Democratic voters in Alabama, California, Colorado, Massachusetts, Minnesota, North Carolina, Texas and Virginia - who they are and how they voted - based on preliminary results from the AP surveys.

Biden's comeback coalition

The former vice president's coalition came into clear focus Tuesday. He did well with moderates and conservatives, voters older than 45 and African Americans. In many states, he enjoyed an advantage among women and college graduates. He ran strong with voters who attend church at least once a month.

And he successfully channelled sentimental attachments to former President Barack Obama. About half of North Carolina voters, for example, wanted a president who would restore politics to a pre-Trump era - and Biden won the majority of this group.

There's little doubt Biden's ties to black voters were critical to his rebound. Biden cleaned up in Alabama on Tuesday night by winning close to 70 percent of African Americans, a majority of Democratic voters in that state.

Voters wait in line to cast their ballot in the Democratic primary at a polling station in Houston, Texas [Callaghan O'Hare/Reuters]

Perceptions about electability may also be key to Biden's supporters. In Minnesota, for example, where Biden benefitted from home-state Senator Amy Klobuchar's exit from the race, about 60 percent of voters said it would be harder for a nominee with strong liberal views to win in the general election.

Even in his California loss, there were signs of newfound energy for Biden. Of voters there who picked their candidate in the last three days, Biden had a slight advantage.

Biden also flashed some crossover appeal in places that matter. In Virginia, a state likely to be a top battleground in November, Biden won more than half of voters who say they voted in the 2016 Republican primary. Mike Bloomberg won close to 20 percent.

Sticking to Sanders

The Vermont senator is holding on to his coalition of liberal voters, Latinos and voters under 30. In states with sizable populations of each, he fared well. In Virginia and North Carolina, where majorities were older and more likely to identify as moderate, Sanders came up short.

Sanders' strength with Latinos was clearest in California, where the group made up about 30 percent of the vote. Sanders won close to half of that group in California - roughly double Biden's share.

Sanders has benefitted from a youth movement. In California and Colorado, he won about 60 percent of voters under 30. His voters there are also somewhat more likely than Biden's to feel as though they're falling behind financially. He won about 40 percent of Texas and Minnesota voters who said that recent economic gains had bypassed them.

Aside from their youth, Sanders supporters stand out for their scepticism of most other candidates in the race. At least half of his voters in North Carolina said they would be unhappy with Biden or Bloomberg as the nominee. Majorities of them in California, Colorado, Massachusetts and Virginia felt the same.

Bloomberg's bust

More than $500m in spending on ads could not buy Bloomberg much love.

Bloomberg fared better among voters older than 65 compared with those younger, and similarly among those who think of themselves as moderate or conservative more so than liberals. But even among these groups, he was edged out by Biden.

The former New York mayor was the Democrat voters were least enthusiastic about nominating.

More than half of voters in Minnesota, Colorado and Massachusetts, his birthplace, said they would be dissatisfied if Bloomberg was the Democratic candidate. Roughly half in California, North Carolina and Virginia would also be displeased. By comparison, majorities in each of those states said they would be satisfied if Sanders was the nominee.

Only in Alabama and Texas would a majority be satisfied by Bloomberg.

Looking towards November

The Democratic contests do appear to be drawing some potential swing voters. Close to 20 percent of voters or more said they are waiting to see who wins the nomination before deciding how they'll vote in November. That was true in seven of the eight states AP surveyed. The share was somewhat smaller, 15 percent, in Minnesota, a state Trump is trying to flip.

Most of this group identified as moderates or conservatives, a sign that they might be open to Trump or consider not voting for any candidate in November.

Another closely watched group is suburbanites, a constituency that could make up more than half the general election electorate. Sanders and Biden battled it out for those voters on Tuesday, neither jumping ahead with a clear advantage.