Primary and caucus voters in 14 states, American Samoa, and Democrats abroad will make their choice today, divvying up 865 delegates between two Democratic candidates and 595 between the five remaining Republicans.
To win the nomination, Hillary Clinton or Sen. Bernie Sanders will need 2,383 delegate votes, while the final GOP contender will need at least 1,237. So far, Ms. Clinton has more than anyone, 546, with Sanders at 87. Republicans are racing to catch up with front-runner Trump, who's won 82, as Senators Ted Cruz and Marco Rubio quibble over second and third place: currently 17 and 16 delegates, respectively.
Super Tuesday could either consolidate leading candidates' delegate support, or turn the tide for hopefuls. Here are three keys to help make sense of the clamor:
The 2016 campaign has consistently defied precedent, particularly as more traditional Republicans battle the rise of political novice Donald Trump. Impassioned voters on all sides of the party are pushing primary turnout to set new records in the first four contests, and the trend promises to continue on Super Tuesday.
"My guess is that it's the Trump phenomenon," Massachusetts Secretary of State Bill Galvin, a Democrat, told the Eagle-Tribune. "The tenor of the Republican presidential contest is different than anything we've seen, and that is driving excitement and interest."
Mr. Galvin predicted turnout would surpass the heated 2008 primaries, particularly for Republicans. 1.8 million Bay Staters went to the polls that year to when choose between Clinton, then-Senator Barack Obama, Mitt Romney, and Senator John McCain.
"The people of Massachusetts clearly understand how much is at stake in this election," he said.
Numerous states have reported an uptick in absentee primary ballot requests, from Arkansas to Massachusetts.
Republican turnout has beat Democrats' in Iowa, New Hampshire, and South Carolina. In Nevada, more Democrats voted, but Republican primaries showed huge growth: 75,000 voters, versus 33,000 in 2012, while Democrat votes were down by 50 percent from the 2012 primaries.
Part of that is political passion, but the sheer number of candidates still left is also motivating primary states to get up and vote. "Turnout rates in presidential nomination contests are highly sensitive to the levels of competition," Prof. Michael McDonald, an expert on election turnout, told The Christian Science Monitor.
For the GOP's top contenders, delegate-rich Texas has been beckoning for weeks: a comeback for home state favorite Senator Cruz, or a powerful sign of support for Mr. Trump or Senator Rubio.
The millionaire's unorthodox campaign has severely cut into conservative demographics that Cruz assumed were his, until Trump's melange of views — some liberal, some extremely conservative, and many unspecified — proved its Southern appeal. Surveys suggest a double-digit win for Cruz, with Trump in second place.
Rubio has stressed himself as the best establishment candidate, as mainstream donors turn to his campaign, but struggles to decisively best Cruz in polls or primaries. While Rubio doesn't need to win Texas, with his own home-state election coming up March 15, his Super PAC supporters at Conservative Solutions have still poured $1.5 million into the Lone Star state in recent weeks. Texan delegate votes are distributed proportionally, meaning Rubio could still gain delegates from districts where he won a majority.
For Super Tuesday overall, they've spent $4.2 million, roughly equal to backers for all the other GOP candidates combined.
Total delegate count
Cruz has pinned his hopes on Texas, meaning that a third-place finish Super Tuesday would offer little hope for a rebound. For Rubio, on the other hand, third place would be a worrying but perhaps reversible result.
Senator Sanders has focused on Colorado, Massachusetts, Minnesota, Oklahoma, and Vermont, which Meet the Press called a "puzzling" decision given that those five contribute just 288 delegates, while the other six Democrat races offer 571.
Reading Super Tuesday's impact on the general election is more complicated.
Primaries have galvanized GOP voters, but it's not a guarantee of higher votes in November, analysts say. Crowded Republican primaries didn't win the White House in 2000, they point out, perhaps because it also winds up inspiring nervous Democrats to vote.
"Every action has an equal and opposite reaction," writes Amy Walter, national editor at the Cook Political Report. "Getting more white and angry voters to the polls with nativist language also helps to turn out passive Democratic voters who might have stayed home but for the threat of Trump’s candidacy."
This report includes material from Reuters and the Associated Press.