Which states will be voting?
This year, 15 states will vote on Super Tuesday: Alabama, Alaska, Arkansas, California, Colorado, Maine, Massachusetts, Minnesota, North Carolina, Oklahoma, Tennessee, Texas, Utah, Vermont and Virginia. One territory, American Samoa, will also vote.
We will also learn the results of the Democratic contest in Iowa, which is being held by mail over several weeks. (Iowa Republicans held their caucuses in mid-January.)
How important will it be this year?
That depends on how Nikki Haley does in South Carolina on Feb. 24 and in Michigan on Feb. 27.
If Ms. Haley wins, or comes very close to winning, one or both of those states, Super Tuesday will be crucial in demonstrating whether she can remain competitive with Mr. Trump on a national scale. Most of the states that will vote on Super Tuesday allow unaffiliated voters to participate in primaries, so there are at least theoretical opportunities for her to recreate the coalition that got her over 40 percent in New Hampshire.
But if she loses decisively in South Carolina and Michigan, Mr. Trump will have run the board in the early-voting states. In that case, it will be hard for Ms. Haley to argue that her candidacy is viable, and Mr. Trump could have the nomination pretty well wrapped up before Super Tuesday. He technically would not have the delegates needed to clinch the nomination, but no serious competitors would be left, and the remaining votes would be academic.
On the Democratic side, nothing indicates that the race is competitive. That is normal for the party whose main candidate is an incumbent running for re-election.