In New Hampshire, which votes Feb. 11, a close four-way race has held for weeks, with Biden, Buttigieg, Sanders and Warren each leading in at least one of the six most recent polls.

Sanders is ahead currently in New Hampshire by 1.3 percentage points in the latest Real Clear Politics average of polls, while Warren has slipped. But New Hampshire has a history in primaries of bucking Iowa, often favoring candidates from New England, perhaps giving an advantage to Sanders or Warren. Both would face pressure to win in New Hampshire if Buttigieg wins Iowa.

Next comes the Feb. 22 caucus in Nevada where Biden is ahead by only 6 to 10 percentage points, according to recent polls. Warren and Sanders — the latter of whom finished close behind Clinton in Nevada in 2016 — are both in striking distance. If, for example, Sanders won New Hampshire, Warren would probably need to win Nevada, or vice versa, for a pattern to emerge of different winners in each state.

One week later, Feb. 29, is the South Carolina primary, where Biden, who remains the only candidate with sizable support among black voters, has dominated from Day 1.