An Iowa Collision in Miami These four — the leading candidates on the stage Wednesday night — have different ideologies, policy proposals and political styles. But they share a common strategy: All are betting big on Iowa. Mr. Booker and Ms. Warren have among the biggest campaign staffs there; Beto O’Rourke, a former representative from Texas, has barnstormed the state; and Amy Klobuchar, a senator from Minnesota, is the neighbor next door. Will any of them make Hawkeye-specific pitches?

Differing Trajectories Ms. Warren and Mr. O’Rourke enter the debate as two candidates who are moving in opposite directions. Mr. O’Rourke has struggled to match the hype surrounding his March entry. Ms. Warren’s course — from a winter swoon to a spring revival— is a reminder of just how fluid the race can be. Mr. O’Rourke, in other words, still has time. But standing next to Ms. Warren, can he answer the question that has nagged at his candidacy: Does he have the substance to compete at the highest level?

In Need of a Breakthrough Perhaps no two contenders at the periphery of the top tier need a big moment more than Mr. Booker and Mr. O’Rourke. Mr. Booker is likely to get airtime for his recent back-and-forth with former Vice President Joseph R. Biden Jr. about segregationists, and he could use that as a jumping-off point to make a splash. Mr. O’Rourke, who has done as many town halls in the last two years as anyone, is quick on his feet and should come prepared after facing off last year against one of the Republican Party’s best debaters, Senator Ted Cruz.

The Two Texans In an election where immigration could prove central — and when migrant children are being held at the border in squalor — these two Texans have the deepest and most personal expertise on the issue. If there is an issue that could help Julián Castro, the former San Antonio mayor and federal housing secretary, break out and connect, it should be this one. But after Mr. O’Rourke’s close Senate race in 2018, some Democrats want one or both candidates to head home to run for Senate instead.

The Ideological Divide This is not the heavyweight ideological showdown that we expect to see in Thursday night's debate. But the gulf between Ms. Warren and Ms. Klobuchar in terms of policy positions is striking and something both candidates may seek to highlight. For now, Ms. Klobuchar is drafting behind Mr. Biden in the centrist lane. Can she make a move here without him on stage?

Single-Issue Candidates There is no mistaking why Jay Inslee, the governor of Washington State, is running: climate change, climate change, climate change. It is an issue of huge concern to Democrats, and Mr. Inslee hopes that pushing the whole field on it will boost his candidacy. Tulsi Gabbard, a representative from Hawaii, is expected to emphasize peace, an especially resonant topic as the United States considered strikes against Iran in recent days. Can her dovish rhetoric (including her controversial defense of some hostile nations like Russia and Syria) make a mark?