Missouri voters will decide a consequential slate of primary contests and constitutional ballot measures on Tuesday.

The results could not only reshape the state’s tax structure and its process for amending its constitution, but also serve as a litmus test for the national Democratic Party’s ideological divide.

Here’s what to watch.

A House primary features a rematch between a ‘squad’ member and a moderate.

The marquee primary on Tuesday is a fiery rematch in Missouri’s First Congressional District, where Representative Wesley Bell, a moderate first-term Democrat, faces off against former Representative Cori Bush, a progressive member of the Democratic Socialists of America.

In 2024, Mr. Bell — a former prosecutor and member of the Ferguson, Mo., City Council — ousted Ms. Bush, a former Black Lives Matter organizer who was part of a progressive group of congresswomen known as “the squad.” Two years later, Ms. Bush, who represented the St. Louis area for two terms, is attempting a comeback in a race that highlights the rift between the Democratic Party’s moderate and progressive wings.

Mr. Bell has maintained a distinct fund-raising advantage, and holds key endorsements from House Democratic leadership, including Representative Hakeem Jeffries of New York, the minority leader, and the Congressional Black Caucus PAC. Ms. Bush has the backing of Senator Bernie Sanders of Vermont and the Democratic Socialists of America.

Ms. Bush has anchored her campaign on vocal opposition to the war in Gaza and criticism of pro-Israel groups, betting that Democrats have further soured on Israel since she was voted out in 2024.

The United Democracy Project, the main super PAC of the American Israel Public Affairs Committee, has poured more than $3 million into advertising opposing Ms. Bush’s bid this cycle, a repeat of its heavy spending against her in 2024.

Republicans hope to make citizen-led constitutional amendments harder to pass.

Missourians are voting on whether to require that citizen-initiated constitutional amendments must win in each of the state’s eight congressional districts, rather than by a simple statewide majority.

The measure, backed by Republicans, would make it significantly harder to pass even popular changes to the state’s founding document, given the widely differing ideological makeups of each district.

Missourians have used citizen-initiated amendments to enshrine the right to abortion, expand access to free health insurance and legalize marijuana and sports betting. None of those would have passed under the new standard, which was proposed by the Republican-led legislature.

If the measure passes, constitutional amendments proposed by the legislature would continue to require only a simple majority, or more than 50 percent of the vote. This would make Missouri the only state whose processes for passing citizen-led and legislature-led ballot initiatives are different.

A ballot measure could eliminate income taxes.

In fiscal year 2025, money from individual income taxes made up more than 60 percent of the state’s general revenue.

A proposed constitutional amendment would instruct the legislature to phase out those taxes, a drastic overhaul of the state budget. The measure would authorize lawmakers to drive up sales taxes to replace the lost revenue, but it does not specify how that should be done.

The state’s governor, Mike Kehoe, a Republican, says the move would catalyze economic growth. But opponents say it would increase taxes for lower-income Missourians.

If it passes, the measure would make Missouri the 10th state in the country to exclude income taxes as a revenue source.