How to Follow Tuesday's Voting in Arizona, Idaho and Utah

Republican presidential candidate Donald Trump speaks at a campaign rally in Salt Lake City on Friday. He has struggled to gain support from Mormons in Utah. Jim Urquhart/Reuters

Residents began casting ballots in Arizona's primary and will caucus in Idaho and Utah for the fourth straight week of voting this month in what the media has branded "Western Tuesday."

Both parties are holding primaries in Arizona and caucuses in Utah. Democrats also are caucusing in Idaho and Republicans in the U.S. territory of American Samoa. The first polls are expected to close at 9 p.m. ET.

National security has been one of the hot-button issues on the campaign trail. But it surfaced to the forefront earlier on Tuesday, after deadly twin explosions rocked Brussels, killing at least 34 people and injuring dozens more. All of the remaining five candidates have spoken out about the attacks. Republican front-runner Donald Trump said he would reinstate waterboarding to learn all of the necessary information from Salah Abdelslam, the lead suspect wanted in connection to the November Paris attacks who was captured Friday by Belgian authorities. Meanwhile, Texas Senator Ted Cruz said police should patrol and secure Muslim neighborhoods "before they become radicalized."

Arizona, viewed by both parties as the most important state participating in Tuesday's contests, could lead Trump and Hillary Clinton even closer to their respective nominations. It's a state that traditionally has been rough on immigration politics and is a winner-takes-all primary for the Republicans.

Trump is aiming to continue his slew of victories by taking all of the Arizona contest's 58 delegates. He has the support of Joe Arpaio, the controversial Arizona sheriff who—among other abuses—has been found guilty of unlawful enforcement of immigration laws and of racial profiling in federal court. Polls show Trump with a double-digit lead over his two opponents.

In the Democratic race, Vermont Senator Bernie Sanders is hoping to make a comeback in Arizona after losing all five of last week's contests. Leading up to Tuesday, he campaigned for a week and spent more than $1.5 million on TV ads in the Grand Canyon State, where polls indicate Clinton holds a double-digit lead. There are 75 delegates up for grabs in the Democratic primary.

Most Arizona polls will close at 7 p.m. local time. The votes will be available on the secretary of state's website, after 8 p.m. local time.

In Utah, recent polls show Cruz in first, Kasich in second and Trump in third. Cruz got a boost in the state with the recent endorsement of Utah Senator Mike Lee. Trump, on the other hand, has struggled to gain support from Mormons, the majority Republican group in the state. His continued attacks on some religions have led many Mormons to align themselves against his rhetoric and policies.

Last Friday, the real estate tycoon questioned the Mormon faith of Mitt Romney, who was the 2012 Republican presidential nominee. Earlier in the day, Romney publicly stated he would caucus for Cruz, and encouraged others to follow his lead. Support for Kasich, he said, will guarantee Trump wins the Republican nomination. Outside of a Trump rally in Salt Lake City the next day, a couple hundred protesters clashed with the candidate's supporters.

In Utah, there are 40 available delegates for Republicans and 33 for Democrats, awarded proportionally on final results. If Cruz wins Utah by more than 50 percent, he could walk away with all of the state's delegates.

Republicans and Democrats can track returns on their respective Utah party websites.

The next primary events are on March 26, for the Democrats only, in Alaska, Hawaii and Washington state.