Trump quickly went on offense today. (Drew Angerer/Bloomberg)

A stunning terrorist attack in Europe. Reaction from U.S. presidential candidates that falls along familiar partisan fault lines, with few policy specifics.

Today's Brussels attacks provided the worst sort of deja vu. 

Ted Cruz issued a statement arguing that the United States should "empower law enforcement to patrol and secure Muslim neighborhoods before they become radicalized." Later, reported Sean Sullivan and Katie Zezima, Cruz's campaign said that law enforcement agencies should target terrorism as they do threats from gangs and human trafficking.

He also accused Donald Trump of wanting to withdraw the United States from NATO. "NATO should join with the United States in utterly destroying ISIS," Cruz told reporters in D.C., "and I would note that NATO is ready to act in a way that our president is not."

Trump himself went on the offensive early. In the morning, he said the United States should "close up our borders"; in the afternoon, he denied he'd said the borders should be shut. He spent most of the day focused calling for more aggressive military and law enforcement responses to terror attacks, again saying that as president he would "use waterboarding” and "try to expand the laws to go beyond waterboarding.” He was so focused on that message, in fact, that it took four television interviews, five Brussels tweets and a pointed question from CNN before he remembered to express sympathy for the victims of the attacks.

And his campaign quickly pushed back on Cruz's claim that the front-runner was calling for a "retreat from the world."

Cruz's charge that Trump had called for U.S. withdrawal from NATO wasn't quite accurate (Trump questioned the need for U.S. involvement, but hadn't said he would end it.) Still, there's an increasing isolationism in Trump's foreign policy message -- and an ideology which might best be described as "America Wins," say Philip Rucker and Dan Balz. It "puts him at odds with decades of Republican beliefs," but it hasn't been a political net-negative; his call for a temporary ban on Muslim immigration has been extraordinarily popular with GOP primary voters -- so far, popular enough to overcome his lack of familiarity with the details of those issues. 

Of course, he hasn't yet been pitted against, say, a former secretary of state (and his foreign policy prep may not be proceeding all that quickly; this week, we learned that one of his foreign policy advisers is a 2009 college grad who lists Model UN as a credential, and that members of that team say they have yet to speak with him.) A focus on terrorism may have helped Trump in the primary -- but it could work to Hillary Clinton's advantage in the general election, says Philip Bump. 

"Trump is more trusted on the economy," he writes, pointing to recent Washington Post-ABC polling. "Clinton, who led Trump by nine points overall, is more trusted on terrorism, immigration and in a crisis. But what's worth noting is the extent to which even Republicans view her positively on those fronts. On the economy, 82 percent of Republicans think Trump would do a better job. On an international crisis, though, only 60 percent think Trump would do better; 30 percent -- again, this is Republicans -- think Clinton would be stronger.)

Clinton's campaign, in a tweet that was not signed by the candidate herself, subtweeted Trump and Cruz's reaction to the day's events:

Clinton herself spent much of the day pointedly avoiding public reaction to Trump's latest attacks (he's returned to the "stamina" hit.) "I don't want to respond to his constant stream of insults," she said, calling the claim "absurd."

(John Kasich's response resembled the Democratic field's more than it did his Republican rivals.')

Polling conducted well after November's terrorist attacks in Paris suggests the gulf between Republican and Democratic voter responses to the ongoing problem is as wide as the reaction by their leading presidential candidates: When Gallup asked people to identify the most important issues this year, Republicans put terrorism at the top, tied with the economy. It was also one of the top four concerns of Democrats, but 82 percent of Democrats cited it, versus 92 percent of Republicans. In the Iowa caucuses, only 6 percent of Democrats listed terrorism as their most important issue for voting -- compared to a quarter of Republicans.

The Brussels attacks overshadowed Election Day for voters in Arizona and Utah, and for Democrats in Idaho.

The Western terrain means results will land late for East Coast viewers, with the earliest results in at 9 p.m. ET/7 p.m. MT -- and online caucus voting in Utah continuing until 1 a.m. ET. You can find full coverage here.

THE DEMOCRATIC OUTLOOK: Hillary Clinton was expected to win Arizona, and Bernie Sanders to claim victories in the Democratic caucuses in Idaho and Utah. Even Clinton campaign manager Robby Mook has predicted as much, even as he's said it is still highly improbable that Sanders can ever catch up to her delegate lead.

THE GOP OUTLOOK: Trump was highly favored to win Arizona, according to recent polls, but Cruz has been polling ahead of Trump in Utah ahead of the caucus vote there. If Cruz can capture more than 50 percent of the vote in Utah -- and he was within striking distance in recent polling -- he would take all 40 delegates at stake and offset Trump’s showing in Arizona’s winner-take-all Republican voting. If no candidate tops 50 percent in Utah, delegates would be apportioned by voting totals.

TODAY'S BIG (PRE-RESULTS) NUMBER:

Three-quarters of the Republican primary voters in the CBS/NYT also said they expect Trump to be their party's nominee.

More good news for Trump in CNN's latest poll: Six in 10 GOP voters in that survey said that if no candidate wins a majority of the delegates in the first ballot at the Republican convention, then the delegates should vote for "the candidate with the most support in the primaries and caucuses."

Meanwhile, the pro-Trump Great America PAC released its first TV ad today; the spot praises the mogul as a unifier. 

SPOTTED ON THE TRAIL: This guy.

TRAIL MIX: GOP delegate drama has already begun. The Virgin Islands GOP, citing party regulations, replaced six elected delegates with alternates (the move results in one additional delegate each for Trump and for Cruz.) The delegate math is even trickier in a Georgia county where Trump won by 12 points -- but Cruz supporters make up 90 percent of the convention delegation. 

And in a reminder that every delegate counts this year: Republican candidates are vowing to visit American Samoa -- which also votes today -- if elected president; it would be the first trip by a sitting commander-in-chief in half a century.

--Donald Trump's campaign, in a rare admission of error, conceded that a charity donation to Florida's attorney general was "a mistake." 

--Super PACs backing Clinton, Kasich and Cruz are getting big checks from opaque/"ghost corporations."

--His campaign manager Corey Lewandowski said that he might sue over a Buzzfeed story on sexual harassment allegations.

--Trump himself said half of his controversial statements in the past were made for "show business" reasons.

--Sarah Palin may wind up back on-screen in a Judge Judy role

--RIP Toronto mayor Rob Ford: Here’s a remembrance of his one-of-a-kind political career. 

COMING ATTRACTIONS: On Tuesday, House Speaker Paul Ryan plans to deliver an address on the "State of American Politics" (Per his office, it's not an endorsement. Or an announcement he's running for president.)

Also coming, per the Stop Trump PAC:

YOUR DAILY TRAIL PIT STOP (And based on today's trail reaction, coming soon to a GOP campaign ad near you, probably): At a ballgame in Cuba, President Obama and Raul Castro do the wave.