Donald Trump gave American voters an opportunity by boycotting Thursday night’s Republican debate: to see the other major presidential candidates in action without his domineering presence.

The result was revealing. Jeb Bush relaxed and owned his GOP “establishment” identity. He was, dare we say, almost joyful. Rand Paul returned to his libertarian roots. Chris Christie got to be the blunt-talking Jersey guy.

But the main show centered on Ted Cruz and Marco Rubio – the top two Republican candidates in national polls after Mr. Trump – who pummeled each other over immigration and the fight against the Islamic State.

“This sans-Trump debate was much more substantive, which might not help ratings, but it certainly helped voters,” says Republican strategist Ford O’Connell.

Whether voters were able to follow the immigration debate, in particular, is an open question. Senators Cruz of Texas and Rubio of Florida got so in the weeds in their skirmish over immigration reform, anyone could be forgiven for getting lost. Rubio formerly championed a path to citizenship for undocumented immigrants; Cruz, too, was once less hard-line than he is today. Both were confronted with video clips showing their shifts in view.

That exchange gave Governor Christie of New Jersey the opening to jump in and present himself as the top “outsider” on the Trump-free stage.

“I watched the video of Senator Cruz. I watched the video of Senator Rubio. I heard what they said. And this is why you need to send someone from outside of Washington to Washington,” Christie said to applause from the audience in Des Moines. “I feel like I need a Washington-to-English dictionary converter, right?”

In one go, the profound impact of Trump on the 2016 campaign was readily apparent, even as he held a separate event nearby. From the opening moments of his improbable presidential bid, when he singled out Mexican immigrants, Trump’s posture as an outsider with no governing experience has dominated the race – and put immigration front and center.

Trump’s absence put Cruz at center stage, which could have been a gift to the onetime Princeton debate champion. But without Trump to go after, Cruz made a major blunder, one that goes to likability and temperament: He tried aggressively to interject himself into the discussion when it wasn’t his turn, repeatedly interrupting Fox News moderator Chris Wallace.

“Sir, I know you like to argue about the rules, but we’re going to conduct a debate,” Mr. Wallace insisted to Cruz.

Political analysts pounced on Twitter.

“Ted Cruz proved for the billionth time in debate history why it makes ZERO sense to argue about the rules,” tweeted Chris Cillizza, a Washington Post politics blogger.

Cruz’s exchange with Wallace also brought to mind a famous comment by the late Supreme Court justice Oliver Wendell Holmes, who once said that President Theodore Roosevelt had “a second-class intellect, but a first-class temperament.” Cruz might be the opposite on both counts. (Some historians think Holmes was referring to Franklin Roosevelt, but he was much closer to the Rough Rider, who appointed him to the Supreme Court.)

Rubio, too, like the other candidates, gave voters a sense of his temperament Thursday night, showing his ability to disagree without being disagreeable. The skirmish on immigration didn’t help him; it could, in fact, be his Achilles’ heel in his quest for the nomination. But in all seven GOP debates, he has turned in consistently competent performances, even if appearing a bit overeager.

He also made a naked plea for evangelical votes, a key GOP voting bloc in Iowa that Trump and Cruz have dominated. When moderator Bret Baier noted, in a question on electability, that Time magazine had once called Rubio “the Republican savior,” the senator pivoted toward his faith.

“Well, let me be clear about one thing: There’s only one savior and it’s not me,” Rubio said. “It’s Jesus Christ who came down to earth and died for our sins.”

That may not do Rubio much good in New Hampshire, which holds the second nominating contest on Feb. 9 with its primaries and where open expressions of faith are not part of the culture. But the result in Iowa will have an impact on subsequent primaries and caucuses. Faith comes back as a factor in the third contest, the South Carolina primary. Finishing in the top three in the early contests has been critical to viability for the long haul.

In the end, Rubio may have done himself some good Thursday, especially in relation to Cruz.

“It’s not so much that Rubio won as no one else within striking distance of the nomination turned in such a standout performance that it matters at this juncture,” says Mr. O’Connell.

As for Trump, he has once again defied conventional wisdom by daring to skip the final debate before the Iowa caucuses. He didn’t face the buzz saw of Cruz’s debating skill, and instead thumbed his nose at the Republican establishment and got CNN to air his alternate event, a fundraiser for veterans. The early line on Trump’s latest gambit is that, once again, he comes out ahead.