Gov. Chris Christie of New Jersey, playing the Washington outsider, landed some deft attacks against Hillary Clinton, earning applause for promising that he would never let her “get within 10 miles of the White House.” Mr. Paul, who is so far behind in the polls that he did not qualify for the last prime-time debate, was aggressive and passionate as he made the case for his brand of libertarian politics, explaining that he was opposed to abortion rights but would leave the issue to the states. Ben Carson, the retired neurosurgeon, was almost as faded a presence as Mr. Trump.

Still, the debate gave Iowa voters a chance to see all seven men one last time before the caucuses. With a good share of Republicans still undecided or open to changing candidates, the lack of scene-stealing by Mr. Trump, who held his own rally across town, could pay some dividends for the candidates here or in New Hampshire, where many of them are competing for second place after Mr. Trump.

Mr. Cruz, who ascended in the Republican race in part thanks to his effective debate performances, found himself absorbing the fiercest attacks yet as he looked to regain lost ground in Iowa. While Mr. Trump had often been a punching bag at past debates as moderates and rivals challenged his policy ideas, Mr. Cruz became the main target this time around, facing skeptical questions from the moderators on immigration, ethanol subsidies and his personal style.

His rivals were even more severe, painting him as a holier-than-thou politician. “Ted, you worked for George W. Bush’s campaign; you helped design George W. Bush’s immigration policies,” Mr. Rubio said in a rhetorical torrent. “Then, when you got to the Senate, you did an interview with CBS News — it wasn’t even part of the video — where you said, on the issue of people that are here illegally, we can reach a compromise.”

Yet Mr. Rubio was also a frequent target over his past support for a comprehensive immigration overhaul. Mr. Bush, whose allies have unleashed a multimillion-dollar ad campaign attacking Mr. Rubio over his support for the overhaul, accused him of having “cut and run because it wasn’t popular among conservatives, I guess.”

Mr. Rubio, referring to a book on immigration that Mr. Bush wrote and had just mentioned, quickly returned fire. “That is the book where you changed your position on immigration,” he said, noting that Mr. Bush had moved from supporting a path to citizenship for undocumented immigrants to favoring only a path to legal status.