For once, Donald Trump was understated: “I guess the bromance is over.”

After Thursday night’s Republican presidential debate, it was also a statement of the obvious. Mr. Trump and Texas Sen. Ted Cruz, for months acting like best friends even while vying for the same job, finally took off the gloves and went after each other. The end of the buddy act was inevitable – and instructive.

Trump showed that he could take a punch. And more important, he demonstrated a nimbleness and a disarming honesty that reinforce why he’s the Republican frontrunner. When challenged by Senator Cruz over his “New York values,” Trump didn’t deny that he once held socially liberal views. Instead, he came back with a stirring defense of his hometown, invoking New Yorkers’ resilience in the face of 9/11. Even Cruz applauded.

“The people in New York fought and fought and fought...,” said the billionaire real estate developer. “And we rebuilt downtown Manhattan, and everyone in the world watched and loved New York and New Yorkers.”

If Trump and Cruz dueled to a draw, Trump comes out ahead, as polls show him leading nationally by a wide margin and in a dead heat with Cruz in Iowa, which holds the crucial first nominating contest Feb. 1. If Trump wins both Iowa and New Hampshire, whose primary is Feb. 9, his march to the GOP nomination could be unstoppable.

“This was without a doubt Trump's strongest debate performance,” says Republican strategist Ford O’Connell. “But Cruz showed he has the backbone and skills to go toe-to-toe with 'the Donald.’ ”

Once a champion debater at Princeton, the Canadian-born Cruz effectively defended his eligibility for the US presidency, as the child of an American citizen. And he dispensed with a damaging New York Times story about unreported campaign loans from Goldman Sachs and CitiBank – by attacking the Times, an easy hit for a conservative.

But it was Trump’s night to show how he has matured as a politician and why his supporters back him so enthusiastically. There was less mugging for the cameras and a continued willingness to give honest answers to questions that, at first blush, may seem counterproductive – but in the end, add to his unquantifiable aura of authenticity.

When Cruz accused Trump of flip-flopping on the Texan’s eligibility for the presidency, because of the businessman’s decline in Iowa polls, Trump ultimately pleaded guilty as charged. Just a few months ago, Trump had his lawyers look into the matter and determined Cruz was eligible. Thursday night, he was unapologetic in his response to why suddenly he had changed tack.

‘Because now [Cruz] is doing a little bit better,” said Trump. “Hey look, he never had a chance. Now, he's doing better.”

The exchange also allowed both men to show some humor. Trump floated the idea of putting Cruz on the ticket with him, and Cruz offered the perfect comeback: “If this all works out, I'm happy to consider naming you as V.P. So if you happen to be right [about the eligibility question], you could get the top job at the end of the day.”

Trump said thanks but no thanks: “I think I'll go back to building buildings if it doesn't work out.”

“Actually, I'd love to get you to build a wall,” Cruz quipped, alluding to the wall Trump wants to construct along the US-Mexico border – paid for by Mexico.

All the witty comebacks aside, the debate amplified issues that could loom large in the final weeks before voting starts. And now that Trump and Cruz are openly at war, they are likely to keep pounding away at each other. Trump’s previous liberal views on abortion, gay rights, and single-payer health insurance may harm his standing among conservatives, particularly evangelicals, who are a crucial GOP constituency in Iowa.

The Cruz birther charge also isn’t going away. Cruz’s argument about his former Harvard Law professor, Laurence Tribe – who has said Cruz’s eligibility isn’t settled – probably defused the issue a bit. (Cruz called Professor Tribe a “left-wing judicial activist” and “a major Hillary Clinton supporter.”) But as long as doubts linger, and Democrats threaten to sue if Cruz is on the ticket (which some have), then it’s an issue for the Texan.

Florida Sen. Marco Rubio’s strong debate performance, accusing Cruz of shifting policy positions (on immigration and ethanol, for starters), keeps Senator Rubio in the game as well. And Rubio’s presence is good for Trump. An NBC News/Wall Street Journal poll released Thursday shows in that in a head-to-head matchup, Cruz beats Trump, 51 percent to 43 percent. But in a three-way contest, Trump wins with 40 percent, Cruz is second at 31 percent, and Rubio in third at 26 percent.

The morning after the debate, Trump didn’t waste time in refreshing his message. He released the second TV ad of his campaign, to be aired in Iowa and New Hampshire.

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Called “Our Country,” the 30-second spot shows Trump going positive, talking about what he would do as president – fix health care, fix the military, help veterans, and, of course, make the country “great again.”

Friday’s TV ad contrasted sharply with the first one, released early this month – a 30-second montage of negative and frightening images.