If Marco Rubio suddenly hits a wall at next week’s New Hampshire primary, his performance at Saturday’s Republican debate will be the reason why.

Fresh off his strong showing at the Iowa Caucus, Rubio has been positioning himself as the establishment alternative to Ted Cruz and Donald Trump. He’s been rising in the polls and has secured the endorsement of big-name Republicans like Rick Santorum, South Carolina’s Tim Scott, and Pennsylvania’s Pat Toomey. But on Saturday he didn’t look like the frontrunner-to-be. He was stilted and continually retreated to well-worn talking points. He looked nervous, off his game.

And it all started with a grilling from Chris Christie, a candidate who’s been lagging in New Hampshire polls even though he’s practically set up residency in the state.

The New Jersey governor saw his chance to strike after Rubio, a first-term senator, attempted to dodge a question regarding his relative inexperience from moderator David Muir. Rubio tried to dismiss the question, saying that while President Obama was inexperienced before he was elected to the White House, he’s done a very good job at changing the country for the worse, and that politicians like Joe Biden shouldn’t be president despite spending decades in the Senate.

“Let’s dispel once and for all with this fiction that Barack Obama doesn’t know what he’s doing. He knows exactly what he’s doing,” he said. “Barack Obama is undertaking an effort to change this country, to make America more like the rest of the world.”

Muir then noted that Christie has attacked Rubio’s lack of executive experience. And Christie, right on cue, pounced.

“You have not been involved in a consequential decision where you had to be held accountable,” he said. “You just simply haven’t. And the fact is—when you talk about the Hezbollah sanctions act that you list as one of your accomplishments, you weren’t even there to vote for it. That’s not leadership; that’s truancy.”

Rubio quickly replied that New Jersey’s bond rating got downgraded a bunch of times during Christie’s tenure before pivoting again to Obama.

“Let’s dispel with this fiction that Barack Obama doesn’t know what he's doing,” Rubio said, falling back on the talking point he’d used just moments earlier. “He knows exactly what he’s doing. He is trying to change this country.”

Christie basically rolled his eyes.

“This is what Washington, D.C., does,” he said. “The drive-by shot at the beginning with incorrect and incomplete information and then the memorized 25-second speech that is exactly what his advisers gave him. Marco, the thing is this: When you’re president of the United States, when you are a governor of a state, that memorized speech doesn’t solve one problem for one person.”

A visibly shaken Rubio hastily replied, “Chris, your state got hit by a massive snowstorm two weeks ago—you didn’t even want to go back. They had to shake you into going back. You stayed there for 36 hours and then he left and came back to campaign. Here’s the bottom line: This notion that Barack Obama doesn’t know what he’s doing—”

Then Christie interrupted.

“There it is, there it is,” he said. “The memorized 25-second speech.”

It was a brutal moment, and Rubio was left stammering on the stage.

It was more than an ill-timed stumble for Rubio—a candidate who has been behaving like the frontrunner even though he came in third in Iowa. The exchange wasn’t just a bad debate moment—it magnified a vulnerability that his campaign has worked hard to dispelled: that he’s just not ready to be president.

And Christie wasn’t done.

Rubio’s major policy weak-spot throughout the primary has been his co-authoring of the Gang of Eight comprehensive immigration reform package, which would have provided a path to citizenship for immigrants in the country illegally. When Muir asked him if he had abandoned that legislation due to pressure from conservatives, Rubio again attempted a dodge, saying that his bill didn’t work because Americans don’t trust the federal government to secure the border with Mexico. After Muir tried to get more of an answer from the Florida senator, Christie stepped in and nailed Rubio for not answering the question.

“This is the difference between being a governor who has to be responsible for problems and not answering a question,” Christie said, tearing in. “The question was, did he fight for his legislation. It’s abundantly clear that he didn’t. It’s abundantly clear that he didn’t fight for the legislation.”

Even when Christie wasn’t charging in to him, Rubio was off his game, and when faced with an answer he didn’t know the answer to he kept going back to President Obama.

“I think anyone who believes that Barack Obama isn’t doing what he’s doing on purpose doesn’t understand what we’re dealing with here, OK?” Rubio said, after looking down at his notes, in what essentially amounted to a non-sequitur later in the debate. The audience, apparently tiring of the talking point, began to boo.

“Barack Obama is, indeed, trying to redefine this country,” he added.

It wasn’t all bad for Rubio. He still clearly knows his stuff when it comes to foreign policy and handled a question on abortion rights well enough. But for a guy who had seemed to play the expectations game so well in recent weeks, he sure didn’t meet them Saturday night.

Betsy Woodruff contributed to this story.