Mitt Romney may be the Republican Party’s presidential nominee and Paul Ryan the ­running mate, but Chris Christie takes a back seat to no one in the GOP.

The New Jersey governor was once a possible rival to Romney. Although he insisted that he didn’t think he was ready to be president, Christie had to keep swatting away speculation and encouragement to jump into the race at a time when the party was still learning to love Romney.

Christie had a tough assignment on Tuesday night. He was the wrap-up speaker on opening night of the Republican National Convention and had the misfortune of appearing after Ann Romney, whose testimony in behalf of her husband connected with the audience, and then the visit to the convention hall by the nominee.

But it was Christie who helped inject some much-needed energy into an arena that had been surprisingly subdued through the early evening. He came on stage punching the air. He clapped as he approached the lectern, returning the welcome he received from the delegates as if to say: Wake up, Republicans. He demanded that they stand up, and they did.

The governor is a politician and an entertainer. On Tuesday, he was on better behavior at the Tampa Bay Times Forum arena than he sometimes is when he is on the loose in his state. He was more dialed down than over the top.

But Tuesday’s stage was the biggest of his political career, and as he spoke he was being measured not just as a surrogate for the Republican ticket but as part of the GOP’s rising generation and a possible future presidential candidate, depending on what happens in November.

His assignment was to make a case for Romney, something he has done repeatedly nationwide but never to an audience as large as he had on Tuesday. He offered up the trademark combination of Jersey pride, humor, direct talk and sharp words aimed at President Obama — things that have made him a folk hero to conservatives.

The speech was as much about Christie and what he has done in New Jersey as it was about Romney, his record and his vision. It was also a story many other Republican governors tell, through their own experiences, as they provide a model of the kind of conservative governance — cutting spending and taxes, challenging public employee unions and shrinking government — Christie said Romney would bring to Washington.

He argued that it’s better to be respected than loved — which is one way to persuade voters to back a Republican nominee who trails the president in likability. He said the campaign should be about big things — just what the Romney team has tried to argue at the same time it has been thrown off stride by smaller matters.

He called the election a test of whether Americans are ready to hear the truth about the nation’s future and he said he is confident that those who challenge the voters will be rewarded. He said his record in New Jersey proves that point.

“The disciples of yesterday’s politics underestimated the will of the people,” he said. “They assumed our people were selfish. . . . Instead, the people of New Jersey stepped up and shared in the sacrifice. They rewarded politicians who led instead of politicians who pandered.”

Christie framed the contest as one of ideas and said Democrats are on the wrong side of the debate. “I know this simple truth and I’m not afraid to say it,” he said. “Our ideas are right for America and their ideas have failed America.”

He saved his criticism of Obama for the end of his speech. The president, he said, should spend less time reading polls and more time trying to shape public opinion. “Real leaders don’t follow polls,” he said. “Real leaders change polls.”

Romney and the Republicans, Christie said, would do that in Washington. “It’s time to end this era of absentee leadership in the Oval Office and send real leaders to the White House,” he said.

In so many ways, the New Jersey governor is the antithesis of the Republican nominee. Christie the politician is the direct opposite of Romney — a bluff, brash New Jersey prosecutor who likes nothing better than to verbally attack his attackers. Romney is modest and reserved, and has attacked rivals in debates but more often shows his sometimes awkward politeness to the people he meets on the campaign trail.

Christie has reveled in his reputation as a blunt-talking governor, rarely passing up an opportunity to tell an audience about his most recent confrontations — whether with Democrats in the New Jersey legislature or constituents who challenged him. Romney has found it difficult to talk about himself or bring undue attention to his actions. It is a measure of how opposites attract that Romney wanted Christie to make a case for him in Tampa.

Still, the governor is no mere sidekick. He still can’t go far without someone asking about his presidential ambitions. On NBC’s “Today” show Tuesday, Matt Lauer asked, “A lot of people look and say this may be the moment that he launches his bid for the next nomination. Is there truth to that?”

The answer was not definitive. “I doubt it,” Christie replied, “ ’cause I think Mitt Romney will be at the convention in 2016 being renominated for a second term. And so then you’re talking about 2020 — long, long way away.”

Before that, he had to deny a report in the New York Post that said he had turned down the vice presidential nomination because he didn’t think Romney would win.

At some point, Christie may be able to stop batting away speculation about his aspirations and begin to act on them. That could be soon or not so soon, depending on what happens in November. He may be his own man, but for this time and place, he played the role he was assigned.

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