There was one thing Cory Booker wasn't lacking as he took the stage of the Democratic National Convention Monday — energy.

The former Newark mayor and Democratic senator from New Jersey — considered by many observers a contender for Hillary Clinton's vice presidential nominee before her selection of Tim Kaine— had in 2012 delivered a full-throated defense of President Barack Obama's first term that was widely praised for rallying that year's convention crowd.

And Monday night, Booker spoke at times with a soaring optimism about a nation with the potential to come together. Tolerance, he said, was not enough.

"But love – love knows that every American has worth and value, no matter what their background, race, religion, or sexual orientation. Love recognizes that we need each other, that we as a nation are better together, that when we are divided we are weak, we decline, yet when we are united we are strong – invincible," Booker said.

It was, at its start, a sharp contrast from the darker message delivered by Donald Trump and other RNC speakers — that America is at a time of crisis out of which only Trump was qualified to lead the country.

"We are not a zero-sum nation, it is not you or me, it is not one American against another," Booker said. "It is you and I, together, interdependent, interconnected with one single interwoven American destiny."

But the tone wasn't always positive.

Where Booker started with an exhortation to togetherness and love, it didn't take long before his message became harsher — as he told the crowd Trump isn't the candidate to bring people together.

In his speech, Booker hammered Trump on the same message that brought Clinton to Atlantic City, and that formed the foundation for an anti-Trump ad featuring the retired owner of the Freehold Music Center — that if Trump runs the country the way he's run his businesses, everyday Americans will get hurt.

"Well, I’m from Jersey, and we have seen the way he leads," Booker said. "In Atlantic City, he got rich while his companies declared multiple bankruptcies. Yet without remorse, even as people got hurt by his failures, he bragged, 'The money I took out of there was incredible.'

"Yes, he took out lots of cash but he stiffed contractors – many of them small businesses, refusing to pay them for the work they’d done. America has seen enough of a handful of people growing rich at the cost of our nation descending into economic crisis.

"Americans, at our best, stand up to bullies and fight those who seek to demean and degrade others. In times of crisis we don’t abandon our values – we double down on them."

The 20-minute speech ended with much of the crowd on its feet. Online, the speech received mixed-to-positive reviews, with many noting its energy and the shift in tone from the RNC:

But the speech had at least one significant detractor:

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