Why Trump's performance on the 'most important night of the whole campaign' was especially brutal

HEMPSTEAD, New York - For David Plouffe, the defining sequence in the most important night of the presidential campaign came when Hillary Clinton delivered a brutal one-two step to Donald Trump.

She first gave an effective response to a bizarre suggestion from Trump - that she had prepared too hard for the debate. She retorted, somewhat awkwardly, yet still persuasively, that she had prepared not only for the debate, but for being president.

Then they segued into an exchange that properly displayed the contrast Clinton was trying to exploit - a long back-and-forth over the issue of President Barack Obama's birthplace, an issue that Trump tried to make go away weeks ago to no avail. Clinton used the word "racist" thrice in her response without ever directly dubbing Trump racist, while Trump floundered and tried to bring back memories of her alleged disrespect toward Obama in 2008.

"That's where," said Plouffe, Obama's former campaign manager, when asked what he thought was the standout moment of the debate.

Monday night's first debate didn't have the same one-sidedness as the memorable first 2012 affair between Obama and then-Republican nominee Mitt Romney. But it is every bit as brutal, if not more, to Trump's chances of becoming president.

That was evident from the instant polls and focus groups that dubbed Clinton the clear winner. And it was evident from the contrast in reactions on each side in the massive "spin room" after the debate. Clinton surrogates, like Plouffe, glowed about her performance. They guffawed at Trump's missteps. They suddenly stood confident, the jitters of the past few weeks of polls in the rearview, at least for one night, quoting Trump's debate lines with apparent glee.

"Some of his stuff was - weird," Sen. Claire McCaskill, a Clinton supporter and Democrat from Missouri, told Business Insider.

"I thought she was clearly prepared - I mean, that was the starkest contrast," she added. "She thinks it's important to be prepared to be president. I don't think so much he does."

And to that end, as a result of Clinton's deft ability to go on the offensive, Trump surrogates, like senior adviser Sarah Huckabee Sanders, were forced to spend the night defending their candidate over some of the more outlandish debate moments - like the "birther" exchange. They praised his supposed restraint for not bringing up - and then bringing up the fact that he didn't bring up - Bill Clinton's sex scandals.

They criticized the debate moderator, NBC's Lester Holt, as former New York City Mayor Rudy Giuliani said he "should be ashamed of himself."

"If journalism has ethics, Lester Holt acted unethically," Giuliani said.

But Trump's surrogates couldn't even get on the same page in their defenses of the candidate. Huckabee Sanders said that overall, Holt did a good job moderating with the "weight of the world on his shoulders." Kellyanne Conway, Trump's campaign manager, told Business Insider she thought Holt did a "fine job."

At another moment in the spin room, Huckabee Sanders was grilled by several reporters over Trump's frequent interruptions of Clinton. She argued that Clinton had also interrupted Trump several times and complained of a double-standard.

But Rep. Chris Collins, a Trump supporter and Republican from New York, told Business Insider that he thought it was actually strange that Clinton didn't interrupt Trump.

"It was like, I don't know, maybe she over-prepared," Collins said. "There it was again!

Perhaps Trump's problem was one of over-confidence. He came into the debate with momentum - perhaps his best-ever shot at winning the race. He was tied or leading in national polls, and he had broken the lock on an Electoral College disadvantage, pulling close in states he needs to get to 270 votes. Pennsylvania. Colorado. New Hampshire.

But Trump has a much narrower path to victory than Clinton. And that's where a rough sequence, and a rough overall debate, can potentially be fatal.

"She doesn't have to win all the battleground states to win the presidency. He does. He has to run the table. Which means, tonight was the most important night of the whole campaign, really," Plouffe said. "And he didn't take advantage of it."

Allan Smith contributed reporting.