Last chance, no backsies. The third and final presidential debate, held in Las Vegas on Wednesday night, was the last real chance for either Hillary Clinton or Donald J. Trump to shift the momentum of the 2016 campaign. Wherever the race is now, it is highly likely to stay that way for the next 19 days. So what happened? Check out our takeaways.

Trump took a chill pill

For much of the debate, viewers could — sort of, if they squinted — see the Donald Trump that his advisers and coaches had been trying to summon since the spring. He was less impulsive. He interrupted less often. Gone was the thin-skinned, jittery counterpuncher of the first two showdowns, when Mr. Trump could not resist lashing out whenever Mrs. Clinton rolled a grenade down the hall. There were times when he even seemed to remember the facts and talking points he had evidently been drilled on: missing State Department funds, for example, and a WikiLeaks email in which a top Clinton adviser lamented her bad instincts.

He knows which voters he needs

Mr. Trump seemed intent on stopping his bleeding among habitual Republican voters, whose support he needs to regain if he is to have even a slim hope of beating Mrs. Clinton. He spit out reasonably focused attacks on Mrs. Clinton’s support for the right to late-term abortion and promised to appoint conservative Supreme Court justices. For his own base of disenchanted working-class voters, there were riffs on the North American Free Trade Agreement and trade-related job losses. He promised tax cuts that would unleash prosperity for all — standard Republican fare that may prove comforting to some who are wavering on his candidacy.