LAS VEGAS — The fundamental question in Wednesday's debate is whether Donald Trump gets a third chance to convince Americans he is a plausible president. Trump's people believe he has such a chance, that a large group of voters will be open-minded all the way to Election Day. But the polls suggest he has an impossible job ahead.

The first presidential debate was Trump's best opportunity to make his case. The day of that debate, Sept. 26, Trump trailed Hillary Clinton by 2.3 percentage points in the RealClearPolitics average of national polls.

Trump didn't take advantage of his opportunity. By the day of the second debate, Oct. 9, Trump's deficit had doubled, to 4.6 percentage points. Still, given Clinton's enduring unpopularity, Trump got a second chance to win support. His performance improved, but his polls didn't.

Now, as Trump prepares for the third debate, he trails Clinton by 7.5 points in the RCP average.

Clinton's lead has grown at a remarkably regular pace: 2.3 points at the first debate, 4.6 at the second, 7.5 at the third. Trump has fallen farther and farther behind.

Can he make it up? In the most volatile period of the race, June 17 to July 27, Trump went from 38.3 in the RCP average to 45.7 — a gain of 7.4 points. That's what he needs now — except that rise took 40 days and Trump has just 20 days until the election.

In another volatile period, Trump stood at 39.9 in the RCP average on Aug. 9 and climbed to 45.0 on Oct. 2. That's a 5.1-point rise — and it took 54 days to accomplish.

So to win, Trump would have to climb higher, faster, than he ever has. And he has to start in Las Vegas, with his last chance to speak to the American people.

Increasingly, Trump appears to pin his hopes on a belief that the polls, which he used to cite with great relish, are now totally wrong. "This is another Brexit," he said during a debate-eve appearance in Colorado. "I don't believe the polls anymore."

Brexit is an important part of Trump's view of the race. In Trump's understanding of Great Britain's vote to leave the European Union, all the polls were wrong, and the result was a great shock to the political class. But Trump has it only half right.

In fact, the Brexit polls were very close. The Economist's poll tracker on voting day had Remain and Leave tied at 44 percent. The Financial Times' poll of polls showed Remain at 48 percent and Leave at 46 percent. Any reasonable politico would have said the vote could go either way. The fact that so much of the British political class was shocked was because many had let their own wishes cloud their analysis of what was happening.

In any event, the U.S. presidential race is not Brexit — not unless Trump is able to climb into a tie by election day.

On the eve of the debate, I asked several Republican political types, none of them involved directly in the presidential campaign, a simple question: Is the race over?

Some said yes. Some didn't want to say yes but gave answers that pretty much said yes. One said there's still a chance. All saw Trump's position as dire.

"Yes, I think it's over," said one. "I don't think it's going to be the blowout most predict, but I think it's over."

Another declined to speak on the record, saying it's bad form for a Republican to slam his party's nominee by name and in public. That said, he unburdened himself on background:

Because of all the unconventional insanity that has occurred in this election, everyone is too scared to say anything conclusive. But this campaign is over. Oh, sure, if Hillary passed out during the debate, or something equally bizarre happened, then it could change. But barring something extraterrestrial, this is over.
Trump was on a glide path to win three weeks ago. Then he had a poor first debate, followed by a debate on the exact weight of Miss Universe, followed by the famous bus ride with Billy Bush, followed by 10 women coming forward, followed by a terrible first 15 minutes of the second debate, followed by the brilliant "rigged election" strategy. This goose has cooked himself.
It is now out of Trump's hands. He could have a brilliant and disciplined final three weeks, and he still would not win. The damage has been done, the race has taken its final turn.
As the Gordon Lightfoot song says, "at 7 p.m. a main hatchway caved in." That's where we are, sinking into the icy waters of Lake Superior.

Another veteran operative was less imaginative, but no less pessimistic. "The race is over, and has been for a long time, since he is incapable of focusing on a message beyond himself," he said.

"He has made no progress in the battleground states, and his map is narrowing," wrote another. "Wednesday night, he needs a closing argument, and to really frame this up better. No personal attacks — everybody knows she is horrible, which is why this race is still [as close as it is]. I am just not sure Trump and his merry band of pranksters are up for this."

"Opposition to DC is alive and well, but it is a long shot — one in 10 — and a very weak organization," said a politico in a deep-red state.

Finally, there was the operative who argued Trump still has a chance. "The national averages are meaningless," he wrote. "Yes, Trump could still win. He'd need to toughen up and stop going down all these rabbit trails."

"Hillary Clinton's 30 years of failed policy ideas and her potential control of the federal court system is more then enough for him to win, if he wants to win," the operative continued. "Clinton's corruption and change is a solid message. It won't be easy but 20 days of error-free ball will make success more likely."

Of course, that could have been said at many points in the campaign, and the extended period of error-free ball didn't happen. Now Trump has less than three weeks left. In a long campaign, he's never had to make up as much ground in so little time. Given the record of the campaign so far, there's no reason to think he can.