Clinton's average lead in recent national polls featuring third-party candidates Gary Johnson and Jill Stein has grown to 3.5 points, up from 1.6 points on the day of the first debate. She leads in an average of recent polls in swing states Colorado, Florida, New Hampshire, North Carolina, Pennsylvania and Virginia, at least one of which Trump would have to win to take the White House. Trump has a recent advantage in Iowa and Ohio.
A strong showing in the debate could certainly give Trump a boost. A record of more than 80 million people watched the first debate at Hofstra University, and Sunday's could also draw a huge audience.
It will feature a town hall format, different from the first in which the candidates fielded questions on stage from NBC News anchor Lester Holt.
It remains to be seen whether Trump comes into Sunday more prepared or looks more comfortable in the town hall format. He held a town hall event in New Hampshire on Thursday night, which he insisted was not preparation despite it featuring a timer on his answers. He fielded relatively easy questions from conservative radio host Howie Carr.
Sunday's debate also follows a vice presidential debate in which most commentators said Trump's running mate Mike Pence got the better of Clinton's running mate Tim Kaine. Pence deflected consistent attacks on Trump from Kaine, though he denied Trump said some statements he made in the past and avoided defending some of Trump's most outrageous remarks and policies.
However, multiple reports indicated that Trump felt upstaged by Pence's debate success after his own struggles.
Focus on Friday has shifted away from politics, as a potentially devastating hurricane bears down on Florida.
Watch the full debate and CNBC's analysis at CNBC.com starting at 9 p.m. ET on Sunday.