A recent New York Times/Siena College poll found a sizable share of likely voters (28%) and registered voters (31%) feel they need to know more about Harris as a candidate. Those numbers were even higher among independent voters: 41% of registered independents and 38% of likely voters who identified as independent said they needed to learn more about her.

Her campaign is well-aware that a large slice of the critical voting bloc feels they don't know Harris well enough, ABC News Chief White House Correspondent Mary Bruce reports.

Tonight, she'll want to fill in the blanks in what is her highest-profile appearance since her acceptance speech at the Democratic National Convention last month. But beyond that, their real goal is to make clear the stark choice in this election between what she wants to do as president and what Trump intends to do.

The Harris campaign released its list of surrogates who will be in the spin room during the debate.

Four Democratic governors -- Roy Cooper of North Carolina, Michelle Lujan Grisham of New Mexico, Gavin Newsom of California and Josh Shapiro of Pennsylvania -- are among the group, as well as Sens. Laphonza Butler of California, Tammy Duckworth from Illinois and Chris Murphy of Connecticut.

Four Democratic House members -- Jason Crow from Colorado, Veronica Escobar of Texas, Robert Garcia from California and Ted Lieu of California -- will also be joining, as well as retired Brig. Gen. Steve Anderson, a former Republican; Reproductive Freedom for All President and CEO Mini Timmaraju and Khizr Khan, a Gold Star father.

As he left the White House for New York, where he will watch tonight's debate, President Joe Biden said he's spoken with Harris.

Biden wouldn't divulge what advice he had for her, but said she appeared "calm, cool and collected."

"I think she’s going to do great," Biden said.

The Trump campaign posted a short clip of the former president boarding his plane, saying he's en route to Philadelphia. Harris arrived in the city Monday evening.

-ABC News' Soorin Kim

Fact Check: Mostly True.

Harris correctly describes what the Nobel laureates said about inflation during a Trump presidency: “There is rightly a worry that Donald Trump will reignite this inflation.” But while the group describes Harris’ agenda as “vastly superior” to Trump’s, their letter doesn’t specifically predict a recession by the middle of 2025. Rather, the group wrote: “We believe that a second Trump term would have a negative impact on the U.S.’s economic standing in the world and a destabilizing effect on the U.S.’s domestic economy.”

The 16 economists are George Akerlof, Angus Deaton, Claudia Goldin, Oliver Hart, Eric S. Maskin, Daniel L. McFadden, Paul R. Milgrom, Roger B. Myerson, Edmund S. Phelps, Paul M. Romer, Alvin E. Roth, William F. Sharp, Robert J. Shiller, Christopher A. Sims, Joseph Stiglitz and Robert B. Wilson.

— PolitiFact's Louis Jacobson