Nightengale added that Phillies owner John Middleton does not want to leave Las Vegas until a deal with Harper is in place.
Jim Salisbury of NBC Sports Philadelphia and MLB.com's Todd Zolecki reported Friday that Middleton's plane was on the ground in Las Vegas. MLB Network's Jon Heyman later reported that Harper and his team were in Vegas as well, which supported the notion that he was in heavy negotiations with the Phillies.
Heyman noted that while the Phillies may be the favorite to sign Harper, he was negotiating with two teams Saturday and there are "more to follow."
The Phillies were also believed to be in on All-Star infielder Manny Machado, but after he signed a $300 million deal with the San Diego Padres this week, Harper has become their primary focus.
The Nats may be out of the mix, though, as team owner Mark Lerner told NBC Sports Washington's Todd Dybas on Friday that he has "moved on" from the idea of bringing Harper back.
Harper, 26, spent his first seven seasons with the Nationals and was named an All-Star on six occasions. He is also a one-time Silver Slugger award winner and one-time National League MVP.
Last season, Harper hit just .249, but he led the majors with 130 walks and also hit 34 home runs while driving in a career-high 100 runs. If the Phillies sign him, Harper would combine with Rhys Hoskins, J.T. Realmuto, Jean Segura, Odubel Herrera and Andrew McCutchen to give Philadelphia what could be one of the most potent lineups in baseball.
It will likely take a hefty contract to land Harper given what Machado landed from the Padres.
Harper's agent, Scott Boras, likely wants a deal that will exceed the $325 million extension Giancarlo Stanton signed with the Miami Marlins in 2014, according to Salisbury, and the Phillies have shown a willingness to spend this offseason.
The Phillies have missed the playoffs in seven straight seasons, but signing Harper would be a move designed on ending that drought in 2019.