Odubel Herrera hit a three-run homer, and the Philadelphia Phillies went deep five times while handing the Cubs their 10th straight loss with a 13-3 romp on Monday night.
Andrew Knapp, Didi Gregorius, Rhys Hoskins and Alec Bohm also connected.
The Cubs remained winless since Zach Davies and three relievers combined to no-hit the Dodgers in Los Angeles on June 24. They also extended their worst losing streak since a 12-game slide in May 2012.
The Cubs were tied with Milwaukee for the NL Central lead prior to their tailspin. Now, they are 8 1/2 games behind the first-place Brewers and third in the division.
Herrera's drive against Kohl Stewart capped a six-run eighth that broke open a 4-2 game. Bohm also drew a bases-loaded walk and two more came in when third baseman Eric Sogard misplayed Ronald Torreyes' grounder.
Knapp led off the third with his second homer, and Gregorius added a solo shot against Davies in the fourth. Hoskins and Bohm went back to back against Sogard in the ninth.
Andrew McCutchen had three hits and scored two runs. He delivered a tiebreaking double in the sixth, and the Phillies scored twice in the inning after Ross got ejected by plate umpire Nic Lentz following a heated argument.
The Cubs (42-43) fell below .500 for the first time since they were 19-20 in mid-May. With All-Star Kris Bryant, Anthony Rizzo and Baez in the final seasons of their contracts, Chicago is looking more like a potential seller than buyer prior to the July 30 trade deadline.
Jake Marisnick homered for Chicago, and Davies lasted five innings, allowing two runs and four hits.
Jake Arrieta (5-8, 5.57 ERA) tries again to get back on track for Chicago, while Aaron Nola (5-5 4.44 ERA) gets the ball for Philadelphia. Arrieta got tagged in a 15-7 loss at Milwaukee last week after a seven-run first by the Cubs. He is 0-4 with a 7.82 ERA in seven starts since winning at Pittsburgh on May 25. Nola gave up seven runs in a loss to Miami last week.