The hosts, meanwhile, could move off the foot of the table if they secure their second successive victory on home soil.
Match preview
After ending a 10-game winless run with a 1-0 victory over DC United in their most recent home match, Chicago Fire were condemned to their eighth league defeat of the season when they lost 2-0 away at Houston Dynamo last weekend.
Despite recording 17 shots on goal, the Fire were unable to breach Houston's backline, while the hosts scored twice in the first half courtesy of strikes from Thorleifur Ulfarsson and Darwin Quintero.
Ezra Hendrickson's side are already nine points behind the playoff places and currently sit level on points at the foot of the table with DC United. Their form will need to improve quickly if they wish to finish inside the top seven for the first time since 2017.
Goals have been hard to come by for Chicago as they have found the net just 14 times in 16 league matches, the fewest of all 28 MLS teams so far this term.
The Fire have, however, scored in four of their last five meetings against Philadelphia, albeit winning only once during this period. Chicago's 2-0 home win over the Union in September 2019 is in fact their only victory in their last nine competitive encounters against them.
Philadelphia secured all three points in dramatic fashion when they beat New York City FC 2-1 on home soil last weekend.
Heading into their clash with the reigning MLS Cup champions, the Union had won only once, lost twice and drawn seven of their previous 10 matches across all competitions. However, they were able to get over the line and secure three crucial points to leapfrog their title rivals to the summit.
NYCFC thought they had claimed a point when Valentin Castellanos scored an 89th-minute equaliser from the penalty spot to cancel out Mikael Uhre's ninth-minute opener, but a 96th-minute winner from substitute Cory Burke sealed the victory for the hosts.
Jim Curtin's side, who have extended their unbeaten league run to 10 matches, now sit two points clear of NYCFC at the top of the Eastern Conference, although the latter have a game in hand.
Philadelphia now have three successive games against the bottom three sides in the Eastern Conference, with Thursday's encounter against Chicago up first. The Union have only won one of their last six away fixtures but victory at Soldier Field will see them remain at the top of the table for now.
Chicago Fire Major League Soccer form:
Philadelphia Union Major League Soccer form:
Philadelphia Union form (all competitions):
Team News
Chicago Fire star Xherdan Shaqiri has missed the last two matches with a thigh injury and is doubtful for Thursday's fixture, while Miguel Navarro (COVID-19), Kendall Burks (knee), Wyatt Omsberg (ligament), Jairo Torres (hip) and Chris Brady (ankle) all remain sidelined.
Former Philadelphia striker Kacper Przybylko is set to lead the line for the hosts, with support to be provided on the flanks by Stanislav Ivanov and Fabian Herbers.
A back four of Boris Sekulic, Carlos Teran, Rafael Czichos and Jonathan Bornstein is set to remain intact, shielding Chelsea and Real Madrid-linked goalkeeper Gabriel Slonina.
As for Philadelphia, they have a clean bill of health and Curtin will have a fully fit squad to choose from on Thursday.
Curtin is set to stick with a 4-3-1-2 diamond formation, with Daniel Gazdag expected to operate in the number 10 role in front of centre-midfield trio Leon Flach, Jose Martinez and Alejandro Bedoya.
After scoring the winning goal off the bench on Sunday, Burke will be hoping to force his way into the first XI ahead of Julian Carranza.
Chicago Fire possible starting lineup:
Slonina; Sekulic, Teren, Czichos, Bornstein; F. Navarro, Gimenez; Ivanov, Gutierrez, Herbers; Przybylko
Philadelphia Union possible starting lineup:
Blake; Harriel, Glesnes, Elliott, Wagner; Flach, Martinez, Bedoya; Gazdag; Burke, Uhre
We say: Chicago Fire 0-2 Philadelphia Union
Philadelphia have drawn at lot of games in recent weeks and have also drawn their last two encounters with Chicago, but last weekend's victory over NYCFC should give them plenty of confidence to claim maximum points on Thursday.