Dempsey spent five years with the Cottagers before joining Tottenham in 2012.
The USA midfielder moved back to America this year to join Seattle Sounders.
He is hoping to join Fulham for January and February as the club battle for Premier League survival.
Manager Rene Meulensteen said: "He's having a medical over the last couple of days and the next couple of days, so hopefully that will all work out.
"Clint has spent so many years playing in the Premier League.
"We know where his strengths lie whether it is coming in from the left or playing as a deeper striker or a number nine depending on who you are playing against. The most important thing is that he will be a good boost for us."
The move could go through when the transfer window opens on January 1, with Dempsey returning to the United States for the start of the new MLS season on 8 March.
Dempsey scored 50 goals in 184 league matches for Fulham before moving to Spurs, a decision that did not please many Cottagers fans.
Meulensteen, however, believes the fans will give him a warm welcome.
"I'm sure they will," he said. "It speaks volumes for him that in his loan opportunity he wants to come back here."