Fillon, a former front-runner who is now lagging in polls following a financial scandal, had earlier this week accused Socialist President Francois Hollande of orchestrating a plot against him.
In comments reminiscent of US President Donald Trump's accusations against former president Barack Obama, Fillon and his team are now raising allegations of wiretapping.
"It's extremely likely," lawmaker Eric Ciotti, a close campaign aide, told Europe 1 radio and CNEWS television in an interview on Sunday, asked about reports that Fillon himself on Saturday said he was likely to have been wiretapped in the course of the probe into accusations of misuse of public funds.
"It wouldn't be illegal but that would, once more, be a democratic scandal," Ciotti said.
As with other accusations made by the Fillon camp in recent weeks, the wiretapping comments triggered some criticism.
"Conspiracy theories ... keep one from looking at one's own responsibilities," he said of Fillon.
Fillon, the 63-year-old former prime minister, had looked sure of winning the presidency after he won the candidate-selection contest in his Republicans party last November.
But he has fallen to third place - meaning he faces first-round elimination on April 23 - since media revelations prompted magistrates to open an inquiry into allegations that he paid his wife and children hundreds of thousands of euros of public money for work as parliamentary assistants they may not have done.
(Reporting by Jean-Baptiste Vey and Ingrid Melander; Editing by Greg Mahlich)