"For us, he's a great candidate (to face in the election)," the National Front's Florian Philippot told Reuters. "His project is so sharply different from ours."
Under the leadership of Le Pen, who took over from her father Jean-Marie in 2011, the FN has switched from an economically liberal, pro-small business party to one that promises to lower the retirement age and guarantee France's generous welfare safety net.
Fillon plans to slash public spending by 100 billion euros ($106.5 billion) over five years, scrap a tax on the wealthy and push the retirement age to 65 as well as increase VAT sales tax.
"It's a program of chaos. It's impossible that this austerity cure does not trigger chaos," Philippot said.
All eyes now turn to whether the deeply unpopular President Francois Hollande will decide to run for the left-wing ticket in his party's primaries in January, amid signs that his prime minister, Manuel Valls, is considering a bid of his own.
Close aides of Hollande have said he will run despite his deep unpopularity. Fillon lacks the broad appeal of the more centrist Juppe and his more radical economic reform plans could give Hollande a peg on which to base his candidacy.
But on Sunday, Hollande's prime minister Manuel Valls for the first time raised the prospect he could challenge Hollande as the Socialists' candidate in the 2017 presidential election, in a further sign of the left's divisions.