By Brian Love
PARIS (Reuters) - The two candidates vying for the French Socialist party's presidential nomination said on Tuesday they would not shift their policies to the left after a surprisingly strong showing by a populist challenger in a first round vote.
Party leader Francois Hollande and former government minister Martine Aubry are due to face each other in a runoff primary election on Oct 15 after placing first and second on Sunday in the first round of a vote to win the nomination.
Hollande, a moderate socialist, led opinion polls for months and was even seen as possibly in line to win the nomination outright with more than 50 percent in the first round.
But Aubry, seen as leaning to the left of Hollande, did better than expected in Sunday's first round, winning 30 percent of the vote to Hollande's 39 percent in final results released on Tuesday.
An opinion poll published on Tuesday -- the first since Sunday's vote -- saw Hollande's score in round two slipping by four points to 54 percent and Aubry's rising four points to 46 percent.
The biggest surprise of the first round was the performance of Arnaud Montebourg, a 48-year-old populist lawyer who placed third with 17 percent of the vote after campaigning as an opponent of free trade who would take a hard line toward banks.
Montebourg's strong showing could help Aubry, who is seen as having more appeal to his left-wing supporters than Hollande.
Polls show either Aubry or Hollande would be likely to beat President Nicolas Sarkozy in next April's election.
In a letter to both candidates, Montebourg said he and the 450,000 people who voted for him wanted to hear what they had to say about his ideas. But both candidates said they would not change tack to woo Montebourg's support.
"I was never the fly that hovers around pots of jam to see which looks tastiest," Hollande told France Info radio, while studiously avoiding mentioning Montebourg. "I won't take any lessons from anybody on my position."
Aubry told France 2 television: "I only propose what I believe in, so I won't be changing."
The two candidates face off in a televised debate on Wednesday.
In his letter to the candidates, Montebourg said taxpayers were fed up with paying for mistakes by banks, and Hollande and Aubry should provide assurances they would carry out a Socialist pledge to separate retail banking from investment banking.
He said companies which moved operations to low cost countries should be made to pay penalties and proposed tighter trade rules to enforce environmental and labor standards.
(Additional reporting by John Irish; Editing by Peter Graff)