By Andrew Callus and Ingrid Melander

PARIS (Reuters) - The race for France's conservative presidential nomination looked tighter than ever on Saturday, with voting due to begin within 24 hours and polls suggesting whoever emerges on top will make it all the way to the Elysee Palace.

Ahead of Sunday's vote, which will select two candidates for the decisive Nov. 27 second round, centrist Alain Juppe had lost most or all of his early polling lead as his fellow former prime minister Francois Fillon enjoyed a late surge.

After Britain's shock "Brexit" vote in June and last week's election of Donald Trump as U.S. president, the French election next spring will be the next test of strength between weakened mainstream political forces and rising populist insurgents.

Opinion polls have for months suggested that far-right National Front leader Marine Le Pen will make it to the decisive run-off in May, but that Juppe would beat her if he won the conservative Les Republicains nomination.

His lead, however, has been eroded by two party rivals to his right - ex-president Nicolas Sarkozy and Fillon, who served as Sarkozy's prime minister from 2007-2012.

"I can sense a surprise coming," Fillon told supporters at a rally on Friday in Paris. He urged them to "shake up" the primaries, winning wide applause and shouts of "Fillon for president" from a crowd of over 3,000.

Long trailing in the polls, Fillon has come from behind in the past week, making the race even harder to call. He was judged the winner of Thursday's final televised debate before the weekend vote, an opinion poll showed.

"I'M NOT HILLARY"

Anyone can vote in the primary, in which there are seven candidates, which opens an already unpredictable contest to tactical participation by left-leaning and far-right voters.

Lack of confidence in pollsters, who failed to predict Trump's win and Britain's vote to quit the European Union, has deepened uncertainty surrounding the conservative and Socialist primaries as well as the election itself.

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But Juppe was confident on Friday that no such upset will happen. "I am not Hillary Clinton," he said on public radio, "and France is not America".

Under unpopular President Francois Hollande, France's ruling Socialists are deeply divided and seen as unlikely to get past the presidential election's first round next April. That would clear the way for whoever clinches the conservative nomination to face Le Pen in the deciding vote.

Juppe is seeking to draw support from centrists and left-wing voters determined to prevent a Sarkozy comeback or Le Pen's National Front from winning power.

Sarkozy, whose campaign has emphasized law and order, mocked Juppe at a rally in Nimes, southern France, for being too "soft". "I will be the president who re-establishes the authority of the state," he said.

Some market analysts have started taking more seriously the possibility Le Pen could become president, an event some believe could weaken or break up the European Union and the euro zone.

But polls have consistently shown her falling short. The electoral system requires her to win over 50 percent of votes in a second-round run-off, and she has persistently polled only around 30. French pollsters have in recent elections also tended to overestimate her appeal rather than underplay it.

But should Sarkozy or Fillon emerge as her conservative opponent, polls and analysts suggest, Le Pen's electoral prospects would improve.

In a note after Thursday's TV debate, Charles Lichfield of Eurasia Group gave Le Pen a 25 percent likelihood of victory against Juppe. But her chances would jump to 35 percent against Sarkozy or Fillon, he said, reflecting their lack of appeal beyond the right-wing electorate.

(Additional reporting by Maya Nikolaeva and Brian Love; Editing by Laurence Frost and Dale Hudson)