By Ingrid Melander

PARIS (Reuters) - The top two candidates for the French conservatives' presidential ticket lashed out at each other on Thursday, with Nicolas Sarkozy saying Alain Juppe would be too soft to carry out reforms and Juppe retorting Sarkozy's strategy was suicidal.

With less than three weeks before the primaries that will appoint the 2017 nominee, pressure is growing on Sarkozy, a former president, to find ways to close the gap with former prime minister Juppe, the frontrunner in opinion polls.

Whoever wins the Les Republicains party's two-round primaries on Nov.20 and 27 will most likely become France's next leader and succeed the deeply unpopular Socialist President Francois Hollande, opinion polls show.

"I really don't want a weak change of power," Sarkozy told a televised debate in a criticism of Juppe, whom he said would move too slowly, not carry out necessary reforms and be hostage to center-right ally Francois Bayrou.

"This attitude is suicidal," Juppe said, adding that refusing an alliance with Bayrou would boost far-right National Front chief Marine Le Pen's score and allow her to top the first round of the April 23/May 7 presidential election.

"I want to bring everyone together, the right, the center, those who are disappointed with Hollande ... and those who are disappointed with the National Front," Juppe said.

Bayrou, a centrist who voted for Hollande instead of Sarkozy in 2012, is a hated figure for some grassroots Les Republicains supporters. Sarkozy has been targeting him for days in a bid to boost his election chances.

A snap poll by Elabe pollsters after the debate showed that this strategy might have some success amid the party's rank and file but not necessarily be enough.

Some 34 percent of voters considered that Juppe won the debate, while 24 percent thought Sarkozy was the victor. But amid centrist and conservative sympathizers, Sarkozy edged ahead with 31 percent vs 28 percent for Juppe.

Sarkozy, who has been campaigning on a hardline law-and-order platform, is unhappy about a party rule that allows anybody willing to pay two euros and sign a declaration that they share center-right values to vote in the primaries.

That is thought to favor Juppe, who has more support among centrist voters as well as left-wing voters tempted to take part in primaries many effectively view as the presidential election's first round.

(Additional reporting by Richard Lough, Simon Carraud and Sophie Louet; Editing by Andrew Hay)