PARIS (Reuters) - Alain Juppe is holding onto his comfortable lead in the race for France's Republican party presidential nomination, a poll showed on Thursday, hours before the former prime minister locks horns with rivals in a televised debate.

The OpinionWay survey showed that 42 percent of respondents intended to vote for Juppe, a moderate conservative, in the primary contest's first round of voting, compared with 28 percent for his chief rival, former president Nicolas Sarkozy.

In a run-off, support for Juppe rises to 62 percent while Sarkozy trails with 38 percent, the poll showed.

With his pitch as the candidate who will restore national pride by acting tough on immigration and defending France against militant Islam, Sarkozy has been alienating centrist voters.

On Wednesday, the UDI party of centrists threw their backing behind Juppe, who regularly ranks as one of France's most popular politicians but struggles to steal media limelight from the energetic but often abrasive Sarkozy.

Roughly half the respondents to the survey who said they would back two other candidates, Francois Fillon and Bruno Le Maire, in a first round, intended to swing behind Juppe while fewer than a third planned to support Sarkozy in a run-off. The remainder did not state their intention.

The poll surveyed 700 people who said they would vote in the center-right primary between Oct 7 -11.

Unemployment was the main election issue for voters, the poll showed, with immigration and security close behind.

Turnout will be critical to the outcome of the two-round primaries on Nov. 20 and Nov. 27. A high turnout from a broad center-right voter base is seen as favorable to Juppe, while a low turnout in which only core party members turn up is seen benefiting Sarkozy.

Anyone willing to cough up 2 euros and sign a document saying they share the values of the right and center can take part in the primaries.

Opinion polls indicate that the winner of the Republican primary vote will be favorite to win the presidential election in April, most likely in a run-off against far right National Front leader Marine Le Pen.

France's Socialist Party is riven by divisions and President Francois Hollande has not yet declared he will seek a second term.

(Reporting by Richard Lough; Editing by Raissa Kasolowsky)