On Thursday evening, 500 Republican activists meeting in a Rochester, N.Y., banquet hall could have a big say over who succeeds retiring Rep. Amo Houghton (R).

At the very least, the vote of the Monroe County GOP nominating convention should cut the 29th district’s sizable Republican field in half.

Perhaps equally important, the results could determine which candidate in the hotly contested primary receives the endorsement of the Club for Growth, the group of hard-line fiscal conservatives that has played a growing role in GOP primaries nationwide, sometimes to the chagrin of national Republican officials. Club leaders have already announced that they will spend heavily to defeat the nominal GOP frontrunner, state Sen. Randy Kuhl.

“We’re waiting to see what Monroe County does,” said David Keating, executive director of the conservative, anti-tax group.

Six candidates are vying for the GOP nomination in a district that runs like an upside down “T” from just south of Rochester through lightly populated regions to the Pennsylvania border. Four of the six candidates hail from Monroe County, which encompasses greater Rochester, making it the district’s main population node.

Because of this, local party leaders are set on forging unity behind one of the four. If it can be done, a three-way regional primary on Sept. 14 will emerge.

The four Monroe County candidates — county Legislator Mark Assini, local transportation authority chairman Bill Nojay, businessman Geoff Rosenberger and county Legislature Majority Leader Bill Smith — have agreed in writing to defer to the winner of Thursday’s convention. That would send the victor into the primary against Kuhl, who hails from the Southern Tier, the vast region along the Pennsylvania border, and Assemblyman Brian Kolb (R), who is from Ontario County, located just south of Monroe.

Measured by population, Monroe County accounts for about one-third of the district’s Republican electorate.

GOP officials say they’re reasonably confident that the four Monroe candidates will honor their pledge. But privately they concede that anything is possible.

The New York Conservative Party, in an attempt to sway Republican convention-goers, has already signaled its intention to offer its ballot line to Assini in November.

“We’re fairly optimistic that Mark on the first or second ballot is going to get the designation,” said Thomas Cook, chairman of the Monroe County Conservative Party.

Cook acknowledged, however, that the Conservatives were unlikely to back Assini if he did not win the Monroe County GOP’s blessing.

Several observers who are monitoring the race said it is difficult to predict who will win on Thursday, suggesting that two or even three ballots may be necessary before someone gets a majority of the votes.

“I think it’s a real horse race,” Rosenberger said. “Everybody has a shot at this.”

Assini — who entered the race a year ago and had prepared to challenge Houghton even if he’d decided to seek a 10th term — has the support of many social conservatives. He also may be the best known candidate among party activists, simply because he has been campaigning for so long.

Nojay, a lawyer and political activist who has ties to Gov. George Pataki (R-N.Y.), is well known in the Rochester area for appearing in government-funded TV ads for the transportation authority he heads.

Rosenberger — the founder of an investment firm and the 1996 GOP nominee against Rep. Louise Slaughter (D-N.Y.) back before redistricting split Monroe County into four different districts — has already spent more than $300,000 of his own money to build his name recognition through TV ads.

Smith, a lawyer who has served in local government for eight years, appears to have several town GOP committees sown up.

While Kuhl and Kolb assembled their organizations and crisscrossed the district in search of primary support, the four Monroe candidates have run very targeted campaigns to date.

“At this stage, at least until we get out of this convention, this is like campaigning for a town council seat,” Smith said. “It’s very much politicking on the retail level.”

Victory on Thursday night hardly guarantees success in the GOP primary. New York Republicans agree that for a Monroe County candidate to have a chance of winning the primary, the runners-up on Thursday must honor their pledge to drop out of the race.

Kuhl has the most establishment support by far. His political base mirrors that of Houghton — the scion of the Corning Glass Works company, based in the Southern Tier city of Corning — and he has been endorsed by Houghton, Rep. John McHugh (R-N.Y.), and several state legislators, including state Senate Majority Leader Joseph Bruno (R) and state Sen. James Alesi (R), who hails from Monroe County.

Ira Treuhaft, a spokesman for Kuhl, said the candidate plans to stump for primary votes in Monroe County regardless of who emerges from Thursday’s convention.

“If the party bosses don’t want us to go into Monroe County, we’ll go to the people,” he said.

But Kuhl can also expect to have a big target on his back. The Club for Growth has vowed to defeat him, citing several votes Kuhl cast in favor of tax increases in the Republican-held state Senate.

The club has been in touch with the five other candidates to varying degrees, and most of the field would probably be palatable to the organization’s leaders, insiders say. Kolb, who has the smallest geographic base of all the candidates, could benefit from the fact that his campaign manager, Scott Armstrong, is a former leader of Change New York, a now-defunct Empire State spin-off of the Club for Growth.

The club’s involvement in the 29th district all but guarantees an entry by the Republican Main Street Partnership — a group dedicated to electing centrist Republicans that Houghton helped found. If so, Kuhl would be the likely beneficiary.

Any of the candidates — even Kuhl — would be significantly to the right of Houghton on most issues. That helps explain why national Democrats hold out some hope that their likely nominee, 26-year-old political operative Samara Barend, can win, even though the district gave George W. Bush a 10-point victory over Al Gore in 2000.