Paris’s Zinith concert hall is usually reserved for the likes of Public Enemy, the Beastie Boys and B.B. King. On Wednesday night, French presidential candidate Francois Bayrou bounded on stage, basking in the kind of adulation that rock stars expect but most politicians only dream of. The overflow crowd cheered, clapped and stomped feet for a candidate who gets toy tractors as welcome gifts from supporters (he grew up on a farm), whose name many French mispronounce and who was polling in the single digits only a few months ago.

In recent weeks, Bayrou (pronounced bye-roo) has scaled the public opinion surveys to become a formidable challenger to the two front-runners — ruling party candidate Nicolas Sarkozy and Socialist Segolene Royal. With the election only four weeks away and with as many as half of potential French voters undecided, Bayrou of the small Union for French Democracy has transformed what was a two-person presidential campaign into an unpredictable three-way sprint for the Elysee Palace.

Though all three main candidates — each a member of the first post-World War II generation to lead France — are advocating change, Bayrou’s swift rise as an alternative candidate is a barometer of voter dissatisfaction with the country’s main political parties.

“What France needs is for these two parties in power for 25 years to be thrown out of power — out of the comfort of power and the comfort of the opposition,” Bayrou told his audience Wednesday night.

Bayrou, 55, is casting himself as a tractor-driving country boy who can lift France out of a lingering national malaise brought on by a floundering economy and the growing fear that globalization will erode the cultural and gastronomic arenas where the French still excel.

“With one candidate who would like France to be America and one who would like it to be Scandinavia, it is time to show France wants to be France!” Bayrou said Wednesday night. Sarkozy has stressed his US ties, while Royal wants to strengthen the welfare state. The audience exploded: “Bayrou, president!”

Two weeks ago, Bayrou pulled even in the polls with Royal, who is attempting to become France’s first female president. In the past few days, she has edged a few points ahead of him. In a poll published by the daily newspaper Le Parisien on Friday, Sarkozy and Royal each received the support of 26 percent of respondents, and Bayrou was favored by 21 percent.

But nearly every voter survey in the past two weeks indicates that if Bayrou can win enough votes April 22 to get into a top-two runoff election May 6, he will beat his opponent, whether it is Sarkozy or Royal. “He’s an old leader from the right, but he’s having success because the right’s candidate, Nicolas Sarkozy, is a little too right, and the Socialist candidate sometimes seems incompetent,” said Jean-Luc Parodi, research director at the Center for the Study of French Political Life.

Parodi and other pollsters say that despite a political ideology that is more closely linked to the ruling party than the Socialists, Bayrou is siphoning more voters from Royal than Sarkozy.

“In my family everybody votes for the left, and I used to support the Socialists. But this year I’ll vote for Bayrou,” said Emilie Montessuit, 28, an education adviser from the northeastern Paris suburb of La Courneuve. “I can’t choose Royal. I can’t be represented by someone who doesn’t have any charisma and whose platform is not realistic at all. I believe Bayrou is more honest. He’s from the country, he’s a real person and someone you can identify with.”

Bayrou, who speaks in a folksy southern France drawl, describes himself as the man in the middle. He says he wants to improve education and give French Arab and African minorities greater opportunities to succeed, but he also wants to boost small businesses and make France a louder voice in the European Union.

He says he doesn’t want to spend as much money on social programs as the Socialists and doesn’t want to be quite as draconian on immigration matters and security issues as the ruling party, the Union for a Popular Movement.