LILLE — Three weeks ago, left-wing firebrand Jean-Luc Mélenchon was an entertaining sideshow in France's presidential race — fun to watch on TV thanks to his cutthroat speaking style but ultimately no threat to the frontrunners.

Nobody is laughing now.

Less than two weeks before the first round of the presidential election, the anti-EU, pro-Moscow Mélenchon is busting open what was already an unpredictable race and threatening to sink the campaign's only viable pro-EU candidates.

Benefitting from the collapse in support for socialist candidate Benoît Hamon, Mélenchon enjoys the support of 18.5 percent of votes. That's on a par with conservative former prime minister François Fillon and only 3.5 percentage points behind centrist frontrunner Emmanuel Macron, according to a daily poll of first-round voting intentions by Ifop.

Mélenchon is still no shoo-in for the presidency. But his increasing popularity is becoming a problem for the two frontrunners, because a large swathe of his backers — 42 percent of them, according to Ifop — say they won't vote in the second-round run-off between the top two candidates if Mélenchon is no longer in the picture.

Far-right leader Marine Le Pen, who is almost certain to make the second round, can rejoice.

"Abstention among Mélenchon voters can turn into a determining factor," Ifop's Jérome Fourquet said. "Many people in this group are going to find it impossible to vote for 'Macron the banker' — how many exactly it's hard to say. What's clear is that the tactical voting we saw in 2002 that kept Jean-Marie Le Pen out of power is not going to work as well this time."

'Operation Abstain'

On Wednesday night, at a massive rally in the northern city of Lille, Mélenchon offered a glimpse of his appeal for millions of French people.

Dressed in his signature Mao-style jacket, the candidate put on a freewheeling performance that alternated between mocking commentary of news articles, snippets of stump speeches and diatribes against rivals.

Hands-free thanks to the kind of microphone Madonna would wear, he moved around the small stage to make sure he addressed everyone. Outside, thousands who were unable to get in stood in the cold watching him on a giant screen for more than two hours. The candidate's voice, amplified by powerful speakers, could be heard half a kilometer away.

Mélenchon explained the thrust of his speech as seeking to show "two visions of the world that oppose one another."

But the substance was devoted to his rivals, namely Macron, whom he repeatedly referred to as a "banker." If either Macron or Fillon were elected, Mélenchon bellowed at one point, then they would make ordinary men and women "spit blood" from overwork.

"In 2008 we both had a membership card for the Socialist Party," he went on, referring to Macron. "I turned my card in to defend my ideas. He turned his card in to join the Rothschild bank. That's about as much as you need to know."

The crowd was vocal. Every time Macron's name was mentioned, they booed long and loud. Mélenchon let them do so, saying at one point: "Doesn't it feel good to shout?" But when they hissed at the mention of the Communist Party, which has not thrown its support behind Mélenchon, he cut them short. "None of that," he snapped. "They are our friends."

The anti-Macron message was being heard loud and clear.

Eight out of 10 supporters questioned by POLITICO said if Mélenchon did not make it into the election's final round, they would abstain — even if that meant the head of the National Front, whom most otherwise opposed, became France's next president.

'The Trojan Horse of finance'

"Macron represents capitalism, which along with financial lobbies is shaping Europe," said a 60-year-old art teacher who only wanted to be identified as Mathilde. "He is the Trojan horse of finance."

She added: "If in the second round Mélenchon is not there, so be it, I will not vote. We'll have the far-right. I don't care. I can promise you that there are many, many people in my situation. We are just completely fed up."

Lucas, an 18-year-old preparing to vote in his first presidential election, was on the same page. "If it's Macron against Le Pen in the second round, I am not voting."

Ditto Lydie and Fabrice from Lille. "What I will do is take two pieces of paper with Mélenchon's name on them and slip them in the ballot box at both rounds," said Lydie, a retired postal worker. "I could not bring myself to vote for [Macron]," echoed Fabrice.

Mélenchon offers such a similar vision of world affairs to the National Front chief.

A massive refusal of Mélenchon backers to switch to Macron in the election’s second round would seriously harm his chances.

While polls show Macron prevailing in the final round, he will need supporters of rivals on the left and right to help him over the finish line. Much of his second-round support is implied as part of a tactical voting scheme known as the "Republican Front."

However, conversations with several Mélenchon backers suggested they had little interest in forming a Republican Front. A reporter from a local newspaper who had voted against Le Pen in a regional election in 2015 said she would be unable to do so this time around against Macron.

Others shrugged when asked if they would be able to accept a Le Pen presidency. One said he would vote for Le Pen against Macron in the final round. Two hinted that a President Le Pen was preferable to a President Macron, because she represented a "real change" whereas the former economy minister was "the same old recipes."

If some Mélenchon backers are ready to accept a Le Pen presidency, it may be partly because the candidate offers such a similar vision of world affairs to the National Front chief.

Both advocate a confrontation with the EU, and a referendum if they don't get what they want; both are skeptical of the United States, friendly toward Russia and want to redefine France's relationship with NATO.