PARIS — A French court on Thursday sentenced Nicolas Sarkozy, the former president, to a year in jail for illegally financing his failed 2012 re-election campaign by wildly exceeding France’s strict electoral spending limits.

Mr. Sarkozy, 66, was president from 2007 to 2012. Though he is no longer active in politics and continues to be dogged by multiple legal entanglements, he is still an influential voice on the French right. He has not yet said whether he would appeal the conviction.

It is the second time that Mr. Sarkozy has been sentenced to prison. In March, he became the first former president in France’s recent history to be sentenced to actual jail time after he was convicted on charges of corruption and influence peddling for trying to illegally obtain information from a judge on a legal case against him.

Mr. Sarkozy has appealed that conviction, which put the sentence on hold. Still, it made him only the second former president in France’s modern history to be convicted of a crime — Jacques Chirac was found guilty in 2011 of embezzling and misusing public funds when he was mayor of Paris.

The verdict against Mr. Sarkozy on Thursday came after a yearslong investigation and a trial in May and June, both of which focused on his 2012 re-election campaign and on France’s stringent electoral rules.

Under French law, spending on electoral campaigns is capped to ensure candidates compete on a level playing field. In 2012, the limit for presidential campaigns, per candidate, was about 16.8 million, or about $19.7 million, in the first round of the elections, and about 5.7 million, or about $6.7 million, on top of that in the second round for the two top vote-getters, who included Mr. Sarkozy.

But suspicions that his campaign had exceeded those limits arose after the election. Prosecutors began an investigation in 2014, causing turmoil within Mr. Sarkozy’s political party.

Ultimately, prosecutors determined that the campaign had spent at least 42 million, or about $50 million — almost twice the legal limit.

The case became known as the Bygmalion affair, named for the public relations and event planning company suspected of issuing false invoices to Mr. Sarkozy’s political party for rallies that were actually for Mr. Sarkozy’s presidential campaign. Prosecutors argued that the goal of the fraud was to hide the overspending from the electoral authorities.

Mr. Sarkozy has denied being aware of any false billing, and he was not charged with wrongdoing in that regard. Instead, the charges of illegal campaign financing relate only to the overspending, for which he has already paid a fine.

During the trial, Mr. Sarkozy rejected the prosecution’s portrayal of a lavish campaign, suggesting that the false invoices had been used instead to enrich Bygmalion — led at the time by close friends of Jean-François Copé, the president of Mr. Sarkozy’s party and one of the former leader’s political rivals.

Mr. Sarkozy also claimed that in 2012 he had been extremely busy with his presidential duties and had barely been involved with the campaign’s budgeting and logistics.

“I was president, head of the Group of 20, and in the campaign, I was directing political strategy,” Mr. Sarkozy told the court in June. “Organizing rallies, the sound systems, the lighting — I had better things to do.”

But prosecutors asserted that Mr. Sarkozy had neglected warnings from his aides, especially over a profusion of campaign events, some of them expensive, large-scale rallies. As a veteran politician with years of experience, prosecutors argued, he could not have ignored signs that his campaign’s spending was out of control.

Thirteen other people were also accused of involvement in the fraud, including former campaign staff members, party officials, aides close to Mr. Sarkozy and former executives at Bygmalion. But prosecutors concluded that there was not enough evidence to determine who had masterminded the false billing scheme in the first place.

Mr. Sarkozy has repeatedly denied any wrongdoing in the web of legal cases that has plagued him since he left office. Some of them have been dropped, including one in which he was accused of manipulating the heiress to the L’Oréal cosmetics fortune into financing his 2007 presidential run.

But Mr. Sarkozy is still dogged by accusations that his campaign received illegal financing from the government of the Libyan strongman Col. Muammar el-Qaddafi, who died in 2011. The investigation into those accusations, the most serious one against him to date, is still continuing.

Despite a failed comeback attempt in 2016, Mr. Sarkozy is still popular with the base of his conservative party, Les Républicains, which has yet to settle on a candidate for the 2022 presidential elections. Mr. Sarkozy’s endorsement is coveted by many of those jockeying for the position.

Constant Méheut contributed reporting.

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