On March 1, the 66-year-old became France's first post-war president to be sentenced to prison when he was given a three-year term, two years of which were suspended, for corruption and influence peddling.
That case was one of several hanging over him since he left office.
Sarkozy has denied any wrongdoing, saying he is the victim of a vindictive judicial system with which he tangled while in power between 2007 and 2012.
In the trial opening Wednesday, which he is not expected to attend, the divisive rightwinger is accused of overspending on his failed 2012 re-election bid to the tune of 20 million euros ($24 million).
The money was spent on lavish US-style rallies in the final days of the race, as Sarkozy scrambled to fend off an unexpectedly strong challenge from his Socialist rival Francois Hollande.
Prosecutors say accountants had warned him that the campaign was set to blow the 22.5 million euro ($26.7 million) cap on spending between the first and second rounds of voting, but Sarkozy insisted on holding more events.
Investigators say his total spending on the second round came to nearly 43 million euros.
To hide the spending, the PR firm behind the campaign, Bygmalion, and officials in Sarkozy's UMP party (since renamed Les Republicains) are accused of conspiring to have the UMP foot the bill through a system of fake invoices.
The former president says he was unaware of the fraud -- unlike some of the defendants he is not charged with fraud, but with the lesser offence of illegal campaign financing. He fought for years to avoid a trial.