The prosecution claims Sarkozy greatly exceeded a spending limit of 22.5 million euros (USD 24 million) by using false billing from a public relations firm called Bygmalion.
The source said one of two judges in charge of the case, Serge Tournaire, had decided on February 3 that the case should go to trial after the failure of Sarkozy's legal efforts to prevent it in December.
Bygmalion allegedly charged 18.5 million euros to Sarkozy's rightwing party -- which at the time was called the UMP, but has since been renamed the Republicans -- instead of billing the president's campaign.
Only one other president -- Jacques Chirac -- has been tried in France's fifth republic, which was founded in 1958. He was give a two-year suspended jail term in 2011 over a fake job scandal.
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Questioned by police in September 2015, Sarkozy said he did not recall ever being warned about the accounting and described the controversy as a "farce", putting the responsibility squarely on Bygmalion and the UMP.
While the so-called Bygmalion case is the most pressing, 61-year-old Sarkozy has been fighting legal problems on several fronts since losing the 2012 election to President Francois Hollande.
In a surprise result, he was eliminated in November in the first round of a primary contest, trailing the eventual winner Francois Fillon and former prime minister Alain Juppe.
The son of a Hungarian immigrant father, Sarkozy was nicknamed the "bling-bling" president for his flashy displays of wealth.