Fighting an uphill battle to win a second term in an election that starts in less than three weeks, Sarkozy had previously declined to comment on new developments in a judicial probe into links between his party and France's richest woman.
French voters are accustomed to headlines about suspected corruption and the latest twist in a probe that has been under way for years has shown no sign yet of diverting voter focus away from the economy and jobs.
But the speculation prompted Sarkozy to break his silence on the issue, telling TV channel Canal+: "As usual ahead of a presidential election, there are a few stink bombs.
"It's classic, it will surprise nobody but it's not, for all that, very pleasant."
Sarkozy said his 2007 accounts had been independently checked and given an official stamp of approval at the time. "Why should there be a question of illegal financing when every millimeter of campaign income and outlays was subject to checks?" he asked.
Questions about Sarkozy's 2007 funding are part of a wider probe by investigators into his party's relations with billionaire Liliane Bettencourt, the elderly heiress to the L'Oreal cosmetics fortune.
Pressure mounted on Sarkozy when investigator Michel Gentil placed Patrice de Maistre, Bettancourt's former wealth manager, in custody last week.
Gentil wants to know more about de Maistre's ties to the then-treasurer of Sarkozy's party, former Labour Minister Eric Woerth, who, like de Maistre, is formally under investigation.
Investigations focus on Swiss cash withdrawals of as much of 800,000 euros ($1.06 million) in the months prior to Sarkozy's May 2007 election victory.
Sarkozy said there was nothing suspicious about visits he had paid to the Bettencourt home because they lived in a western Paris suburb where Sarkozy had been mayor for many years.
Socialist candidate Francois Hollande, who is tipped by all polls to win the May 6 election runoff, has steered clear of accusing Sarkozy of wrongdoing, but his ex-partner and former presidential candidate Segolene Royal - who lost to Sarkozy in 2007 - has not held her punches.
In a recent television interview, Royal said one reason Sarkozy needed re-election was to secure another five years of legal immunity, a privilege conferred on French presidents in office. ($1 = 0.7518 euros)
(Reporting By Brian Love; Editing by Robin Pomeroy)
Reuters
