Paulius Lukauskas, head of the staff of Simonyte, the candidate of the conservative Homeland Union–Lithuanian Christian Democrats, voiced suspicions that Nauseda might have spent around 50 percent more money than he had officially raised, and complained to the Central Electoral Commission.
Lukauskas said his suspicions were based on "available advertising monitoring data". The same suspicions were earlier voiced by Andrius Kubilius, a former conservative prime minister.
Nauseda's staff hit back, accusing Simonyte's team of "throwing unfounded suspicions, a common strategy in Lithuania's political life".
The independent candidate's staff said they had been spending money in an efficient and disciplined way, buying only the necessary services and relying on volunteers to do a lot of work for the campaign.
Figures from the Central Electoral Commission show Nauseda has raised over 196,000 euros and Simonyte has more than 533,000 euros in her campaign account.
The Homeland Union has transferred 330,000 euros to Simonyte.
Nauseda's staff say Simonyte's publicly-available campaign spending report shows that the party's donation, which was accepted last week, had been planned in advance.
The candidates will have to submit their audited campaign financing reports by August 26.