Pheu Thai Party spokesman Prompong Nopparit on Thursday filed a lawsuit charging Democrat chief adviser Chuan Leekpai for defamation.
"Chuan, despite his seniority for serving as the prime minister twice, has made irresponsible remarks to tarnish my reputation," he said.
Prompong said he wanted to salvage his credibility as spokesman after Chuan had labelled him as a "daily lier".
Chuan made his remarks as a rebuttal after Prompong told reporters that a senior Democrat, whose initial is Ch, lobbied for the aspiring candidate from the New Politics Party not to enter the race for the by-election in Bangkok Constituency 6.
The Criminal Court is scheduled on August 26 to rule whether to accept the case for judicial review.