Italian prosecutors are investigating two former senators who switched to then prime minister Silvio Berlusconi's coalition in December 2010 and backed his government in a key confidence vote.
Domenico Scilipoti and Antonio Razzi, former members of the centre-left Italy of Values party, are suspected of corruption, prosecutors said.
Both politicans deny any wrondoing in the case.
The case was opened after complaints by party founder and former graft-busting magistrate Antonio di Pietro.
"I have always acted in the public interest," Scilipoti said, while Razzi dismissed the allegations against him as "laughable".
"I have never sold myself and I never will," Razzi said, adding that he was happy to cooperate with Italian magistrates.
In a separate case, Berlusconi is being investigated on suspicion of paying former senator Sergio De Gregorio three million euros to switch to his People of Freeom party in 2006, a move allegedly aimed at bringing down the centre-left government of Romano Prodi.
The Prodi government, which had a wafer-thin majority in the Senate, fell in 2008.