By Steve Kingstone
BBC News, Sao Paulo
Brazil has launched a parliamentary inquiry to consider allegations that the ruling Workers' Party offered bribes in return for political support.
Mr Jefferson will be among the first witnesses to be called
The claim was made by the Labour Party leader, Roberto Jefferson.
He said the Workers' Party paid MPs up to $12,000 (£6,570) a month in return for remaining loyal to the government's parliamentary coalition.
The Workers' Party official alleged to have paid the bribes has publicly denied the claims.
The investigation team consists of five lawmakers from the lower house of Brazil's parliament.
It is there that the alleged vote-buying is said to have taken place.
Mr Jefferson will be among the first witnesses to be called.
His claim was formally denied for the first time on Wednesday by Delubio Soares, the Workers' Party treasurer and the man accused of organising the payments.
Mr Soares said he would make his tax and banking details public. He said Mr Jefferson, who faces his own corruption allegations, was trying to blackmail the government.
This potentially explosive affair is now at a pivotal moment.
Mr Jefferson has let it be known that he has taped evidence incriminating members of the government. Opponents say it is a desperate bluff.
For President Luiz Inacio Lula da Silva, who leads the Workers' Party, there are more anxious days ahead.