The Democratic nominee for state attorney general said Monday that an independent prosecutor should be appointed to take over the investigation of alleged corruption in the Legislature from Republican incumbent Tom Corbett.

John Morganelli, the Northampton County district attorney, said Corbett has a conflict of interest because of contributions he received for his 2004 campaign from Republican legislative leaders who were in power during the period that is being investigated.

Morganelli also questioned whether Corbett’s office adequately protected evidence in Republican hands while pursuing the first round of charges against House Democrats.

‘‘This investigation has been botched from the beginning,’‘ Morganelli told several dozen people at a Pennsylvania Press Club luncheon.