Tagg is one of several partners at a private equity fund (PE A for simplicity sake) that invests in other private equity funds.
One of the PE funds (PE B) they invest in has invested in the voting machine company but here is why it isn't true. The monies PE B used to invest in the voting machine company are not from PE A. When an entity invests in a PE company (be it a person or an entity like PE A) that entity can direct those investments towards specific parts of a portfolio i.e. a specific fund. From all accounts while PE A invested in PE B its monies were directed towards a fund that did not invest in the voting machine company.
Now it is true that PE B's co-founder worked with Romney so they have some control but according to a story the Cleveland Plains Dealer (hardly a R leaning paper) tampering with the voting machines is not all that easy because the voting machine company's machines are stand alone scanners used to scan paper ballots.
