"Common law is all of the court decisions at any level that fill in the gaps ... that the lawmaking bodies, the writers of statutory law, could not have reasonably been expected to cover, or simply didn't get around to covering," Deitzler said.
The 1982 ruling said the attorney general couldn't make decisions -- like appointing special attorneys -- that would fall under common law because the Legislature exclusively had the power to decide what happened with the office.
The ruling was tweaked in 2002, when the court said the Legislature could not get rid of the "inherent powers" of the attorney general.
Tuesday's ruling goes further, stating those inherent powers include common law powers. That means the Attorney General has the right to appoint and pay special assistants, because nothing in law says they cannot.
The opinion goes on to say the companies failed to show the private attorneys used in the case caused any harm.
"We have not and will not interfere with or disqualify a party's counsel merely because of allegations of improper conduct that has not occurred," the opinion states.
The Supreme Court also ruled a private attorney appointed as a special assistant attorney general is not an employee of the office, so the state ethics act does not apply.
Morrisey said the ruling is a "major victory" for his office. It allows the Attorney General to continue to prosecute consumer protection cases effectively and efficiently, he said
Companies involved in the lawsuit include Citigroup, Discover Financial Services, Bank of America, GE Money Bank, HSBC Card Services and JP Morgan Chase.
The companies can appeal to the U.S. Supreme Court. It wasn't immediately clear if they planned to do so.
Other Top HeadlinesSchool, library levies to be merged in November special election