Standard & Poor’s, carrying out an earlier threat, downgraded Ameren’s Illinois utilities to junk-bond status today. Moody’s, a rival debt-rating agency, had taken similar action already. Both agencies have expressed concern about a move in the Illinois Legislature to freeze Ameren’s rates.

The Illinois Senate passed a rate-freeze bill last week, but the state’s contorted political alliances mean that this particular bill has little chance of becoming law. The House has passed a rate-freeze bill that affects both Ameren and Exelon of Chicago, but Senate President Emil Jones insisted on exempting Exelon from the Senate bill.

Regardless of the politics, S&P doesn’t like the threat that the bill’s passage represents. S&P analyst Barbara Eiseman says in a statement:

If signed into law, SB1592 would result in a significant revenue shortfall and materially affect the liquidity of Ameren’s Illinois utilities.

For its part, Ameren insists that the rate-rollback bill is unconstitutional. It says that the Senate vote jeopardizes assistance that it was planning to provide for Illinois customers.

S&P also downgraded the credit ratings of the parent Ameren, of Missouri subsidiary Union Electric and of AmerenEnergy Generating by one notch, to BBB-minus. That’s S&P’s lowest investment-grade rating.