Just days after a Philadelphia judge rebuffed a legal challenge to fund-raising caps aimed at preventing deep-pocket donors from driving city policy, the U.S. Supreme Court sounded what might be the death knell for another key provision of the city's campaign law.

On Thursday, the high court tossed out federal rules designed to level the playing field when rich candidates spend their own money. Under the so-called millionaire's amendment, candidates are permitted to raise over-the-limit donations against a big spender.

That same principal is enshrined in the city's campaign law, which triggered higher giving limits for Michael Nutter and three other candidates in 2007 when personal spending by mayoral contender Tom Knox rose into the millions.

Will the city's millionaire rule face a successful constitutional challenge? Hard to say. But the local election watchdog group Committee of Seventy says that's a real danger now.

At least the city campaign law's core protection remains intact: On June 20, city Judge Gary F. DiVito dismissed a lawsuit that sought to free the failed mayoral campaign of U.S. Rep. Robert A. Brady (D., Pa.) from fund-raising limits of $5,000 for individuals and $20,000 for political committees.

Brady faces paying off campaign legal fees of nearly $450,000. That would be easier if he could collect big checks above the limits. As the city's Ethics Board says, though, such unlimited fund-raising after an election would gut the campaign law's safeguards against pay-to-play politics. Candidates would merely run up debts while seeking office, then tap big donors.

Late last week, Seventy smartly reminded Mayor Nutter of his pledge to launch a full review to fine-tune city campaign rules. That's a good step toward keeping local political reform alive.