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“Our money would be tripled against us,” said Mr. Voeller. “So if we wanted to support a candidate, we’d have to think long and hard about whether it would do any good.”

Even so, the law has proven popular among Arizona politicians. About two-thirds of all candidates for state office took advantage of the public-financing option through the 2008 election, said Michael Becker, voter education manager of Arizona’s Citizens Clean Elections Commission, which administers the law.

The Supreme Court suspended the matching-funds provision prior to the 2010 election in anticipation of a decision, which dropped participation to about 50 percent of all candidates, said Mr. Becker.

Funding for the public financing comes mainly from a 10 percent surcharge on criminal fines and penalties, such as speeding tickets, as well as from voluntary contributions. In order to qualify for public funding, candidates must first collect $5 donations from a specified number of donors, depending on the office.

The ruling leaves in place some provisions of the Arizona law. Publicly funded candidates will continue to receive a base-level grant from the state after meeting certain thresholds for both the primary and general elections.

That could change, however, as a result of a referendum to strike down the entire law slated to appear on the 2012 ballot. The measure, referred to the ballot by the Republican-controlled state legislature, would eliminate all public funding in Arizona elections.

The high court’s ruling Monday “is really the first break in the dam on public financing in Arizona,” said Mr. Voeller.