WASHINGTON - The Supreme Court's conservative justices signaled during oral arguments that they would vote to strike down another campaign funding law and make clear that states and cities may not try to "level the playing field" between candidates for public office.

The justices objected to part of an Arizona law that provides public funds to candidates for state office if they agree to forgo private fundraising. The disputed provision gives extra "matching funds" to candidates who face a well-funded and free-spending opponent.

Conservatives and libertarians in Arizona sued and argued that the extra funds unfairly penalized candidates who relied on private fundraising. They lost in the U.S. Ninth Circuit Court of Appeals, which upheld the law and said it did not limit the free speech rights of the candidates who depended on private funds.

The Supreme Court has been split 5-4 on campaign funding cases, with the conservative bloc in the majority, and that split was evident again Monday.

Last year, the conservative majority said spending on political campaigns cannot be limited by law, even to prevent big corporations from using their profits to sway elections. Those justices sounded just as determined Monday to make clear that governments cannot seek to "equalize" spending between a well-funded candidate and a challenger who is trying to keep pace.

Arizona's voters adopted the Citizens Clean Elections Act in 1998. It offers state candidates a basic grant to run for office and extra "matching funds" if their opponent is spending heavily with private funds. For example, a candidate for the state legislature who receives a grant of $21,000 to run in a general election can receive up to twice that amount in extra funds to match the spending of an opponent.

Several states, including Maine, Connecticut and Florida, adopted similar laws, as have at least a dozen municipalities, including Los Angeles and New York.

"Under our precedents, leveling the playing field is not a legitimate state purpose," said Chief Justice John Roberts. "I checked the Citizens Clean Election Commission website this morning, and it says that this act was passed to 'level the playing field' when it comes to running for office. Why isn't that clear evidence that it's unconstitutional?" he asked.

Justice Samuel Alito voiced the same view a few minutes later.

The court's three other conservatives -- Justices Antonin Scalia, Anthony Kennedy and Clarence Thomas -- have regularly voted to strike down campaign funding restrictions.

The court's liberal justices spoke up in defense of Arizona's law, and an Obama administration lawyer joined the argument on Arizona's side.