The Supreme Court on Monday struck down part of Arizona’s public campaign finance law, the latest in a series of its rulings holding that the right of political speech trumps government efforts to restrain the power of money in elections.

The court rejected Arizona’s system of providing additional funding to publicly funded candidates when they face big-spending opponents or opposition groups. The system has been used in every statewide and legislative election since voters approved it in 1998, after a rash of political scandals in the Arizona capitol.

But the court, in a 5 to 4 ruling, said the law impermissibly forces privately funded candidates and independent political organizations to either restrain their spending or risk triggering matching funds to their publicly financed opponents.

“The First Amendment embodies our choice as a nation that, when it comes to such speech, the guiding principle is freedom — the ‘unfettered interchange of ideas’ — not whatever the state may view as fair,” wrote Chief Justice John G. Roberts Jr.

Under Roberts, the court has sided every time with those who challenge government restrictions on campaign finance. The court’s conservative majority — Roberts and Justices Antonin Scalia, Anthony M. Kennedy, Clarence Thomas and Samuel A. Alito Jr. — has declared that major portions of the McCain-Feingold campaign finance act are unconstitutional.

And the loosening of spending constraints on corporations and unions in last year’s Citizens United v. Federal Election Commission was a game-changing decision that roiled the midterm elections.

“The court’s decision today, like other recent decisions, makes clear that the First Amendment is not an exception to campaign finance laws; it is the rule,” said Institute for Justice lawyer Bill Maurer, who argued the challengers’ case.

Monica Youn, who heads the money in politics project at New York University’s Brennan Center for Justice, said the court appears to be creating “a new set of rights” that she characterized as “the right to speak without response” and the “right to preserve monetary advantage.”

New Justice Elena Kagan, reading her dissent from the bench for the first time, seemed to emerge as a spokeswoman for the minority, saying that states have an interest in combatting “the stranglehold of special interests on elected officials.”

“The First Amendment’s core purpose is to foster a healthy, vibrant political system full of robust discussion and debate,” Kagan said. “Nothing in Arizona’s anti-corruption statute, the Arizona Citizens Clean Elections Act, violates this constitutional protection.”

Four states — Maine, New Mexico, North Carolina and Wisconsin — have similar “trigger funds” in their public campaign finance laws, as do some cities, including New York and San Francisco. These laws are now open to challenge.

Advocates of campaign finance restrictions found a silver lining in Roberts’ assertion that “we do not today call into question the wisdom of public financing as a means of funding political candidacy.” The court ruled in 1976 that the presidential public financing system was constitutional.

Bradley S. Phillips, who argued the case for Arizona, said the ruling was a narrow one and that “there are all sorts of ways that public funding laws can be crafted.”

The court made it clear last summer that it had concerns about the Arizona system, ordering the state not to distribute the matching funds in the midst of the 2010 elections.

Arizona’s system provides qualified candidates with an initial grant of money. It also gives them “matching funds” if they face privately financed candidates who spend more, or if an independent group spends money against them or for their opponents.

Challengers, including a conservative political group and candidates who have raised their own campaign funds, said the matching funds are unconstitutional. They say the system forces candidates to curtail their spending, and thus their political speech, to avoid triggering more funds for their opponents.

In its 2008 decision in Davis v. FEC, the court in a 5 to 4 vote struck down the “millionaire’s amendment” in the McCain-Feingold campaign finance act. That provision had allowed a congressional candidate to raise money in excess of contribution limits if his or her opponent was spending large sums of personal wealth.

The state says that about two-thirds of Arizona candidates opt for public funding and that campaign spending has increased over the years. It contends there is no reliable evidence that candidates curb spending to deny matching funds to their opponents.

Roberts went to lengths to describe the decision as following decades of the court’s jurisprudence. “The dissent criticizes the Court for standing in the way of what the people of Arizona want,” Roberts wrote. “But the whole point of the First Amendment is to protect speakers against unjustified government restrictions on speech, even when those restrictions reflect the will of the majority.”

Kagan countered, “We have never, not once, understood a viewpoint-neutral subsidy given to one speaker to constitute a First Amendment burden on another.”

The two sparred over whether Arizona’s law is intended to thwart corruption by reducing politicians’ reliance on campaign contributions or to “level the playing field” for all candidates. The former, the court has found, is a compelling state interest, but not the latter.

By its own admission, Arizona wants to level the playing field, Roberts said. “ ‘Leveling the playing field’ can sound like a good thing,” Roberts wrote. “But in a democracy, campaigning for office is not a game. It is a critically important form of speech.”

Kagan said that Arizona’s law “fostered both the vigorous competition of ideas and its ultimate object — a government responsive to the will of the people.” She concluded, “Truly, democracy is not a game.”

The combined cases are Arizona Free Enterprise Fund v. Bennett and McComish v. Bennett.