In a 5-4 vote won by conservative justices, the court struck down limits in federal law on the total amount of money a contributor can give to candidates, political parties and political action committees.
The decision wipes away the overall limit of $123,200 for 2013 and 2014. It will allow the wealthiest contributors to pour millions of dollars into candidate and party coffers, although those contributions will be subject to disclosure under federal law. Big donors, acting independently of candidates and parties, already can spend unlimited amounts on attacks ads and other campaign efforts that have played an increasingly important role in elections.
The justices left in place limits on individual contributions to each candidate for president or Congress, now $2,600 per election.
The court's conservative majority, under the leadership of Chief Justice John Roberts, continued its run of decisions back to 2007 rejecting campaign finance limits as violations of the First Amendment speech rights of contributors. The most notable among those rulings was the 2010 decision in Citizens United that lifted limits on independent spending by corporations and labor unions.
Roberts said the aggregate limits do not act to prevent corruption, the rationale the court has upheld as justifying contribution limits.
The overall limits "intrude without justification on a citizen's ability to exercise 'the most fundamental First Amendment activities,'" Roberts said, quoting from the court's seminal 1976 campaign finance ruling in Buckley v. Valeo.
Justice Clarence Thomas agreed with the outcome of the case, but wrote separately to say that he would have gone further and wiped away all contribution limits.
Justice Stephen Breyer, writing for the liberal dissenters, said that the court's conservatives had "eviscerated our nation's campaign finance laws" through Wednesday's ruling and the Citizens United case.
"If the court in Citizens United opened a door, today's decision we fear will open a floodgate," Breyer said in comments from the bench. "It understates the importance of protecting the political integrity of our governmental institution. It creates, we think, a loophole that will allow a single individual to contribute millions of dollars to a political party or to a candidate's campaign."
Roberts said the dissenters' fears were overstated because other federal laws prohibit the circumvention of the individual limits and big donors are more likely to spend a lot of money independently in support of a favored candidate.
Reaction to the ruling generally followed party lines, with advocates of capping money in politics aligned with Democrats in opposition to the decision.
Republican National Committee Chairman Reince Priebus called the Supreme Court decision "an important first step toward restoring the voice of candidates and party committees and a vindication for all those who support robust, transparent political discourse."
The GOP and Senate Republican Leader Mitch McConnell of Kentucky had argued that other decisions relaxing campaign finance rules had diminished the influence of political parties.
Democratic Sen. Chuck Schumer of New York said, "This in itself is a small step, but another step on the road to ruination. It could lead to interpretations of the law that would result in the end of any fairness in the political system as we know it."
Congress enacted the limits in the wake of Watergate-era abuses to discourage big contributors from trying to buy legislative votes with their donations and to restore public confidence in the campaign finance system.
But in a series of rulings in recent years, the Roberts court has struck down provisions of federal law aimed at limiting the influence of big donors as unconstitutional curbs on free speech rights.
In the current case, Republican activist Shaun McCutcheon of Hoover, Ala., the national Republican Party and Senate GOP leader McConnell challenged the overall limits on what contributors may give in a two-year federal election cycle. The limits for the current election cycle included a separate $48,600 cap on contributions to all candidates.
McCutcheon gave the symbolically significant amount of $1,776 to 15 candidates for Congress and wanted to give the same amount to 12 others. But doing so would have put him in violation of the cap.
Relatively few Americans play in the big leagues of political giving. Just under 650 donors contributed the maximum amount to candidates, PACs and parties in the last election cycle, according to the Center for Responsive Politics.
The court did not heed warnings from Solicitor General Donald Verrilli Jr. and advocates of campaign finance limits that donors would be able to funnel large amounts of money to a favored candidate in the absence of the overall limit.
The Republicans also called on the court to abandon its practice over nearly 40 years of evaluating limits on contributions less skeptically than restrictions on spending.
The differing levels of scrutiny have allowed the court to uphold most contribution limits, because of the potential for corruption when donors make large direct donations to candidates. At the same time, the court has found that independent spending does not pose the same risk of corruption and has applied a higher level of scrutiny to laws that seek to limit spending.
If the court were to drop the distinction between contributions and expenditures, even limits on contributions to individual candidates for Congress, currently $2,600 per election, would be threatened, said Fred Wertheimer, a longtime supporter of stringent campaign finance laws.
The case is McCutcheon v. FEC, 12-536.
Associated Press writer Steve Peoples in Boston contributed to this report.
Follow Mark Sherman on Twitter at: @shermancourt
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