WASHINGTON (AP) — The Supreme Court is tackling a challenge to limits on contributions by the biggest individual donors to political campaigns.

The case being argued at the high court Tuesday is a test of the Roberts court's readiness to take its most aggressive swipe at campaign finance laws since its Citizens United decision in 2010 took the lid off independent spending by corporations and labor unions.

Supporters of campaign finance laws say the case poses a threat to the contribution limits that Congress first enacted in 1974, in the wake of Watergate abuses.

Republican activist Shaun McCutcheon of Hoover, Ala., the national Republican Party and Senate GOP leader Mitch McConnell of Kentucky want the court to overturn the overall limits on what contributors may give in a two-year federal election cycle. The total is $123,200, including a separate $48,600 cap on contributions to candidates, for 2013 and 2014.

The limit on individual contributions to any candidate for Congress in any given election, currently $2,600, is not at issue in this case.

McCutcheon, owner of the Coalmont Electrical Development Corp. in McCalla, Ala., said he will spend a few hundred thousand dollars in the current election cycle, including large donations to so-called super PACs that are not affiliated with candidates.

In 2011 and 2012, McCutcheon gave the symbolically significant $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.

"It's a very important case about your right to spend your money how you choose," he said.

Solicitor General Donald Verrilli Jr. told the court that donors "could potentially funnel massive amounts of money to a favored candidate" in the absence of the overall limit. One donor, giving the maximum allowed to every congressional candidate, political party and political action committee, he said, would exceed $3 million in contributions in a single election cycle.

The Republican challengers are asking the court to take an even more aggressive approach than merely overturning these particular limits. McConnell is leading the charge to urge the justices to ditch their 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 in 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 the per-election contribution limit of $2,600 to any candidate for Congress would be threatened, said Fred Wertheimer, a longtime supporter of stringent campaign finance laws.

"This is not about some abstract concept of aggregate limits. It's about re-establishing a system where huge contributions can be given directly to benefit candidates and parties, and it will create opportunities for the corruption of governmental decisions," said Wertheimer, president of the nonpartisan Democracy 21 group.

In recent years, opponents of campaign finance laws have won a string of victories at the Supreme Court, typically by a 5-to-4 vote with the court's conservative justices in the majority. Their winning streak dates to the replacement of Justice Sandra Day O'Connor, who often voted to uphold campaign limits, by Justice Samuel Alito, who is far more skeptical of the restrictions.

The sweep of the decision could come down to the willingness of Alito and Chief Justice John Roberts to join their three conservative colleagues, Anthony Kennedy, Antonin Scalia and Clarence Thomas, in pushing for a tougher review of laws that limit contributions. Those three justices already have said they would consider doing away with the distinction between campaign giving and spending.