The U.S. Supreme Court appears likely to toss out restrictions on how much money some of the richest individual donors can give to political campaigns, opening the way for more cash to pour into U.S. elections.

In a hearing Tuesday, the second day of its new term, the court's conservative justices voiced repeated skepticism about overall limits on the amount individuals may contribute in a two-year federal election cycle.

Chief Justice John Roberts, possibly the pivotal vote in the case, said that telling an individual she can give the legal maximum of $2,600 US per election to only a handful of candidates for Congress "seems to me a very direct restriction" on constitutional free speech rights.

The court did not appear willing, however, to call into question all contribution limits. 

Supreme Court Campaign Finance

Republican activist Shaun McCutcheon wants the court to overturn the overall limits on what contributors may give in a two-year federal election cycle. (Susan Walsh/Associated Press)

U.S. President Barack Obama waded in, saying the elimination of a cap on some political contributions would be akin to telling big-dollar donors that anything goes.

Obama said no politician is innocent on campaign finance. But he said politicians should insist on rules to ensure voters are more important than donors.

Obama spoke at a news conference as the U.S. federal government shutdown entered its second week.

The top court's nine justices appeared closely divided as they weighed a challenge by Republican donor Shaun McCutcheon, an Alabama businessman, and the Republican National Committee to the overall limit on campaign contributions that donors can make to individual candidates and committees over the  two-year federal election cycle.

The court waded back into the politically-charged debate for the first time since its sweeping Citizens United v. Federal Election Commission decision in 2010. In that, the high court, split 5-4, lifted limits on independent expenditures, not co-ordinated with individual politicians or parties, by corporations and unions during federal election campaigns. The ruling greatly increased the amount of outside expenditures during the 2012 presidential election, experts said.

In Tuesday's argument, some justices suggested they might vote to lift the restrictions in question, but there was no sign of a desire to go further in weakening a key 1976 ruling, called Buckley v. Valeo, which upheld limits on campaign finance donations while also describing how courts should analyze such regulations in future.

Chief justice seeks middle ground

Chief Justice Roberts, who was in the majority in the 2010 case, appeared to be looking for some middle ground that would allow individual donors to give to more candidates without opening the floodgates to multimillion-dollar contributions.

McCutcheon can only give the maximum amount allowable to a total of nine candidates before he hits the cap under the current law.

"We are telling him he can't make that contribution no matter how modest," Roberts said.

The court's four Democratic-appointed justices appeared reluctant to lift the restrictions. Justice Anthony Kennedy, the court's regular swing vote, was suspicious of the government's current limits, but did not signal a strong interest in taking any dramatic steps.

In the current two-year election cycle (2013-14), an individual can give $2,600 US to a candidate or committee and $32,400 to a political party. But donors cannot exceed the $123,200 overall limit during that period. There is also a $48,600 cap on total donations to candidates and a $74,600 cap on donations to political action committees and parties.

Supporters of campaign finance laws say the new case poses another threat to the contribution limits that Congress first enacted in 1974, in an attempt to restore the public's faith in government after President Richard Nixon's resignation.

Solicitor General Donald Verrilli, the Obama administration's top Supreme Court lawyer, argued that the overall limits serve as a check on corruption. Without them, Verrilli said, donors could write checks of more than $3.5 million.

In recent years, opponents of U.S. campaign finance laws have won a string of victories at the country's 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 in 2006 of retired justice Sandra Day O'Connor, who often voted to uphold campaign limits, by Justice Samuel Alito, who is far more skeptical of the restrictions.

With files from The Associated Press