JUNEAU — Republican U.S. Senate hopeful Dan Sullivan said Tuesday that he supports the U.S. Supreme Court decision that allowed unlimited spending by unions and corporations in political races even as he proposed a way to curb outside influence in his contest.

Sullivan called on Democratic Sen. Mark Begich to sign a pledge asking special interest groups to stop TV and radio ads by July 4 that support or attack their campaigns. If they don’t stop, his pledge says the candidate benefiting from the ads must contribute half the amount of the ad buy to a charity his opponent chooses.

Begich spokesman Max Croes called Sullivan’s comments hypocrisy. Campaign manager Susanne Fleek-Green said in a statement that if Sullivan wins his primary, “it will be a stark contrast between his put-corporations-first position and Senator Begich’s support for real campaign finance reform, including support of a constitutional amendment throwing out Citizens United,” the 2010 court ruling.

The campaign did not directly say whether Begich would sign the pledge.

There is much at stake in the race — Republicans have focused on Alaska as critical to their efforts to win control of the U.S. Senate. It’s also the state’s first major race in the era of super political action committees.

During a conference call with reporters, Sullivan, a former state attorney general, was asked about the Citizens United decision and said he was a strong supporter of the First Amendment and was not trying to limit free speech with his proposal.

Sullivan said in a follow-up call with The Associated Press and in a campaign tweet that he supported the court decision but that his proposal wasn’t about that.

“It’s about how Mark Begich and I are going to conduct this race,” Sullivan said.

Sullivan singled out Begich for the pledge, modeled on the “People’s Pledge” signed by Republican Scott Brown and Democrat Elizabeth Warren in the contentious 2012 Massachusetts U.S. Senate race. It helped limit attack ads on the airwaves, but groups turned to mass mailings. The race was still the most expensive in Massachusetts’ history.

Sullivan still must beat his August primary opponents, including Lt. Gov. Mead Treadwell and Joe Miller, the 2010 GOP Senate nominee in a race that Republican Lisa Murkowski won with a write-in campaign.

Sullivan told reporters that he and Begich so far have been singled out for attacks by third-party groups. Outside ads have been a fixture on Alaska airwaves for months, and groups have reported reserving millions of dollars in air time ahead of the elections. Sullivan said he and Begich are the only candidates who can put a stop to it.

Leading gubernatorial candidates have complained about the difficulty of buying air time because the airwaves are crowded with ads for the Senate race and an August ballot question to repeal Alaska’s new oil tax structure, the Fairbanks Daily News-Miner reported.

Under the pledge, outside groups have incentive to stand down because the campaigns would be penalized if the ads continued, said Sean Cairncross, a Sullivan campaign attorney and former counsel to the National Republican Senatorial Committee.

Sullivan shares a name with Anchorage Mayor Dan Sullivan, who is running for lieutenant governor as a Republican.