HELENA -- U.S. Sen. Conrad Burns, R-Mont., will return the estimated $150,000 in campaign donations he has received from indicted lobbyist Jack Abramoff, his co-workers and tribal clients.

Burns said Abramoff's donations, while "legal and fully disclosed, have served to undermine the public's confidence in its government."

"I call on my colleagues, both Democrats and Republicans alike, including the state and national committees, to do the same," Burns said in a prepared statement released Thursday evening.

Burns, who is up for re-election in 2006, received more Abramoff-related campaign contributions than any other lawmaker, some $146,590 between 1999 and 2004, a Washington Post tally shows.

The announcement comes just two days after a Burns spokesman said the senator would not return the money because it had already been spent.

Sen. Byron Dorgan, D-N.D., announced Tuesday he is returning $67,000 he received in Abramoff-related campaign donations.

Mark Baker, chairman of Burns' re-election campaign, said Burns' Abramoff-related money had been spent and some of it was donated to campaign committees that no longer exist. Still, Burns' campaign had just over $3 million in the bank at the end of September. The reimbursements will come from that account, Baker said.

Abramoff is at the heart of an ongoing Justice Department investigation alleging the former lobbyist and his partner, Michael Scanlon, bilked casino-rich American Indian tribes out of millions of dollars when he served as their lobbyist. The agency is also looking into whether Abramoff pressured the tribes to make donations to key lawmakers, possibly in exchange for legislative favors.

Burns is one of four lawmakers reported to be part of the investigation, the Washington Post and Wall Street Journal have reported, citing anonymous sources.

The Montana Republican has come under scrutiny for making decisions favorable to certain Abramoff clients around the time he received donations from the lobbyist, his associates or clients. In addition, some of his former aides left to take jobs with Abramoff.

Burns said he will return all the money to its original donors, except for the thousands contributed from Abramoff and his associates. That money Burns will donate to a Native American charity. Burns said he's asked his staff to work with leaders from Montana and Wyoming tribes to identify the charity.

"I can think of no better use of Mr. Abramoff's contributions than to assist a charity that serves the very folks he wronged," the statement read.

"From what I've read about Jack Abramoff and the charges which are pending or about to be brought against him, he massively deceived and betrayed his clients," his statement read. "Plus, Abramoff appears to have deliberately lied to dozens maybe hundreds of members of Congress, Democrats and Republican alike."

Matt McKenna, a spokesman for the Montana Democratic Party, said the party would search through its books to see if it had received any donations from the Republican-connected lobbyist.

"The major difference between the Montana Democratic Party and Conrad Burns is that we don't know Jack Abramoff and we never did any favors for him in exchange for campaign contributions -- like Conrad Burns did," he said.

Craig Wilson, a political science professor at Montana State University Billings, said Burns' decision to return the donations may not help the senator avoid continued scrutiny in the investigation.

"He did the appropriate thing, but at the moment, this issue isn't going away," Wilson said. "We have to see how it plays out over the next few months to make a determination about how much damage has been done."

Burns' link to the ongoing Abramoff story is not the only hurdle the senator must overcome in the 2006 campaign, Wilson said. A recent MSU-B poll showed that Montanans are growing dissatisfied with the war in Iraq and President Bush.

"In Montana, you've definitely got a Republican base and Democratic base and what the Republicans and Bush are losing is the middle," he said.

Still, Burns has been effective and funneling federal dollars into the state, Wilson said, and will enjoy all the benefits of already being in office.

In his statement, Burns may also have quashed a growing rumor: That the senator will pull out of the race before Election Day.

"I am fully committed to running for re-election in 2006 based on my strong and positive record for Montana and the mission at hand," he said.

Burns faces a challenge from Bob Kelleher of Butte in the Republican primary. Four Democrats have lined up to run against him: state Auditor John Morrison of Helena, state Senate President Jon Tester of Big Sandy, former state Rep. Paul Richards of the Boulder area and Clint Wilkes, a Bozeman businessman.