(CNN) - Longtime Sen. Max Baucus, D-Montana, will not seek re-election next year, he said in a statement Tuesday.
"After much consideration and many conversations with my wife Mel and our family, I have decided not to seek reelection in 2014. I will serve out my term, and then it will be time to go home to Montana," he said.
During the remainder of term, Baucus pledged to fight the nation's fiscal issues and work for highway and farm bill that will support jobs in his state.
"I want to focus the next year and a half on serving Montana unconstrained by the demands of a campaign," he said. "Then, I want to come home and spend time with Mel, my son Zeno, and our family enjoying the Montana public lands we've fought hard to keep open and untarnished."
Meanwhile, former Democratic Gov. Brian Schweitzer is leaning towards running, said multiples Democrats with knowledge of Schweitzer's thinking. The officials asked to remain anonymous to speak more freely.
Baucus, now in his sixth term, becomes the sixth Democrat overall to retire from the Senate after his term is up. He's the third from a red state to not seek re-election. Sen. Jay Rockefeller of West Virginia and Sen. Tim Johnson of South Dakota both made their announcements earlier this year.
Three others from less conservative states are Sen. Tom Harkin of Iowa, Sen. Frank Lautenberg of New Jersey, and Sen. Carl Levin of Michigan.
The Democrats control the Senate 55-45 (with two independents caucusing with the party), but as of now they are defending 21 of the 35 seats up for grabs in November 2014.
Meanwhile, two Republicans in the upper chamber will not seek re-election: Sen. Saxby Chambliss of Georgia and Sen. Mike Johanns of Nebraska.
A knowledgeable Democratic source said he made the decision because of several factors: he's getting older, he is not that happy in the Senate these days, and he is relatively newly married and is actually happy in his personal life
He raised more than $1.5 million in the first three months this year and has almost $5 million in his campaign war chest, according to Federal Election Commission reports. He was expected to face a tough re-election bid next year.
In recent news, Baucus was one of the few Democrats who voted against a bipartisan compromise to expand the background check system to include private sales at gun shows and online.
Gun control advocates vowed to aggressively target Democrats up for re-election next year who voted down the background check proposal.
Progressive Change Campaign Committee co-founder Stephanie Taylor said she was happy to see the Democratic senator go.
"Good bye, Senator K Street. Max Baucus has a history of voting with corporate interests and not the interests of Montana voters," she said in a statement. "Montana will finally have a chance to have a senator with its best interests at heart, and we hope Brian Schweitzer jumps into the race immediately."
Separately, some senior Democrats say they are thrilled about the idea of Schweitzer potentially jumping into the race. One called it an "upgrade."
Another source said Schweitzer is being heavily recruited but is not yet 100% certain he'll make the Senate bid.
Before being elected to the Senate in 1978, Baucus served in the U.S. House for two terms and in the Montana House of Representatives for one year.
The senior senator from Montana serves as the chairman of the Senate Finance Committee, a highly influential post in the Senate.
In 2008, he was elected by a wide margin, 73%-27%, over his Republican opponent, Bob Kelleher.
In last year's Senate race in Montana, the junior senator, Democrat Jon Tester, narrowly won re-election against Republican challenger, Rep. Denny Rehberg.
The National Republican Senatorial Committee sees the Baucus news as another sign that the Democrats will have hard time keeping control of the Senate next year.
"The 2014 electoral map is in free-fall for Democrats, who were already facing a daunting challenge," said NRSC Executive Director Rob Collins, in a statement.
Not surprisingly, the Democratic Senatorial Campaign Committee sees it differently
"Democrats have had a great deal of electoral success in Montana over the last decade, and I am confident that will continue," said Sen. Michael Bennet, DSCC chairman, in a statement. "Democrats built an unprecedented ground game in Montana in 2012 when Senator Tester was reelected, and we will continue to invest all the resources necessary to hold this seat."
The Washington Post first reported the story.
– CNN's Dana Bash and Jessica Yellin contributed to this report.