Rehberg announced his candidacy to an exuberant crowd of about 400 during a speech that hit conservative highlights - from opposition to abortion and gay marriage to bashing Obama administration policies - and promised that fiscal conservatives will really toe that line if given a chance to run the country again.
The undisputed leader of the Montana Republican Party said his candidacy can give the Republicans a chance to take back the Senate. Tester played an instrumental role with his 2006 win in giving Democrats control of the chamber amid voter dissatisfaction with former President George W. Bush and congressional Republicans.
"I know the leadership that conservative Republicans can bring to our state and our nation when we stay true to our principles," said Rehberg. "I know what happens when we lose our way and lose the trust of the American people. And I understand how Republicans can repent, we change our ways, and chart a new path forward for the American people."
Both sides, in the months leading up to the highly anticipated matchup, have also been very public about drawing a distinction on congressional earmarks, which Rehberg once supported to boost federal projects in the state. He since has become a critic in the national debate that has identified earmarks as a symbol of the nation's overspending.
Tester, who called pork barrel spending secretive while running in 2006, now says it's needed to ensure specific spending authority doesn't rest entirely with the president. The Democrat also says a new process makes sure the earmarks are vetted publicly.
Rehberg told the Republicans he is a "reformed earmarker," and that constructive criticism convinced him to change his mind and oppose the REAL ID act that became universally reviled in Montana a few years ago.
"Our entire party has done a bit of soul-searching recently," Rehberg said. "But the American people have responded to our sincere desire to humble ourselves and win back their confidence."
Rehberg's challenge to Tester sets up a contest between a Montana GOP leader and a populist Democratic senator who in his first term has returned home to work on his Big Sandy farm on weekends. Both parties expect a multi-million dollar slugfest that is sure to be a marquee race as Republicans try to take control of the Senate.
Rehberg's speech also blasted the "big government agenda" of the Obama administration and Testers support of it, including the economic stimulus package and the federal health care law. He promised a "hard-fought" race.
After easily fending off a tea party primary challenge last year, Rehberg has cozied up to the movement as he criticizes the federal deficit and earmark spending. His announcement Saturday was made at an event with tea party activist and Minnesota Rep. Michele Bachmann.
"You have to elect him because we have to get a conservative Senate in 2012," said Bachmann.
Tester, who has made advocacy of veterans' affairs a prime issue, also criticized Rehberg for his link to Bachmann, who has advocated for less spending for veterans.
"With 21 months left until Election Day, Jon is building on his strong record of creating jobs, cutting spending, cleaning up Washington, fighting for veterans, and actually getting things accomplished for Montanans," Tester spokesman Aaron Murphy said. "When Montanans choose their candidate a year and a half from now, we look forward to an honest debate, comparing Jon's record as a hardworking farmer against anyone else's."
Tester's office has also criticized Rehberg for opposing a bipartisan plan to rescue the timber industry - a deal that would have mandated more logging while also increasing wilderness area elsewhere.
Democrats have controlled both of Montana's U.S. Senate seats since Tester beat Republican Conrad Burns in 2006. Rehberg had helped elect Burns in 1988, running a campaign that bashed the era's wilderness plans.
Before entering politics, Rehberg, 55, worked a variety of jobs - from teaching gymnastics to selling real estate.
He holds a political science and administration degree from Washington State University. He worked as a staff member for former U.S. Rep. Ron Marlenee before winning election to the state Legislature in the 1980s. He was appointed lieutenant governor under former Gov. Stan Stephens and then elected to that post on a ticket with Marc Racicot.
The race against Tester will be Rehberg's second run for U.S. Senate after losing a bitter election battle with U.S. Sen. Max Baucus in 1996. He first won Montana's sole congressional seat in 2000.
Tester, 54, never strayed far from his family's Big Sandy farm before 2006, the year he delivered Burns a stunning defeat. Tester holds a music degree from the University of Great Falls and served on the local school board before joining the state legislature in 1999.