POLITICO has learned that Rep. Parker Griffith, a freshman Democrat from Alabama, will announce today that he's switching parties to become a Republican.

According to two senior GOP aides familiar with the decision, the announcement will take place this afternoon in Griffith's district in northern Alabama.

Griffith's party switch comes on the eve of a pivotal congressional health care vote and will send a jolt through a Democratic House Caucus that has already been unnerved by the recent retirements of a handful of members who, like Griffith, hail from districts that offer prime pickup opportunities for the GOP in 2010.

The switch represents a coup for the House Republican leadership, which had been courting Griffith since he publicly criticized the Democratic leadership in the wake of raucous town halls during the summer.

Griffith, who captured the seat in a close 2008 open seat contest, will become the first Republican to hold the historically Democratic, Huntsville-based district. A radiation oncologist who founded a cancer treatment center, Griffith plans to blast the Democratic health care bill as a prime reason for his decision to switch parties-and is expected to cite his medical background as his authority on the subject.

While the timing of his announcement was unexpected, Griffith's party switch will not come as a surprise to those familiar with his voting record, which is one of the most conservative among Democrats.

He has bucked the Democratic leadership on nearly all of its major domestic initiatives, including the stimulus package, health care legislation, the cap-and trade energy bill and financial regulatory reform.

He was one of only 11 House Democrats to vote against the stimulus.

"Look at his voting record - he's had substantial differences philosophically with the Democratic agenda here in Congress," said an Alabama ally who is familiar with Griffith's decision. "It's something that's been discussed for the last several months… talking to people in his family. And it genuinely is a reflection of where he feels. It's his own personal conviction."

The Obama administration's decision to scrap plans to build a missile defense shield in Eastern Europe further frustrated Griffith, according to GOP sources, because his district contains the base for Boeing's ground-based missile defense research.

Ironically, a National Republican Congressional Committee spokesman called Griffith a "woefully ineffective advocate for Tennessee Valley jobs" after the decision was announced in September.

Signs of Griffith's dissatisfaction with his party began to surface publicly during the summer recess, when he received an earful of criticism from constituents.

In August - one month after Republicans picked up his former state legislative seat in a special election - Griffith told a local newspaper that he wouldn't vote for Nancy Pelosi to remain as House Speaker because she's too divisive. He joked that if she didn't like it, he'd provide her with a gift certificate to a mental health center.

He added that if the Democratic leadership wouldn't commit to working in a more bipartisan manner, "perhaps we should look at altering that."

Later that month, he was booed at a town hall forum, even though he said he was against his leadership's version of health care reform.

"If I'm agreeing with you, don't fight me," Griffith said to a heckler, according to the Huntsville Times. "I'm on your team."

After the November off-year elections, he told POLITICO that he wanted to be called an independent Blue Dog and not a Democrat. He said the point of the elections was clear: "I should be nervous."

Democratic pollster John Anzalone, an Alabama native, said that Griffith would have faced a difficult re-election, and undoubtedly was worried about his prospects in a conservative district.

"This is never a pleasant thing… you never can really figure out the motivation. Usually, it's political opportunism. Parker Griffith can win as a Democrat, but it's easier to win as a Republican," said Anzalone. "When you run as a Southern Democrat, you run because you have a certain principle of being with the party of the people. You know what you're getting into, there's no surprise on the legislative side. It's a political calculation."

Several Republicans have already stepped forward to challenge Griffith, and it's not clear whether they'll drop their bids in light of his party switch. Madison County Commissioner Mo Brooks has already raised more than $100,000 for the campaign, while Navy veteran Les Phillip also is running.

Brooks's campaign manager, Bruce Tucker, told POLITICO he would remain in the race and called Griffith's move "a desperate political move."

The Democratic Congressional Campaign Committee will now have to scramble to find a candidate to run against Griffith before Alabama's April 2 filing deadline.

Though it has never elected a Republican to Congress, Griffith's seat has a long conservative tradition and has backed Democrats who have a brand independent from the national party. As a result of the district's Democratic heritage, Democrats still hold the majority of state legislative seats within the 5th Congressional District's boundaries.

The district, however, is trending Republican: A wave of new residents is moving into the Huntsville suburbs, where the area's burgeoning aerospace and defense industries have created a miniboom. And those voters, with fewer ties to the area's past politics, have been reliably Republican at the national level.

The district gave John McCain a resounding 61 percent of the vote last year - a tick above the 60 percent President George W. Bush won in 2004.

Even so, last year Griffith managed a narrow win against the strong drag at the top of the ticket in one of the nastiest House races in the nation. He defeated Republican Wayne Parker 51 percent to 48 percent, despite heavy GOP spending against him.

The National Republican Congressional Committee poured in $514,000 to air ads attacking Griffith, including one that suggested he was soft on Islamic terrorism.

And the committee raised questions about whether Griffith engaged in medical malpractice when it released decades-old documentation accusing his cancer center of underdosing patients with radiation so that he could collect more money.

Griffith is the first House Democrat to switch parties since Rep. Rodney Alexander (R-La.) changed parties in 2004. The most recent member of Congress to switch parties is Sen. Arlen Specter (D-Pa.), who announced his decision in April.

Griffith's predecessor, former nine-term Rep. Bud Cramer, had been frequently mentioned as a possible party-switcher - and also as a possible appointee in the Bush administration - but he retired from Congress as a Democrat.

Griffith now has $619,000 in the bank to run as a Republican, a total boosted by contributions from several of the Democratic Party's more liberal leaders. The political action committee of House Majority Leader Steny Hoyer donated $10,000 to Griffith's reelection this year, and even Pelosi chipped in $4,000 - prior to Griffith's August remarks.