Rep. Parker Griffith

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.”