THEY PROBABLY WON’T MISS HIM…. Alabama’s 5th congressional district is among the most conservative in the country to represented by a Democrat. After Blue Dog Parker Griffith abandons his party and becomes a Republican today, the pairing will make a bit more sense.

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.

By any reasonable measure, any time a party gets a member to switch sides, it’s something of a coup. But in this case, Parker Griffith has practically been the definition of a DINO (Democrat In Name Only). Just this year, he voted against the economic recovery package, the federal budget, health care reform, energy policy, and Wall Street reform. The guy even voted against equal pay for women when Congress approved the Lily Ledbetter Fair Pay Act.

For all intents and purposes, Parker Griffith has been a far-right reactionary since the day he took the oath of office. He fit in with congressional Democrats about as well as Dick Cheney would fit in at Netroots Nation.

Chances are, Griffith’s switch was motivated by electoral considerations — his district backed McCain and Bush overwhelmingly in recent elections, and was probably inclined to vote Republican in the 2010 midterms, too. Just as Arlen Specter became a Democrat to improve his chances at re-election, Griffith was likely guided by the same motivation.

Also note, while the Republican Party is no doubt thrilled to add to its caucus, Griffith may still run into serious trouble come Election Day. Erick Erickson is already trashing him, telling readers this morning, “We can pick this guy off and get a real Republican in that seat.” The right wing Club for Growth is thinking along the same lines.

In terms of the bigger picture, the RNC will no doubt crow, but it’s hard to characterize this as some kind of seismic shift — a conservative lawmaker with a conservative voting record will represent a conservative district. He’ll have a different letter after his name, but that’s about the most significant aspect of the development.

That said, Matt Yglesias raises an important point: “[T]his is a reminder that the Democrats’ current huge majority with 257 members isn’t remotely sustainable. To get a majority that big you need to win a lot of districts you just can’t reliably win. Substantial losses in 2010 and/or 2012 are basically inevitable. That said, there are still a few GOP-held House seats that could plausibly be won by a reliably liberal Democrat. The real issue is whether the Democratic majority can add a few seats like that, and contain losses enough to maintain 220-230 reasonably reliable votes and thus the effective ability to govern.”

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Follow Steve on Twitter @stevebenen. Steve Benen is a producer at MSNBC's The Rachel Maddow Show. He was the principal contributor to the Washington Monthly's Political Animal blog from August 2008 until January 2012.