Robert Byrd, at 92 the oldest member of the US Senate, the longest-serving member in Congressional history and the President pro tem, sits in a Washington-area hospital today.

Sen. Robert Byrd (D-W.Va.) has been admitted to a Washington-area hospital and is in “seriously ill” condition, his office said in a news release Sunday.

The statement said that Byrd, 92, “was admitted to the hospital late last week suffering from what was believed to be heat exhaustion and severe dehydration as a result of the extreme temperatures.” The region has experienced a stretch of temperatures in the 90s with high humidity.

Other complications have arisen since his initial admission to the hospital. With his advanced age, any trip to the hospital ought to be concerning.

West Virginia has a Democratic Governor, Joe Manchin. If Byrd were to pass away, Manchin would get to pick a replacement in the near-term. But that new senator would serve until the next “regularly scheduled” general election. One would assume that we’re far enough away from November that a special election to fill out Byrd’s term, which expires in 2012, would take place this year. With West Virginia trending away from Democrats at the national level, that could put one more Senate seat on the playing field for Republicans.

UPDATE: Nate Silver tracked down the relevant information with respect to a vacancy:

Byrd’s current term expires on January 3, 2013. Under West Virginia state law on handling Senate vacancies, “if the vacancy occurs less than two years and six months before the end of the term, the Governor appoints someone to fill the unexpired term and there is no election”. Otherwise, Manchin would appoint an interim replacement, and an special election would be held in November to determine who held the seat in 2011 and 2012.

In other words, we are within a week of the threshold established by West Virginia law. If a vacancy were to be declared on July 3rd or later, there would not be an election to replace Byrd until 2012. If it were to occur earlier, there could potentially be an election later this year, although there might be some ambiguities arising from precisely when and how the vacancy were declared.

UPDATE II: The other thing to deal with, and I don’t want to get crassly political but these things do have implications, is that the Wall Street reform bill was crafted basically to get exactly 60 votes. And if Byrd can’t make the vote on the conference report, which is likely, they may not be able to pass it. Especially because Scott Brown is balking at the $18 billion bank fee levied on them for budgetary purposes and to pay for implementation.