President Obama’s unpopularity is threatening Democrats’ hold on the governor’s mansion in West Virginia.

Sources close to Tuesday’s special election for the remaining year on now-Sen. Joe Manchin’s (D-W.Va.) term as governor say the race continues to move in favor of Republican nominee Bill Maloney, who appears to be winning over the many undecided voters in the state. The race is now looking more and more like a toss-up, though Democrats remain confident they will pull it out in the end.

Regardless of where operatives come down on the final results, both agree that the reason for its narrowing is simple: Obama.

After largely leaving the president out of the race for months, the Republican Governors Association has gone up with a heavy rotation of ads linking Acting Gov. Earl Ray Tomblin (D) to the implementation of Obama’s health care bill.

“It was an interesting decision to go late with it,” said one Democrat close to the race. “I figured they would go earlier, but the data seem to suggest what they did in the closing week had influence.”

It’s not clear just how much money the RGA has spent on the ad, but it could be seen even in the Washington, D.C., media market during NFL football games Sunday afternoon. That’s a market that hits only the Northeast corner of West Virginia at a very, very expensive time to run ads.

It’s further evidence of the RGA’s financial heft. The committee, which routinely outraises its Democratic counterpart two-to-one, has now spent $3.4 million on the race, including $2.7 million in the last two weeks alone.

That compares to $2.4 million spent by a group allied with the Democratic Governors Association.

Democrats now expect the race to be decided by a margin in the low single-digits, while Republicans hold out hope that they can pull off what would be a pretty significant upset but say that have nothing to lose, given the state’s heavily Democratic — if conservative — tilt.

It would indeed be a significant upset because, even as Republicans made big gains elsewhere in 2010, West Virginia stuck by its Democratic roots and returned large Democratic majorities to its state legislature, along with sending Manchin to Washington, despite national Republicans’s best efforts to beat him. And gubernatorial races, which are often decided on state issues rather than national ones, are more insulated from the national winds.

What Republicans have done, though, is transformed the national issue that is Obama’s health care bill into a state one. And given Obama’s approval rating in the state — which has stood in the low-30s for some time — that could be a winning strategy.

The effect of the Tomblin-equals-Obama messaging has been to push undecided voters, which until last week were about one-quarter of all likely voters, into the Maloney camp.

“The momentum in this race is with Bill Maloney,” said RGA spokesman Mike Schrimpf. “Now it comes down to who shows up to vote.”

While Obama has certainly become an issue in the race, as we’ve pointed out before, West Virginia is very much its own political vacuum.

Voters in the Mountain State aren’t nearly as disappointed with the current state of affairs as voters are nationally, which generally suggests that they wouldn’t feel the need to send a message by unseating the acting governor.

And to whatever extent Obama is an issue, it’s largely because Republicans have been able to tie him to state issues.

As we said a few weeks ago, if Democrats lose this race, Obama will have played a role. But that role is neutralized somewhat by the Democratic nature of the state and the fact that its a governor’s race rather than a federal race.

If this was a Senate special election happening in any state besides West Virginia where just one-third of voters approved of the president, Republicans probably would have locked up the race long ago.

The fact that they’re competitive here shows that Obama can be a drag on pretty much anyone.

Like the special election in New York’s 9th district last month, West Virginia poses some very odd electoral circumstances which make it hard to compare to other areas of the country.

But one thing is clear: If they lose, Democrats would again be forced to explain why the president continues to cost them in areas that otherwise lean Democratic.