If recent history is any guide, Democrats will have a hard time holding onto the seat of retiring Sen. Jay Rockefeller (D-W.Va.).

While longtime Democratic incumbents have often been able to hold on to red districts and states in recent years, their retirements have almost always handed their seats to Republicans.

Over the past decade, Republicans have won 17 of 20 races (83 percent) for conservative-leaning Southern seats — both House and Senate — that were vacated by a Democrat. If you include such seats outside the South, the GOP has won 23 of 29 races (79 percent).

Sen. Jay Rockefeller (D-W.Va.) is retiring. (Associated Press)

If you focus just on Senate races, Republicans have won seven of nine races in red states where a Democrat retired or died over the past decade — including four of five in the South. And over the past two decades, the GOP has picked up 16 of 22 Senate seats in red states following a Democrat's exit.

Clearly, Democrats have their work cut out for them.

Now, we should emphasize that West Virginia is something of a special case. As much of the South has gone Republican (even at the state and local level), West Virginia and much of Appalachia has stuck by its conservative Democratic roots — something The Fix has labeled the "Appalachian Bubble."

West Virginia Republicans have made gains in recent years, including taking the state attorney general's office in 2012 and stealing an open congressional seat from Democrats in 2010. But West Virginia Democrats still hold majorities in both state legislative chambers and every statewide office except attorney general.

In addition, the Democratic Senatorial Campaign Committee won open seats in two red states last year — Indiana and North Dakota — and they held onto an open Senate seat in West Virginia in 2010, when then-Gov. Joe Manchin (D-W.Va.) won the seat held for decades by the late Sen. Robert Byrd (D).

But even in victory, Manchin struggled with the federal nature of the race and a tough environment for Democrats. Despite an approval rating that was often in the 70s and facing perennial GOP candidate John Raese, Manchin took 53 percent of the vote — far less than he had taken as governor.

Democrats don't have another candidate of Manchin's quality, but they do have a deep bench that includes some potentially formidable nominees. Be sure to check out Sean's post from earlier today for more on that.

Whoever emerges from this field should have a tough race against Rep. Shelley Moore Capito (R) — or whoever else emerges from the GOP primary.

And they will be fighting against a pretty clear trend in American politics, in which a Southern Democrat's retirement opens the door wide for the GOP.