Sen. Mary Landrieu (D-LA), chair of the Senate Small Business and Entrepreneurship Committee, recently introduced a bill that would let more individuals and employers keep their current plans. (Melina Mara/The Washington Post)

Republicans are now favored by most analysts to take over the Senate in the 2014 election; this much we know.

But in what is shaping up to be an intriguing battle for the Senate, some folks are missing a tantalizing subplot. And that is this: There's a significant chance that control of the upper chamber will be decided not on Election Day, but in December or even January, with all eyes on (and money flowing to) a two-candidate runoff in one state.

Two states holding top Senate races this year hold runoffs if neither candidate attains 50 percent of the vote on Nov. 4: Georgia and Louisiana.

So what are the odds that runoffs in either (or both) of these states might determine who controls the Senate come 2015? Let's take a look at each one:

Louisiana - Dec. 6 runoff

It's much more likely that there would be a runoff in Louisiana than in Georgia. This is because Louisiana's November election is a nonpartisan race in which there will be multiple Republicans splitting the vote.

That means Republicans will likely be battling to make the runoff  — Rep. Bill Cassidy is the favorite, but retired Air Force lieutenant colonel Rob Maness has tea party support and state Rep. Paul Hollis has self-funded $250,000  — and hoping Sen. Mary Landrieu (D-La.) doesn't crack 50 percent.

(It's possible that Cassidy could simply swamp Landrieu and win outright on Election Day, but if that happens, Republicans have probably already won the Senate in a rout.)

In her three Senate races, Landrieu has faced a runoff twice (and took just 52 percent the other time). Given that and the fact that the GOP seems to have momentum right now, it seems quite likely she'll be headed for a runoff.

For now, let's put the odds of a runoff in Louisiana at 75 percent.

Georgia - Jan. 6 runoff

A runoff in Georgia is less likely, because Nov. 4 will feature a more standard general-election matchup between a Republican nominee, a Democratic nominee and third-party candidates. This means: 1) There needs to be a somewhat-viable third-party candidate, and 2) the race needs to be quite close.

On that first count, there is a Libertarian Party candidate  — former Flowery Branch city councilwoman Amanda Swafford  — but nobody else at this point.

The Libertarian nominee in 2008 took enough of the vote to push Sen. Saxby Chambliss (R-Ga.) and Democrat Jim Martin into a runoff, but that candidate  — Allen Buckley  — was more well-known than Swafford. And, of course, Democrats had a favorable national environment that allowed them to compete in a red state like Georgia.

In 2014, Democrats likely need Republicans to nominate a candidate with liabilities. National Republicans worry that Rep. Paul Broun or Rep. Phil Gingrey might open the door to a competitive race, but right now, there's a crowded GOP primary featuring five viable candidates.

So let's say, for argument's sake, that the odds of the Georgia Senate race going to a runoff are about 10 percent.

If you combine our rough estimates for Georgia and Louisiana, the odds that at least one Senate race will be headed for a runoff is 77.5 percent.

From there, the question is whether that one race would be pivotal. For this, let's get a little help from our friends over at the Monkey Cage blog.

For one of these runoffs to matter, it needs to be the difference between Republicans getting 51 seats — a majority — and 50 seats — a tie, under which Democrats would still control the Senate by virtue of Vice President Biden's tie-breaker vote.

According to the Monkey Cage, the odds of the GOP getting to 50 or 51 seats is right around 24 percent (about a 9 percent chance that it's 50 and about a 15 percent chance that it's 51). But the GOP can still get to 50 or 51 even if that final race isn't pivotal; for example, if a Louisiana runoff gets the GOP from 49 to 50 seats or they have 51 seats in hand but lose a Georgia runoff and stay at 51.

The GOP needs to be right on 50 seats when the runoff is declared -- which accounts for two out of four scenarios that can get you to 50 or 51 GOP seats -- which means there's about a 12 percent chance that it would be pivotal.

So if you assume that the odds of a runoff occurring are 77.5 percent and the odds of a single, decisive race are 12 percent, the odds that a runoff would determine control of the Senate is a little more than 9 percent.

And if it starts looking like Republicans aren't such favorites to take the Senate, it becomes even more likely.

Keep an eye on this subplot going forward. If it happens, we will likely see resources flood to a single Senate race (Dec. 6 in Louisiana or Jan. 6 in Georgia) in an unprecedented way.

A political nerd can dream.