Missouri, Montana and Virginia were the three states that gave Democrats the Senate in 2006 — the three seats that were genuinely in doubt on Election Day and went Democratic.

Now, six years later, those same three states could be the ones that determine whether they can hold that majority.

Sen. Jon Tester (D-Mont.) speaks to constituents during his first campaign in 2006. That year, Tester’s race was one of three that handed the majority to Democrats. This year, a Tester loss could hand the majority back to Republicans. (Melina Mara/The Washington Post)

The Democratic Senatorial Campaign Committee this week reserved more than $15 million in ad time in those three states — its first entry into the air wars this year. And the states they chose were no surprise. As in 2006, these are the consensus battleground states of the 2012 election, and at least right now, whoever wins more of those three looks like they will be in the majority.

But the idea that they will again decide the majority on Election Day seems unlikely, given the sheer number of competitive states on the map.

The fact is that, while these three states are certainly crucial ones and rank as (arguably) the GOP’s three best pickup opportunities after likely takeovers in Nebraska and North Dakota, they could just as easily be supplanted by other races by the end of the election cycle.

For example, to this point, there’s not much polling evidence to suggest that they are much better pickup opportunities than open seats in New Mexico and Wisconsin, which are also looking like toss-ups.

Missouri, Montana and Virginia are certainly more GOP terrain than those two states, and Mitt Romney should carry the first two for sure.

But in New Mexico and Wisconsin, Republicans could have a solid candidate with moderate credentials who could over-perform the party’s presidential ticket. (For the record, Republicans have reserved time in all five of those states.) The same goes for an open seat in Hawaii, where former governor Linda Lingle is running.

By contrast, in the other three states, its hard to see the GOP candidate performing better than Romney. (Can you really see an Obama voter also supporting former senator George Allen in Virginia? Neither can we.)

In addition, both Ohio and Florida promise to be very competitive at the presidential level, and even if they don’t top the list of potential pickups, they are certainly well within the realm of possibility for the GOP.

The GOP’s majority could also very well hinge on whether the party is able to hold seats in Massachusetts and Nevada, which are looking like pretty pure toss-ups at this point. Nevada is the sixth state where the NRSC has reserved ad time.

So while Missouri, Montana and Virginia are certainly pivotal states — and it’s not hard to see why both the DSCC and NRSC have both reserved time in those three states early this year — it’s not obvious that they will decide the majority.

Republicans could win all three and still not win the majority if they lose in Maine, Massachusetts and Nevada. Or they could win one of them, hold all their seats except Maine, and win the majority by winning two other seats like New Mexico and Wisconsin.

The possible combinations are nearly endless.