They’ll probably hold the Senate majority Tuesday — with a couple of seats to spare, most analysts believe. But 2012 is a different story.

By then, Republicans will be poised to take control of the Senate — with pickup possibilities scattered across the map and a much narrower base of their own to defend.

It’s not simply the lopsided mathematics — with at least 21 Democratic seats on the table in 2012, including two independents who sit with the Democrats, compared with 10 Republicans. It’s where the seats are located.

Start with Democratic seats in three states where President Barack Obama lost in 2008: Nebraska, North Dakota and Montana.

Then go down a list of where Democrats are poised to lose Senate battles this year — Ohio, Florida and Missouri, for example — and Democrats will be right back at it in 2012, defending seats there again.

Throw in some bona fide tossup states — Virginia and New Mexico — and it’s pretty hard not to picture Republicans picking off the handful of seats needed to take control, if Tuesday goes as well for the GOP as experts expect.

But that’s still two years out. Even National Republican Senatorial Committee Chairman John Cornyn has said he expects the Republican takeover effort to be a “two-cycle process,” continuing into 2012.

There is one X factor: Obama himself. Unlike this year, Democrats expect him to be at the top of the ticket, and they hope some of his appeal to the Democratic base will spread to candidates down the ballot, as it did in 2008. And if Obama can improve his standing with voters, maybe even recapture some of those that independents lost to the GOP this year, then Senate Democrats will be in better shape in 2012.

Democrats note that for all the Senate seats in potentially hostile territory, several are in states that Obama carried in 2008 and would be high on his list to win again in 2012: Pennsylvania, Washington, Connecticut and Michigan.

Florida and Ohio are must-wins for Obama as well, if he hopes to stay in the White House, and all that effort could bring Democratic senators along for the ride.

“We have more incumbents up than they do, but the fact that it’s a presidential year and so many incumbents are in Obama ’08 states will be an important advantage,” said Fred Yang, a pollster who works with several Democratic senators.

There is some precedent for this.

Both Democratic and Republican political operatives pointed to 1994, when then-President Bill Clinton watched his party get shellacked and lose the House. Two years later, Clinton stopped the bleeding for the most part: He won reelection to a second term, Democrats gained a small number of seats in the House and lost a net of two seats in the Senate.

“It’s all in the hands of our esteemed president,” said Republican ad guru Fred Davis. “Look at what Bill Clinton did. If he learns some lessons from this, it could turn around. Bill Clinton turned ’94 around. With no changes in direction, he’s in extreme trouble, as are the rest of the Democrats.”

Of course, the senators up for election matter, too. Three of the traditionally Republican states will feature tough 2012 senate contests whether or not Democratic Sens. Ben Nelson of Nebraska, Jon Tester of Montana and Kent Conrad of North Dakota run for reelection.

In the more Obama-friendly states, other Democrats will be up for reelection next cycle, including Ohio Sen. Sherrod Brown, Florida Sen. Bill Nelson, Pennsylvania Sen. Bob Casey, Washington Sen. Maria Cantwell and Missouri Sen. Claire McCaskill. Independent Connecticut Sen. Joe Lieberman will also be up. But Democrats are struggling to keep seats in many of those states this year.

In Ohio, Democrat Lee Fisher is expected to lose badly in his bid this year for the Senate.

In Missouri, polls show Republican Rep. Roy Blunt leading Democrat Robin Carnahan — a GOP victory that will only boost the party’s spirits for 2012, when McCaskill is up for reelection.

In Washington state, polls have traditionally shown Democratic Sen. Patty Murray to be more popular than her colleague, Cantwell, but even Murray is fighting to fend off a challenge from Republican Dino Rossi this cycle.

In Florida, Republican Marco Rubio is expected to win next week — paving the way for a Republican to challenge Nelson in 2012. Appointed Sen. George LeMieux will step down at the end of this cycle and would be a prime challenger to Nelson in two years.

Democrat Joe Sestak and Republican Pat Toomey are fighting to the finish in Pennsylvania, where Casey will be up for a second term next cycle.

And according to recent polling, Lieberman could have an even harder path to reelection than he did in 2006, when Democrats rejected him in the primary and he won another term running as an independent with support from Republicans.

Some Democrats up for reelection next cycle remain popular in their traditionally Democratic states — a situation that could allow Maryland Sen. Ben Cardin, Hawaii Sen. Daniel Akaka and Vermont Sen. Bernie Sanders, an independent who caucuses with the Democrats, to skate to reelection.

There are a couple of opportunities for Democrats to play offense in the 2012 cycle in an effort to fend off another GOP avalanche. Specifically, Democrats will very likely target the traditionally Democratic state of Massachusetts, where Sen. Scott Brown will be up for reelection to a full term. There also could be an opportunity in Nevada, depending on whether Republican Sen. John Ensign, who was embroiled in a personal and ethical scandal last year, will run again.

But for the most part, the 2012 Senate map shows Democrats will have to defend almost two dozen Senate seats instead — many of which are in traditional tossup states.

In Virginia, former Sen. George Allen is already making moves to run for the seat he lost in 2006. What’s more, some believe it could be an open seat: Sen. Jim Webb has not yet said whether he’ll seek a second term.

There will also be at least two repeat races — New York and West Virginia — in 2012, the candidates for which will depend on who wins Tuesday. In both of those states, local election law dictates the winners of this year’s Senate seat will serve for only two years before running for a full term in 2012. New York Sen. Kirsten Gillibrand is expected to coast to election Tuesday, but in West Virginia, Democratic Gov. Joe Manchin is battling with Republican John Raese.

Nonetheless, with so many Democratic seats up in the air, it’s hard to find someone to take on the top job at the Democratic Senatorial Campaign Committee. The current DSCC chairman, Sen. Robert Menendez of New Jersey, will be up for reelection and therefore can’t take the job.

Minority Leader Mitch McConnell, meanwhile, is expected to stick with continuity and ask Cornyn to stay on board for a second cycle — which, given the Republicans’ prospects in 2012, would seem like an appealing job, indeed.