First thing to know: it's really tough. Redistricting alone won't help Republicans expand their majority, but they were able to shore up quite a few vulnerable incumbents that would be toast in anything but a GOP wave year.
Second thing to know: it's still doable. We need a net gain of 25 seats to retake the House, and as the swings of the past three elections have shown, such numbers are not out of the question when one party has the wind at their backs.
First off, we'll take a look at what I consider a worst-case scenario, one where the 2010 wave has mostly subsided but Obama still struggles in the presidential race, ultimately losing by a smidgen to Romney. This would be consistent with a political atmosphere something like that of the mid Bush years. Not overwhelmingly GOP, but still tough for Dems.
Even still, the House outlook wouldn't be too bad, and with some luck we might even be favored to claw back a few seats at least since we're pretty close to rock bottom as is. In what would best be described as a neutral environment, House victories would come largely on the strength of individual candidates and on the partisanship level of each seat, with multiple seat pickups by both sides.
(Note: short of some kind of national catastrophe or massive scandal on Obama's part, I do not think a landslide Romney win/repeat GOP wave year is worth even considering at this point.).
For ease of counting, I focus only on changes to the number of Dem-held seats. For instance, Louisiana loses a seat this year due to reapportionment (and it will certainly be a Republican), but that alone does not put us one seat closer to 218. It's much simpler to assess the 2012 landscape in terms of how many more or less Democrats each state will send to Congress, then add up all the numbers.
Arkansas: -1 (GOP picks up Ross' seat and sweeps delegation)
California: +2 (Dems only beat Miller and take what's left of Dreier's seat)
Florida: +2 (GOP pretty much gave us a new Orlando seat and the old one of Allen West)
Georgia: -1 (Barrow defeated in tougher seat)
Illinois: +4 (Even in a bad year, Dold, Schilling, Biggert, and Walsh now have unwinnable seats in Dem gerrymander)
Indiana: -1 (Donnelly's old seat is tough to hold)
Iowa: -1 (Latham defeats Boswell in a swingy seat)
Maryland: +1 (Bartlett is not the kind of Republican who can win the new 6th)
Massachusetts: -1 (Lost due to reapportionment)
Michigan: -1 (Peters and Clarke combined)
Missouri: -1 (Clay and Carnahan combined)
Nevada: +1 (only the new 4th is really a slam dunk for us)
New Jersey: -1 (Pascrell and Rothman combined)
New York: -1 (Dems and GOP swap Hochul and Buerkle, Hinchey and Turner seats eliminated)
North Carolina: -4 (GOP gerrymander gets seats of Miller, Shuler, Kissell, and McIntyre)
Ohio: -1 (Renacci defeats Sutton in GOP-leaning seat)
Oklahoma: -1 (GOP finally gets East Oklahoma seat)
Pennsylvania: -1 (Critz and Altmire combined)
Texas: +2 (Dems and GOP split new seats)
Utah: -1 (Matheson defeated in unfamiliar territory)
Washington: +1 (GOP candidate too conservative for Dem-leaning new seat)
All other states: no changes.
Total: -4 Dems. Keep in mind this is pretty much what I think the absolute best (yet realistically possible) GOP year could get them, consistent with a narrow Romney victory through just barely sweeping the swing states.
Now let's look at something consistent with another big Obama win, as the current polls show. Let's say Obama wins every state he did in 2008 by the same or larger margins, and even manages to pick off a few more like Arizona and Missouri. Republicans up and down the ballot struggle and our candidates overperform, changing the map quite drastically.
Arizona: +2 (Dems win the new Phoenix seat plus take back the friendlier 1st)
Arkansas: 0 (yellow dog Dems come back, hold Ross' seat)
California: +6 (Dems sweep the competitive races as Obama wins those districts big)
Colorado: +2 (Dems beat Tipton and Coffman, now in a swing seat)
Florida: +5 (We get West's old seat, plus beat him in a new one, and take out Southerland in a bluer seat, and defeat Rivera)
Georgia: 0 (Barrow might be conservative enough to hold his new seat)
Illinois: +5 (Dems also pick up Johnsons' newly swingy seat)
Indiana: +1 (Donnelly's seat saved, and Bucshon defeated in the perpetually volatile 8th)
Iowa: +1 (Boswell beats Latham, and Steve King defeated by a strong candidate in a swingier seat)
Maryland: +1 (as before)
Massachusetts: -1 (our most unavoidable loss)
Michigan: 0 (Dems beat Benishek in swingy 1st)
Minnesota: +1 (Cravaack defeated)
Missouri: -1 (even in a good year, it's a tough map)
Montana: +1 (Dems are contesting this open seat hard)
Nebraska: +1 (Obama will win the 2nd again, and maybe we'll get the seat too)
Nevada: +2 (Heck also defeated)
New Hampshire: +2 (the swingy state flips back in our direction, both Bass and Guinta booted)
New Jersey: 0 (Runyan's seat still in reach)
New York: +4 (Dems sweep the state save for King and Reed)
North Carolina: -2 (Kissell may very well hang on in a good year)
North Dakota: +1 (Dems also contesting this open seat)
Ohio: +1 (Sutton beats Renacci, and we win back Johnson's ancestrally Dem seat)
Oklahoma: 0 (East OK continues to support conservative Dems)
Pennsylvania: 0 (Fitzpatrick bounced again from swingy seat)
Texas: +4 (Canseco defeated in swingy seat, and Lampson returns to Congress in what was Ron Paul's seat)
Utah: 0 (Matheson again defies GOP attempts to draw him out of Congress)
Washington: +1 (Burner or DelBene cruises to victory)
Wisconsin: +2 (Dems retake Duffy's and Ribble's swing seats)
Total: +39 Dems, giving us easy control of the House.
Is that likely? I don't know yet. My initial impression of this coming election is that Democrats have a big advantage building, but it may not be enough to gain us back the House. I really doubt Romney is going to pose much of a threat to Obama, so I see Democratic gains as a near certainty, but 25 may just be too much to ask for unless the country as a whole sours on Republicans and Dems start picking up seats that were leaning GOP before.
One thing is for sure, the House is definitely in play this election.