It looks increasingly possible that control of Congress will be split for another two years.

With one week to go before Election Day, Republicans are primed to retain a majority in the House, even as their margin will almost surely be cut. Senate control remains too close to call, although momentum has been on the Democrats’ side at least since the national conventions two months ago.

All of these races must be viewed in the context of a presidential contest that’s been a pure tossup for the past month. GOP presidential nominee Mitt Romney’s sustained bump in the polls since his first debate with President Barack Obama on Oct. 3 breathed new life into Republicans’ Senate prospects and dashed any long-shot Democratic hopes of netting the 25 seats needed to retake the House this cycle.

While a solid number of GOP Members of Congress are probably not coming back, Republicans have been on offense in a number of races from New England to California. Democrats are poised to gain only a handful of seats — probably somewhere in the single-digit range.

The Senate landscape has been far more fluid, with states such as Pennsylvania emerging onto the competitive playing field in the past month. The opportunities are certainly there for Republicans to win, which is why few are counting them out just yet. But given the GOP’s own potential loss of seats and candidate missteps in eminently winnable states such as Missouri, Democrats are more optimistic than ever that a narrow majority is theirs for another two years.

In the House, the decennial redistricting process benefited Republicans, who were able to tweak a number of GOP-held seats to be less competitive. Along with having plenty of their own incumbents to defend, that’s part of the reason Democrats have always had a steep uphill slog to take back the Speaker’s gavel.

The overall political environment, the largest single contributing factor to the House landscape, has remained as starkly neutral as the presidential election is bitterly close. Unlike 2006, 2008 and 2010, no partisan wave ever formed. That has been to Republicans’ significant advantage, as Democrats have tried to position themselves to find a path to 218 seats.

Senate races are less influenced by national atmospherics than the House, but this cycle the majority will be won or lost in a cross-section of presidential battleground states and states where the incumbent must overperform the top of the ticket. Sen. Jon Tester (D-Mont.) is hoping to hold off Rep. Denny Rehberg (R) in a state that Romney should win handily, while Sen. Scott Brown (R-Mass.) may have to win around 20 percent of the Democratic vote against Elizabeth Warren (D) in a strong Obama state.

The final week of campaigning in these hard-fought elections kicked off with a bang, at least on the East Coast, as Hurricane Sandy made landfall Monday night. The damage caused by the great storm was expected to be of historic proportions, and its presence affected campaign plans up and down the coast.

Romney and Obama canceled several events in battleground states where they had hoped to make personal appearances in the campaign’s final days. In Virginia, Senate candidates Tim Kaine (D) and George Allen (R) canceled events Monday. Kaine’s campaign manager emailed supporters Sunday night with emergency preparedness tips and a request to remove their lawn signs to avoid the possibility that they become projectiles. Brown also asked his supporters to take down their lawn signs and reminded them in an email of Bay Staters’ “proud tradition of resilience.”

But it’s House Democrats who will need resilience in the face of a different kind of powerful storm.

The GOP wave in 2010 gave Republicans control of not just the House of Representatives, but also a number of state legislatures. That left Republicans controlling the redraw of far more House seats than Democrats and allowed them to cement recent gains. One recent study from New York University estimated that Republicans will be poised to keep long-term control of 11 more seats now than the party would have under the old lines.

A number of retirements in unfavorable districts also bedeviled Democrats this cycle, undermining their ability to chart a path back to control. And well-timed and well-positioned GOP independent expenditure spending — though matched for much of the cycle by outside Democratic spending — has also added to the party’s woes.

“It was always kind of far-fetched to think that Democrats would win back the House in just one cycle,” Democratic pollster Dave Beattie said.

Early on this cycle, it was also a less-than-even proposition that Democrats could retain control of the Senate, but a lot went right for them over the past year, and the party is now well-positioned to keep control. Still, the Senate majority is riding on a long list of close races that, given the uncertainty in the presidential race, makes any predictions tenuous. 

Two Senate seats likely to flip party control are no longer viewed as competitive. Former Maine Gov. Angus King (I) is likely to win a three-way contest for the seat of retiring Sen. Olympia Snowe (R), and both parties expect he will caucus with Democrats. Nebraska state Sen. Deb Fischer (R) is expected to defeat former Sen. Bob Kerrey (D) for the seat of retiring Sen. Ben Nelson (D).

That leaves Democrats’ 53-47 majority in tact. To close that gap, Republicans must defeat Tester in Montana and pick up the open Democratic seat in North Dakota, where Heidi Heitkamp (D) is taking on Rep. Rick Berg (R). Romney will win both states with ease, yet both races are tossups with a week to go.

Should Romney win the White House and assuming the GOP wins both of those seats, Republicans would need to win just one more Democratic seat for a 50-50 Senate — as long as the party doesn’t lose any other seats. The GOP’s most likely possibilities for offense include the open seats in Virginia and Wisconsin, where Rep. Tammy Baldwin (D) is taking on former Gov. Tommy Thompson (R). Ohio would be next, where state Treasurer Josh Mandel (R) is challenging Sen. Sherrod Brown (D) in the most targeted presidential state in the country.

Republican candidates are also making waves in Pennsylvania, where self-funding businessman Tom Smith (R) has closed in on Sen. Bob Casey (D), and in Connecticut, where former WWE CEO Linda McMahon (R) is competing against Rep. Christopher Murphy (D). Both states that were hardly on the radar at the beginning of the year have attracted an influx of outside spending in the past month.

“In many ways the battle for the Senate reflects what we’re also seeing in the presidential race — a lot of very close battles but a continued trend in the Republicans’ direction as more voters focus on the Democrats’ failed economic leadership,” National Republican Senatorial Committee spokesman Brian Walsh said.

The most glaring missed opportunity for Republicans will likely turn out to be Missouri, where the race took a stunning turn in the direction of Sen. Claire McCaskill (D) after GOP Rep. Todd Akin’s comment about “legitimate rape.” Yet the undoing of the party’s quest for a majority may end up being Democrats’ surprising number of offensive opportunities. 

The number of seats the GOP must win for a majority increases with every Republican seat lost. Beyond Maine, Nevada Rep. Shelley Berkley (D) could defeat appointed Sen. Dean Heller (R) in a stubbornly close race, Indiana Rep. Joe Donnelly (D) could capitalize on state Treasurer Richard Mourdock’s (R) remark about rape and pregnancy in a recent debate, and while the race is close, Warren’s prospects for victory have never looked better.

Democrats are even hoping for a win in Arizona, where former Surgeon General Richard Carmona (D) is taking on Rep. Jeff Flake (R) in a state that Romney will carry.

“Democrats are cautiously optimistic we will keep the majority,” Democratic Senatorial Campaign Committee spokesman Matt Canter said. “We have recruited stronger candidates and forced Republicans to spend millions playing defense in five of the 10 Republican-held seats on the map.”