Despite the extraordinary amounts of time, money and political energy being expended by both sides, the 2012 elections are unlikely to resolve the partisan gridlock and paralysis that have defined Capitol Hill during the past year.

A key reason is that the Senate, which will provide some of the most competitive and expensive contests on the election calendar, is likely to function, or not, in January 2013 the way it does now, regardless of which party holds the majority. Neither the Democrats nor the Republicans are expected to make the sort of sweeping gains at the polls that are necessary to take effective control of the chamber, where work has been hobbled by a constant flow of filibusters and other political and procedural gimmicks.

There were early expectations of large Republican gains — based on the disproportionate amount of territory Democrats are defending this year — but those have subsided amid signs of an economic rebound, increased approval for President Obama’s performance and a GOP presidential contest that’s left independent voters feeling uneasy about that party’s issue focus.

While Republicans are still expected to gain seats — they need a net gain of four to secure a clear majority — independent handicappers now forecast an outcome that could produce only the fourth evenly split Senate in history.

“We’re headed for another tied, or 51-49, Senate. Regardless of which party keeps the majority, nobody is going to have anything that resembles control,” said Jennifer Duffy, senior editor at the Cook Political Report and longtime expert on Senate campaigns.

Such an outcome, no matter who claims the White House, could leave the next president in a fairly similar situation to the past 14 months: a paralyzed Senate and an unstable House majority that has transformed Washington governance into something resembling a never-ending hostage negotiation in which each side latches on to an issue and demands some form of political ransom in exchange for its survival.

Because of the Senate’s reliance on rules that allow the minority to sometimes compel 60-vote majorities, this will likely leave Congress in a gridlocked state over most issues next year unless broad bipartisanship develops, an unlikely scenario given the history of the current Congress.

Nevertheless, just beneath the radar of the increasingly intense presidential contest, campaigns are well underway for up to a dozen Senate races that will determine whether Republicans can vault back into the majority for the first time since 2006.

In the two battleground states of Ohio and Missouri alone, Democratic incumbents have already faced a combined $8 million in campaign-style ads from outside conservative groups, going back to last summer’s debate over the national debt. More than eight months ahead of the election, Sen. Claire McCaskill (D-Mo.) returned fire Thursday with her first ad of the year, attacking the “special-interest agenda” of groups such as the U.S. Chamber of Commerce who have run ads against her.

The theme of McCaskill’s ad is likely to be replicated in other states and other contests as Democrats believe tying Republicans to billionaire outsiders financing attack ads can mesh with Obama’s message of fighting for the middle class. “Voters are recognizing that these super donors have an agenda,” said Sen. Patty Murray (Wash.), chairman of the Democratic Senatorial Campaign Committee.

After fighting the political winds since the 2008 elections, Democrats have felt a significant shift in their fortunes as the economy in general and job growth in particular finally began to creep upward.

This came as the Republican presidential primary turned increasingly toward social issues such as contraceptive coverage in Obama’s health-care law, a debate that has drifted into the corridors of the Capitol as well.

On Friday, Murray sent out an e-mail to Democratic Senatorial Campaign Committee donors warning that Senate Republicans “back an amendment that would let any employer deny women basic contraception coverage for any reason.” Republicans acknowledge the tone of the presidential debate has not helped them.

“Those are distractions that have given the president a little bit of a break and a little bit of a bump,” said Sen. John Cornyn (R-Tex.), chairman of the National Republican Senatorial Committee. But Cornyn anticipates that once the nomination battle is concluded, the party will refocus the debate on Obama’s economic record. “I think these races are going to be nationalized. I think that will help us,” he said.

That strategy worked well in the 2010 cycle, when the NRSC secured a seven-seat gain. According to GOP strategists, Democrats held eight seats on the eve of the 2010 elections in which Obama’s approval rating was just 44 percent, or worse, and Republicans won seven of them.

That is bad news for McCaskill and several other Democrats, including Sen. Jon Tester (Mont.) and those running for open seats in North Dakota and Nebraska. In those states, Obama is a drag on other Democratic candidates and is not expected to even challenge the eventual Republican presidential nominee there, leaving those candidates to fight on their own. However, Democrats found a strong challenger to Sen. Scott Brown (R-Mass.) in Harvard professor Elizabeth Warren, who has become a liberal icon for her Obama administration work in regulating big banks.

In addition, Rep. Shelley Berkley (D-Nev.) is giving a strong challenge to Sen. Dean Heller (R-Nev.) in a state Obama is targeting. If Obama can run strong in those states, that could put Warren and Berkley over the top — which would offset the likely losses in states that Obama won’t be contesting.

Overall, Democrats have 23 seats up for grabs while Republicans have just 10.

That disparity, coming off the 2010 GOP wave and recent Democratic retirements, had many insiders certain of GOP success this November, but now the tide has hit a point of odd instability. The public continues to give Congress the worst ratings in history, but it’s increasingly doubtful either party can capture that angst to produce the sort of swings of the last three election cycles in which one party has gained between six and eight Senate seats.

“It’s hard to know if there’s a wave out there. There’s a wave of dissatisfaction with Congress, but it’s hard to see how that wave plays out state by state,” said Stuart Rothenberg, founder of the Rothenberg Political Report.

Rothenberg predicts GOP gains of two to five seats, while Duffy predicts a net gain of three to six seats for Republicans. If Republicans push things to the higher end of those gains, they will end up with a similar-size majority to the current 53 members of the Democratic caucus — and they will have done so by winning seats previously held by the sort of moderate Democrats they would need to enact their agenda.

If GOP gains are on the lower end, the majority could come down to the battles of familiar figures in Virginia and Montana, where all four candidates have held statewide office. Those were the last two states to decide the majority in 2006 also, the last time this class faced off. Despite the legislative gridlock ahead, both sides have dug in for a fight that could end in a 50-50 tie, with the winner of the White House claiming the majority based on the vice president’s tie-breaking vote in the Senate.

Even with that outcome, Murray or Cornyn will claim victory. “I want to keep the majority — that’s my goal,” Murray said.