Sen. Pat Roberts (R-Kan.), left, answers a question during a debate with independent candidate Greg Orman, right, in Overland Park, Kan., on Wednesday. (Orlin Wagner/AP)

For months, the 2014 midterm elections have looked like a deck stacked in favor of Republicans.

But as campaign season heads into its final weeks, some wild cards are on the table in states where the GOP had been expecting easy victories on its way to gaining six seats and taking control of the Senate.

Election Day may not resolve things, either. If the balance stays as even as it appears now, the question of which party holds a Senate majority may not be settled until the last ballots are counted — and perhaps recounted — in far-flung precincts of Alaska. Or maybe until runoff elections that could happen as late as January.

The latest twist has come in South Dakota, a conservative state with an open seat that had not been on the radar screen as a potential loss for Republicans. All of a sudden, former senator Larry Pressler — elected to that job three times as a Republican but now running as an independent — is closing in the polls with GOP nominee and former governor Mike Rounds and Democrat Rick Weiland.

“I think this race can be won with 35 or 38 percent of the vote. There is no runoff, and that is all that may be needed,” Pressler said in an interview. Democrats — who had written off South Dakota — also seem to sense an opportunity, making a late decision this week to put $1 million into advertising and field operations in the state.

Election Lab: See our current forecast for every congressional race in 2014

In deeply red Kansas, where the Democratic candidate has dropped out of the race, longtime GOP Sen. Pat Roberts is in an uphill battle against entrepreneur Greg Orman, an independent who has not said which party he will caucus with if he wins.

Meanwhile, in Georgia, Republican front-runner David Perdue has made some missteps that have helped his Democratic opponent, Michelle Nunn, ascend in the race. There is also a Libertarian candidate, Amanda Swafford. If no candidate gets more than 50 percent in November, the state would hold a runoff on Jan. 6 — which could be days after the new Senate is sworn in.

Even the year’s marquee contest has not settled down as many expected it to. The most recent Bluegrass Poll in Kentucky has Democratic challenger Alison Lundergan Grimes holding a two-point lead over Senate Republican leader Mitch McConnell — a six-point gain for Grimes since the previous poll was conducted in August.

All four Republicans may well win. But at a minimum, their unexpectedly precarious situations have the party sweating and considering whether to divert more resources into states they thought they could take for granted.

The GOP was already facing the possibility of libertarian spoilers in a number of states, including North Carolina, where Sen. Kay Hagan (D) is leading GOP candidate Thom Tillis.

“We don’t know how any of this will turn out, but there are at least some possibilities that didn’t seem likely even a few weeks ago,” Democratic pollster Mark Mellman said. “It shows that Senate races are not just about national trends. The candidates and the local circumstances do matter.”

GOP donors are uneasy. At private fundraising receptions this month for Senate Republicans, some of them brought up concerns about Roberts.

For those of you who’ve been closely watching the midterm elections, sometimes it seems like all the candidates are saying the same things. Well, that’s because they are. (Jackie Kucinich and Julie Percha/The Washington Post)

Sen. Rob Portman (R-Ohio), who serves as vice chairman of the National Republican Senatorial Committee, said he has reassured donors that Roberts has enlisted “top flight professionals” to revive his flagging campaign.

“People want to know what is likely to happen there,” Portman said. “That’s the one state where we had hoped to avoid spending any money.”

Republicans’ fraying nerves have not discouraged donations to the NRSC, which raised $15.5 million in a record-setting September haul. The campaign arm for Senate Democrats has yet to release its September fundraising totals, but it has outraised Republicans overall this year.

“I think they understand the stakes,” Portman said of the party’s wealthiest backers.

In a memo Thursday, NRSC aides struck an optimistic tone, pointing to a series of polls showing “that Republicans are beginning to break away from their Democratic opponents in Arkansas, Colorado, and Alaska,” and taunting Sen. Mary Landrieu (D-La.) for replacing her campaign manager.

Former Massachusetts senator Scott Brown, running in New Hampshire against Sen. Jeanne Shaheen (D), is another candidate seen as a possible GOP upset. A WMUR Granite State poll released this week shows Shaheen ahead by 47 percent to 41 percent.

Late-season surprises are not unheard of in politics.

“In every cycle, races come onto the map toward the end,” said Justin Barasky, communications director for the Democratic Senatorial Campaign Committee. “But in this cycle, it is happening even later.”

On the other side of the ledger, some contests once thought competitive are sliding off the screen. High-profile Republican recruits have failed to break through in states such as Oregon and Virginia.

Facing a need to concentrate on the most winnable states, GOP officials have largely given up on winning the Senate race in Michigan, where Republican Terri Lynn Land and Rep. Gary Peters (D-Mich.) are competing for retiring Democratic Sen. Carl Levin’s seat. This week, the NRSC pulled nearly $1 million in planned ad buys for the contest.

Saul Anuzis, a former chairman of the Michigan Republican Party, said state Republicans are unhappy with the move. “The symbolism sends a bad message,” he said. “The reality is that we all realize they’re playing nationally — we get it — but it would have been better if they hadn’t pulled out.”

Anuzis also said national Republicans have relayed to state party leaders that they will need to rely on outside groups to fill Land’s coffers from here on.

Ending Spending, a Republican-leaning outfit founded by conservative investor Joe Ricketts, whose family owns the Chicago Cubs, has already funneled than $5 million into the state. Liberal groups such as NextGen Climate Action, which is funded by environmentalist and hedge-fund manager Tom Steyer, have spent millions in Michigan as well.

Republicans have also been counting on an “enthusiasm gap” to carry them over the finish line. Polls show their base is more excited about voting than Democrats are.

But in states where establishment Republicans defeated tea party insurgents in primaries, such as Kentucky, Kansas and Alaska, the GOP is struggling to rally activists who feel their favorites were wronged.

Conservative firebrand Chris McDaniel, a Mississippi state senator who was defeated by Sen. Thad Cochran (R-Miss.) in a June primary, said he expects many of his hard-right supporters from around the country to stay home in November in order to send a message to the national party.

“Conservatives feel betrayed by elements within the Republican Party,” said McDaniel, who is challenging Cochran’s victory in state courts. “It’s always regrettable when people decline to take part, but thousands of them will, and I understand their frustration.”