WASHINGTON >> Republicans vigorously pursued their last, longshot chances for taking control of the Senate — Pennsylvania topped their list — as Democrats remained cautiously optimistic that they’d retain their narrow majority after Tuesday’s suspense-filled elections.

In the final days of a caustic campaign marked by negative ads, more than $1 billion in spending and the direct impact of a close White House race between President Barack Obama and Republican Mitt Romney, Democrats who were once considered a sure-fire bet to lose the Senate were upbeat about flipping Republican seats in Maine and Massachusetts and holding most of the 23 seats they’re defending this year.

GOP candidates’ statements on rape and abortion proved to be self-inflicted political wounds that could cost the party Indiana and undermined its chances of taking down a vulnerable Democrat in Missouri. Incumbent Democrats considered in jeopardy at the start of the election cycle managed to counter a barrage of outside spending and were on track to keep their seats in Michigan and Florida.

Democrats hold a 53-47 Senate edge but Republicans are defending only 10 seats Tuesday. Retirements in 10 states proved the most vexing for both parties.

Democrats were counting on holding the open seat in Hawaii, were unsure whether former Sen. Bob Kerrey’s late surge would work in Nebraska and considered Virginia, Wisconsin and North Dakota toss-ups. New Mexico leaned Democratic but was a bit uncertain after the national parties left the two candidates — Democratic Rep. Martin Heinrich and former GOP Rep. Heather Wilson — to fend for themselves.

Among the GOP open seats, Republicans had written off Maine, chalked up Texas and warily but confidently counted Arizona.

In Connecticut, Republican and former wrestling executive Linda McMahon spent more than $42 million of her own wealth on the race for retiring independent Sen. Joe Lieberman’s seat, but it looked like it would be insufficient against the number of Democratic votes. On the final weekend, Obama stepped in with his first campaign ad appeal for Democratic Rep. Chris Murphy.

The ever-tight presidential race, the possibility of a Romney win and the potential boost or drag of the candidates on down-ballot races meant few were offering predictions on Senate control. Both parties grabbed at newfound opportunities like Nebraska and Pennsylvania.

First-term Sen. Bob Casey in Pennsylvania wasn’t on anyone’s lineup of vulnerable Democrats six months ago, especially after Republicans nominated little-known businessman Tom Smith. But the coal executive spent more than $17 million of his own money and cut into Casey’s edge. The National Republican Senatorial Committee and the U.S. Chamber of Commerce jumped in with money and ads in the final week.

“I think a race that everyone here in Pennsylvania assumed was safe for Sen. Casey no longer is,” said Gene Barr, president of the Pennsylvania Chamber of Business and Industry. “It probably leans Democratic. The Casey name here is well-known for decades. His father was governor and Sen. Casey has run statewide before, but it is one that has clearly tightened up.”

Some Democrats suggested that Casey has run a lackluster campaign, a notion his campaign manager emphatically dismissed.

“That is one of the ridiculous phony narratives that have been written,” said Larry Smar. “When the press, pundits thought that this wouldn’t be a race and weren’t covering the race, we were the ones pointing out that Smith had lot of money and he was going to spend it. It’s pretty ridiculous when they weren’t paying attention to the race all year to try to push the blame on us.”

Casey’s latest ad appeals directly to the state’s millions of seniors, promising to strengthen Medicare and “never stop fighting for Pennsylvania.”

A look at the competitive races that will decide Senate control:

—Maine: In a rare case of bipartisanship, Democrats and Republicans agree that independent Angus King likely will win the three-way race against Republican Charlie Summers and Democrat Cynthia Dill. It will be a loss for Republicans who had expected Sen. Olympia Snowe to sail to a fourth term, but the moderate had enough of Washington’s rancor and gridlock. King has not said whether he would caucus with Democrats or Republicans and the outcome of the presidential election is certain to have a bearing on any decision. However, the expectation has been that he would side with the Democrats after Republicans such as Karl Rove’s group and the U.S. Chamber of Commerce spent hundreds of thousands of dollars criticizing him.

—Massachusetts: Democratic challenger Elizabeth Warren has the edge against Republican Sen. Scott Brown in one of the most expensive races in the country — $68 million and it’s all candidate spending as the two agreed to ban outside money. With the backing of the tea party, Brown won a special election in January 2010 to fill the seat of the late Democratic Sen. Edward Kennedy. Brown has vowed to be an independent voice in the Senate, but he’s up against some hard numbers. Obama will win the state handily and there will be 700,000 to 800,000 more voters than in 2010, many of them Democrats and independents who favor Democrats.

—Indiana: Tea party-backed state treasurer Richard Mourdock stunned the GOP in May when he easily knocked out six-term Sen. Richard Lugar in the primary. He is giving Republicans fits again because he could lose on Tuesday even as Romney wins the state and Rep. Mike Pence likely emerges as the next governor. Mourdock had limited goodwill after suing in 2009 to stop the federal government’s bailout of Chrysler. He further damaged his hopes when he said in a debate that pregnancy resulting from rape is “something God intended.” Public and internal polls show conservative Democratic Rep. Joe Donnelly with a lead in the high single digits. The question is whether he can overcome the state’s Republican trend. Libertarian Andrew Horning could affect the outcome.

—Virginia: Former governors, Democrat Tim Kaine and Republican George Allen, are locked in a close race. Allen is trying to reclaim the seat he lost to Democratic Sen. Jim Webb six years ago. Webb decided to retire. Kaine has run slightly ahead of Obama in the battleground state, where the presidential outcome could weigh on the Senate results. The race that has attracted some $50 million in outside spending.

—Montana: Democratic Sen. Jon Tester is fighting for political survival against Republican Rep. Denny Rehberg in a state that narrowly went for John McCain in 2008 but is expected to solidly back Romney. The former music teacher, butcher and current farmer was a surprise winner in 2006. Democrats hope his solid campaign this year will tip the balance.

—North Dakota: Both parties give high marks to the campaign Democrat Heidi Heitkamp has run this year, but it may not be enough against Republican Rep. Rick Berg in the strong Republican state and the race to fill the seat held by Democratic Sen. Kent Conrad.

—Wisconsin: Former four-term Gov. Tommy Thompson is a familiar name to many in Wisconsin. The question is whether the 70-year-old Republican who last ran statewide in 1998 is seen as history. Liberal Democratic Rep. Tammy Baldwin is counting on a strong turnout in her hometown of Madison. If elected, Baldwin would be the first openly gay senator.

—Missouri: Democratic Sen. Claire McCaskill, once considered the most vulnerable incumbent, could be back in Washington if she can hold off Rep. Todd Akin. The Republican severely damaged his candidacy in August when he said women’s bodies have ways of avoiding pregnancy in “legitimate rape,” and GOP leaders, including Romney, called on him to abandon the race. Akin stayed in and is counting on support from evangelicals to lift his prospects in a state that will vote Republican.

—Nevada: Republican Sen. Dean Heller secured the seat after a sex scandal sunk GOP Sen. John Ensign. Ethics questions have dogged his Democratic challenger, Rep. Shelley Berkley, in a race flooded by outside cash and ads. The race is a test of whether Senate Majority Leader Harry Reid’s vaunted political operation can push Berkley to victory. In final push, she loaned her campaign $250,000 last month.

—Ohio: Republican groups swamped Democratic Sen. Sherrod Brown with some $30 million in negative ads, helping to lift the candidacy of Josh Mandel. The state treasurer who just took office in January 2011 equivocated for months over the 2009 auto bailout, critical in the state, before finally saying he opposed it. The outcome of the closely fought presidential race will affect the Senate race.