Optimistic Republicans looked to win control of the Senate, while Democrats struggled to limit their congressional losses Tuesday in a US election that could change the balance of power midway through President Barack Obama's second term.

Democrats weighed down by Obama's low approval ratings kept their distance from him and looked to a costly get-out-the-vote operation in the most competitive Senate races to save their seats and their majority. They were working furiously to reach out to minority, women and young voters who tend to sit-out elections when the presidency is not at stake.

The main prize in a $4 billion campaign was control of the Senate, a contest that sprawled across three dozen states. Republicans need to pick up six seats to gain a Senate majority. But a large number of competitive races combined with the possibility of runoffs in Louisiana and Georgia meant that neither party might be able to claim victory by the day after Election Day.

There was little suspense about the races for all 435 seats in the House of Representatives, beyond the size of the new Republican majority. A gain of 13 seats would give Republicans their largest representation since it stood at 246 in 1946. Democrats concentrated on protecting their incumbents

About 10 Senate races have drawn most of the attention, but Democrats were at a disadvantage because these were either in Republican-leaning states carried by Mitt Romney in the 2012 presidential election or evenly divided swing states. In these competitive states, astronomical spending and cease-less attack ads have dominated campaigning — with few ideas offered on how best to govern the nation. Serious discussions about deficit spending, climate change, immigration, and other knotty issues rarely emerged.

Republicans sought to cast the election as a referendum on Obama's presidency even though he was not on the ballot.

"The president's policies have just flat-out failed," House Speaker John Boehner said Monday, campaigning for a 13th term in Congress and hoping for two more years as the top House leader. He and other Republicans vowed to change Obama's policies, but have offered little in the way of specifics.

Democrats didn't so much defend the president as insist they were independent of him.

"There are two people on the ballot tomorrow, me and Scott Brown," Sen. Jeanne Shaheen of New Hampshire said as she campaigned Monday, referring to her Republican opponent who moved to New Hampshire after losing his Senate seat from neighboring Massachusetts.

Republicans were all but assured of winning Democratic-held seats in West Virginia, Montana and South Dakota, and Democrats held out little hope that Sen. Mark Pryor could win re-election in Arkansas.

Polls suggested that races for Democratic-held seats in Iowa, Colorado and Alaska have tilted the Republicans' way, too — although Democrats said their get-out-the-vote operation made any predictions unreliable.

Democratic incumbents faced competitive races in New Hampshire and in North Carolina where Democrats said they had an edge — and Republicans disagreed.

North Carolina Sen. Kay Hagan was one Democrat in a marquee race quietly accepting a bit of last-minute help from the president. She has spent much of the year distancing herself from Obama, but her campaign sponsored a radio ad featuring Obama calling her a tireless leader "who shares our priorities." It was unclear where Hagan's campaign was airing the ads, but other candidates have used similar ads to boost turnout among African-American voters still loyal to the president.

Strategists in both parties said candidates in Louisiana and Georgia were unlikely to reach the 50-percent threshold needed to avoid a runoff. The wildest wild card of all was in Kansas, where polls said 78-year-old Republican Sen. Pat Roberts was in a close race with independent Greg Orman in a state that has only sent Republicans to the Senate for nearly 80 years.

Democrats had hoped to pick up the Kentucky seat held by Senate Republican leader Mitch McConnell, but recent polls showed him building a lead over Democratic challenger Alison Lundergan Grimes. McConnell would be in line to control the Senate's agenda as majority leader if Republicans win on Tuesday

That left Georgia as the Democrats' best opportunity to pick up a Republican seat, with Democrat Michelle Nunn, whose father served four six-year terms in the Senate, facing Republican businessman David Perdue.

Partisan fighting and legislative inaction have already characterized Washington in recent years, peaking in 2013 with budget fights that shut down the government and raised the specter of a default on the federal debt.

With a Democrat in the White House and Republicans in control of both chambers of Congress, a big question is whether the legislative paralysis would deepen or whether political reality will push both sides to compromise at least on modest goals.

Republicans may be emboldened to intensify conflicts with an unpopular president in the last years of his term, but they risk Obama's veto if they push too hard for lower taxes, fewer regulations and other conservative priorities. Trade agreements that Obama supports, and changes to the immigration system that many Republicans favor, could offer chances for agreements.

Obama was back at the White House after making his final campaign appearances over the weekend. He raised tens of millions of dollars over two years for Democratic candidates, but in the campaign's final days he campaigned mostly for Democratic gubernatorial candidates in states he carried in 2008 and 2012.

Also on the ballot were gubernatorial elections in 36 states, and an unusual number of incumbents from both parties appeared to be struggling.

Among the most closely watched is Wisconsin, where Republican Gov. Scott Walker is in neck-and-neck race with Democratic challenger Mary Burke. Walker, a favorite of conservative Republicans, is often mentioned as a potential candidate in 2016, but his White House chances would almost certainly evaporate if he lost Tuesday.

In another hard-fought race, Florida Republican Gov. Rick Scott is facing a tough challenge from Charlie Crist, a former Republican governor-turned-Democrat.

Early voting topped 18 million ballots in 32 states, and both parties seized on the number as evidence of their own strength.