As Americans vote, Republicans look for gains in the Senate WASHINGTON (Reuters) - Voters in the United States took to the polls on Tuesday, casting ballots in midterm elections that could return control of the Senate to Republicans, complicating President Barack Obama's last two years in office. Obama's low job approval rating, partisan gridlock in Washington and a U.S. economy that is not growing broadly enough to help many in the middle class are the major issues before voters in elections for 36 senators, 36 governors and all 435 members of the House of Representatives. Republicans are expected to pick up seats in the Senate, but polls show eight to 10 races are still toss-ups and it is unclear whether they can gain the six seats they need to control the 100-member chamber for the first time since the 2006 election. The battle for control of the Senate also could extend beyond Tuesday night. Senate races with multiple candidates in Louisiana and Georgia, where the winner must get more than 50 percent of the vote, could be forced into runoffs in December and January, respectively. In Raleigh, North Carolina, Edward Sanders, 59, said the economy was his top issue. After voting for Democrat Kay Hagan for the Senate in 2008, Sanders said he decided this time to go with her challenger, Republican Thom Tillis. "I don't particularly like Tillis, but he seemed more likely to shake things up in Washington," said Sanders, a mechanical engineer. Kyle Stephenson, 26, an accountant, said he recently switched parties from Republican to Democrat. He cited widening economic inequality as his key issue. "It seems like the gap between the really rich and the rest of us is just getting bigger and bigger," he said. It's gotten harder and harder for regular Americans to make a living. Seizing the Senate would give Republicans, who are expected to build on their majority in the House, control of both chambers of Congress. That would constitute the most dramatic political shift since Obama entered the White House in early 2009 and might force the president to make more concessions to his Republican opponents than he would prefer. The White House tried to play down the prospect of sharp changes in strategy after the election, saying Obama would seek common ground with Congress on areas like trade and infrastructure. On other issues, like climate change and immigration reform, Obama is likely to continue to take actions on his own. By the end of the year, he is expected to announce executive action to defer deportations for some undocumented immigrants. Jay Carney, Obama's former spokesman, said he expects Obama to make an "all-out push" on his priorities regardless of the makeup of Congress. "He's very competitive, and he will see it as a challenge, regardless of whether it's a split Congress or GOP-controlled Congress," Carney said in an interview. Democratic senators are battling for re-election in tough races in Alaska, Arkansas, Louisiana and North Carolina, all states won by Republican Mitt Romney in the 2012 presidential election. Democratic Senator Mark Udall also is in a tight race in the swing state of Colorado, and the fight to replace retiring Democratic Senator Tom Harkin in the swing state of Iowa is a toss-up. Republicans are in tight races to retain their seats in Georgia, where Senator Saxby Chambliss is retiring, and Kansas, where independent Greg Orman is challenging Republican Senator Pat Roberts. Republican Mitch McConnell of Kentucky, who polls show has a slight edge over Democratic challenger Allison Lundergan Grimes, would replace Democrat Harry Reid as Senate majority leader if Republicans win the Senate and he hangs on for re-election. Several governors' races were also close, including the Florida contest between incumbent Republican Governor Rick Scott and Democrat Charlie Crist. At Manitoba Elementary School on Milwaukee's south side, Jordan Wescott, 33, voted for Republican Governor Scott Walker on his way to work at a construction company. "I feel he's an honest person and he's very black-and-white on issues, and he stays out of the social issues, which I like," said Wescott, a 14-year veteran of the Wisconsin Air National Guard. Obama's public approval rating of around 40 percent made him a political liability in some states on the campaign trail, where his last campaign appearance was on Sunday in Philadelphia. He stayed in Washington on Monday and is not campaigning on Tuesday, holding private meetings about the international economy and the Ebola response. White House spokesman Josh Earnest said the election was not a referendum on Obama's policies. "The vast majority of voters across the country are making decisions in this election based on the candidates themselves, and not on President Obama," he said, citing polling data. The president plans to watch election returns at the White House on Tuesday night, he said. Obama will face pressure to make changes at the White House if his party loses the Senate. A Reuters/Ipsos poll showed 75 percent of respondents believe the administration needs to "rethink" how it approaches major issues facing the United States (bit.ly/1ph8sLs). Former Obama adviser David Axelrod said on Tuesday he thinks the president should take a close look at his team, many of whom have been at the White House since 2009, to see how he can make the most of his last two years in office. Washington always wants you to throw out bodies after a bad election, so you'll hear that hue and cry. But it's also been a turbulent couple of years," he said in an interview with MSNBC, where he is now a political analyst. Additional reporting by Roberta Rampton, Jeff Mason, Susan Heavey, Tim Ryan and Ian Simpson in Washington; Marti Maguire in Raleigh, North Carolina; David Beasley in Atlanta; Barbara Liston in Orlando, Bill Cotterell in Tallahassee and Zachary Fagenson in Miami Beach; Colleen Jenkins in Winston-Salem, North Carolina; Jonathan Kaminsky in New Orleans; Editing by John Whitesides, Ken Wills and Douglas Royalty