By John Whitesides
WASHINGTON (Reuters) - Republicans must pick up a net of six seats from Democrats to regain control of the U.S. Senate in the November 4 elections. The key battles are likely to play out in these 13 states:
ALASKA - Democratic Senator Mark Begich is running for a second term in this conservative state, and three Republicans will compete in an August primary for the right to challenge him. Former state attorney general Dan Sullivan is the early favorite, and his fundraising has far outpaced that of the other two Republican candidates, Lieutenant Governor Mead Treadwell and 2010 nominee Joe Miller.
ARKANSAS - Democratic Senator Mark Pryor is seeking a third term against Republican Representative Tom Cotton, who holds a slight lead in early polls in an increasingly Republican state.
COLORADO - Things got a lot tougher for Democratic Senator Mark Udall last week when Republican Representative Cory Gardner jumped into the race.
GEORGIA - The race to replace retiring Republican Senator Saxby Chambliss has drawn a big Republican primary field. That could be good news for Democrat Michelle Nunn, daughter of former senator Sam Nunn. Democrats hope Michelle Nunn will face either Phil Gingrey or Paul Broun, congressmen who are favorites of the conservative Tea Party movement and known for making controversial remarks. Others in the Republican primary include Representative Jack Kingston, former Georgia secretary of state Karen Handel and businessman David Perdue.
IOWA - Democrats in this politically divided state have settled on Representative Bruce Braley to replace retiring Democratic Senator Tom Harkin, giving Braley a chance to fatten his campaign accounts and prepare for a November race against the winner of a crowded Republican primary in June.
KENTUCKY - This high-stakes Senate race features a double hurdle for Senate Republican Leader Mitch McConnell, who first must beat Tea Party challenger Matt Bevin in the Republican primary in May. If McConnell wins as expected, he will face Democratic secretary of state Alison Grimes in November. Former president Bill Clinton campaigned for Grimes in Kentucky last week.
LOUISIANA - Democratic senator Mary Landrieu is accustomed to tough election fights, and she will have another one this year. U.S. representative Bill Cassidy leads a field of Republican challengers that includes Tea Party favorite Rob Maness and Republican state representative Paul Hollis. Landrieu could face a runoff in December if she fails to clear 50 percent in the state's unusual open primary on November 4.
MICHIGAN - The retirement of Democratic senator Carl Levin has sparked a surprisingly competitive battle in heavily Democratic Michigan. Ads by the conservative group Americans for Prosperity have hammered the likely Democratic nominee, Representative Gary Peters, over his support for President Barack Obama's healthcare overhaul. Peters has criticized Republican Terri Lynn Land, a former Michigan secretary of state, for her opposition to the auto bailout during the 2008 and 2009 financial crisis. Many analysts credit the bailout with helping to save the auto industry in Detroit.
MONTANA - Democratic senator John Walsh got a head start last month when he was sworn in to replace the retiring Max Baucus, who became U.S. ambassador to China. But polls show Republican representative Steve Daines already has a big lead over Walsh in the November election, and the conservative advocacy group American Crossroads has slammed Walsh in an ad over a 2010 Army inspector general's report that said he improperly solicited other National Guard leaders to join an association he was running. Walsh responded with ads defending his record and accusing Republicans of smears.
NORTH CAROLINA - Democratic senator Kay Hagan has been pummeled by millions of dollars in ads from Americans for Prosperity, attacking her for backing the program known as Obamacare. But Hagan entered the year with nearly $7 million on hand and could be helped by Republicans' having a competitive primary in May.
SOUTH DAKOTA - Republican former governor Mike Rounds is favored to replace retiring Democratic senator Tim Johnson in this conservative, heavily Republican state.
VIRGINIA - Democratic senator Mark Warner is still the heavy favorite in this state, but former Republican National Committee chairman Ed Gillespie has complicated Warner's task by jumping into the race.
WEST VIRGINIA - Republican representative Shelley Moore Capito is favored to replace retiring Democratic senator Jay Rockefeller. Democratic secretary of state Natalie Tennant trails Capito in polls and fundraising.
(Editing by David Lindsey and Prudence Crowther)