While Democrats now hold control of the U.S. Senate, this fall’s election has the potential to shift that leadership to the GOP.

Republicans will take control with a net gain of six Senate seats.

Among the 36 seats on the ballot, seven are held by Democrats in states won by Republican presidential nominee Mitt Romney in 2012.

A look at what’s happening in six competitive Senate races where a change in party is possible, and where that change could help decide which party ends up leading the Senate during the next Congress.

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NORTH CAROLINA

On the Ballot: Democratic U.S. Sen. Kay Hagan, an incumbent elected to her first term in 2008; Republican Thom Tillis, the North Carolina House speaker in his first run for statewide office.

In the Bank: Hagan may be one of the most vulnerable incumbent Democrats, but had raised roughly $11 million and was sitting on $8.6 million as of mid-April. Tillis has raised almost $3.3 million, including a $250,000 personal loan, and has just over $1 million in the bank.

On the Stump: Hagan recently has accused Tillis of denying the existence of climate change and she calls the regulation of greenhouse gases key to protecting the environment. Tillis says the question is whether humans are causing global warming and suggested Hagan and President Barack Obama are using “false science” to promote a “war on coal” that would damage the economy.

On the Air: Hagan ran a radio ad before the GOP primary May 6 reminding voters that Tillis approved severance pay for two former legislative aides who had inappropriate relationships with lobbyists. “Our tax dollars, bailing out the indiscretions of Thom Tillis’ staff. Those may be values, but they’re not North Carolina’s,” the ad said. Tillis responded by accusing Hagan and a political action committee supporting Senate Democrats of trying to interfere in the primary. “Don’t be fooled by Harry Reid,” the ad said. Reid, a Nevada Democrat, is the Senate majority leader.

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ARKANSAS

On the Ballot: Democratic U.S. Sen. Mark Pryor, an two-term incumbent first elected in 2002; U.S. Rep. Tom Cotton, a freshman congressman from south Arkansas.

In the Bank: Pryor, the only Democrat in the state’s congressional delegation, had raised nearly $6.9 million in his re-election bid and had more than $4.1 million in cash through April 30. Cotton, who launched his Senate bid last August, had raised nearly $5.4 million and had almost $2.4 million in the bank.

On the Stump: Last month, Pryor and U.S. Sen. Cory Booker, D-N.J., toured an Arkansas community hit by a deadly tornado and criticized Cotton’s vote against disaster aid for the Northeast following Superstorm Sandy. Cotton has focused on trying to tie Pryor to Obama, who is deeply unpopular in Arkansas.

On the Air: Pryor has spent the spring focusing on Medicare and Social Security, airing television spots that criticize Cotton for supporting changes to the programs that he argues would hurt older people. Cotton has aired ads aimed at introducing himself to the state, with his most recent spot featured the newlywed congressman’s wife and their home in Dardanelle.

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ALASKA

On the Ballot: Democratic U.S. Sen. Mark Begich, a former Anchorage mayor seeking his second term; Republicans Joe Miller, Dan Sullivan and Mead Treadwell are running in the state’s August primary.

In the Bank: Begich had raised more than $4.6 million and had $2.8 million on hand at the end of March. Sullivan, who most recently served as Alaska’s natural resources commissioner and is the best funded of the potential GOP challengers, had raised more than $2.6 million and held close to $2 million in the bank.

On the Stump: Sullivan and Treadwell spoken out last week against Obama’s proposed greenhouse gas regulations, with Treadwell saying it was an attempt to impose a change on Alaskans without a full debate in Congress. Begich has recently opened field offices in the small towns of Bethel, Ketchikan and Dillingham, something he says shows an unprecedented commitment to the state’s rural areas.

On the Air: Begich and Sullivan are talking to each other in their TV ads about each other’s ads. In one, Begich says a steel plant featured in a Sullivan spot has more business because of his work as a senator; Begich then suggests other such places where Sullivan could shoot an ad. In a response, Sullivan replied “I’m not a career politician like Mark, but I thought I’d return the favor” and asks him to explain votes he says mostly line up with Obama’s policies.

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KENTUCKY

On the Ballot: Republican U.S. Sen. Mitch McConnell, the minority leader, elected to his first term in 1984; Democrat Alison Lundergan Grimes, the Kentucky secretary of state, who is making her second run for statewide office.

In the Bank: McConnell, the longest-serving senator in Kentucky history and a prime target for Democrats this year, had raised close to $12 million through the end of April and was sitting on $10.1 million. Grimes had raised more than $8 million and had close to $4.9 million in the bank.

On the Stump: Obama’s recent announcement of stricter standards for greenhouse gas emissions has given McConnell more ammunition in Kentucky, one of the nation’s top coal producers. Grimes has also attacked the new rules and again tried to portray herself as independent of Obama, who has lost by a wide margin every time he has appeared on the ballot in Kentucky.

On the air: Grimes is running a TV ad aimed at military voters, an influential bloc of the electorate in a state that’s home to Fort Knox and Fort Campbell. The ad promotes a Kentucky law, championed by Grimes, that allows military and other overseas citizens to register to vote online. McConnell’s most recent ad featured U.S. Sen. Rand Paul, R-Ky., praising his credentials as a conservative.

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GEORGIA

On the Ballot: Democrat Michelle Nunn, an Atlanta nonprofit executive and daughter of former Sen. Sam Nunn; Republican U.S. Rep. Jack Kingston, from Savannah, and David Perdue, a former corporate CEO in his first bid for office, meet in a July 22 runoff.

In the bank: Nunn had hauled in $6.6 million through April 30 and had almost $3.7 million on hand, an impressive total for a first-time candidate and a reflection of the hopes of national Democrats that she can pull an upset. Going into the primary, Kingston had raised more than $5.6 million and had almost $1.3 million saved, while Perdue had taken in about $4.3 million, a figure that includes about $2.6 million of his personal fortune through loans and contributions.

On the Stump: Kingston has assembled a litany of endorsements from tea party figures and vanquished rivals Karen Handel and Phil Gingrey, arguing he is uniting the state’s conservatives. Perdue tells voters at every stop the federal debt is the nation’s biggest problem and any sitting member of Congress helped create it. Nunn, meanwhile, quietly continues a campaign built around community events and is treading lightly when asked about Obama’s health care overhaul and the new greenhouse gas rules.

On the Air: After a busy primary on television, all three candidates are currently off the air.

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IOWA

On the Ballot: Democratic U.S. Rep. Bruce Braley, a four-term congressman elected in 2006; Republican Joni Ernst, an Iowa state senator and Iraq war veteran making her first run for statewide office.

In the Bank: Braley was viewed as an early favorite to replace retiring Democratic U.S. Sen. Tom Harkin and had raised almost $5.9 million, with $2.3 million in the bank, as of mid-May. Ernst has raised almost $1.2 million, but was left with roughly $100,000 in her accounts after sailing through a five-way primary on June 3.

On the Stump: With no primary competition, Braley has been reaching out to general election voters for more than a year, promoting support for minimum wage increase and recommending fixes to the Affordable Care Act. Ernst has been short on specific proposals, focusing her rhetoric on attacking Obama and Braley. She has taken to referring to the health care bill as “Bruce Braley’s Obamacare.”

On the Air: Ernst has already run the campaign ad of the year, in which she talked about her background castrating hogs on the farm as proof she would cut federal spending. In his first ad after the primary, Braley attacked Ernst for failing to write any legislation to cut spending in Iowa following her election to the state Senate in 2010.

(AP)