Top ten Senate races to watch in 2012

As the Republican presidential nominating contest appears to start winding down, Senate races across the country are just starting to heat up. And there's a lot at stake for both parties. Democrats are attempting to hold on to a slim 53-47 majority in the chamber (including Independents), while Republicans are seeking to seize on what looks like a pretty sizable advantage. Come November, Democrats will be defending 23 Senate seats, including six open seats; Republicans will only have to defend ten. Even so, right now it's impossible to tell which way the wind will blow.

In the following slides, CBS News presents -- in no particular order -- the top ten Senate races to watch going into the 2012 elections.

Top ten Senate races to watch in 2012

MASSACHUSETTS

The battle between incumbent Republican Sen. Scott Brown and Democratic challenger Elizabeth Warren is sure to be among the highest-profile Senate races in 2012. Brown, who was elected with Tea Party support in 2010, is fighting to hold on to his seat in the solidly blue state, while Warren - a longtime consumer advocate and liberal Democrat - attempts to appeal to middle-class and blue collar voters. Both candidates have proven themselves to be powerful fundraisers. So much so that they both signed a pact in January pledging to bar outside money from outside groups from spending money in the campaign.

Warren's outspoken support for issues like gay marriage, consumer advocacy and increased oversight of the financial services sector have earned her a reputation as something of a national liberal icon, but it's unclear how that will play among Massachusetts' working class. And while the two candidates have traded leads in a handful of recent polls, Brown has performed particularly well in recent surveys among independent voters. He also benefits from the built-in advantage that comes with being an incumbent candidate.

Top ten Senate races to watch in 2012

VIRGINIA

The Virginia Senate battle is one of the most highly-anticipated races in the country - and it may be one of the most competitive contests as well. Two former governors - Democrat Tim Kaine and Republican George Allen - will likely go head to head this November for the state's open seat, which is being vacated by outgoing Democratic Sen. Jim Webb, a former Navy Secretary. Allen, who also served in the Senate from 2000 to 2006, lost to Webb in 2006 in the wake of controversy sparked when he called one of Webb's aides a "macaca."

The battle is expected to be expensive, protracted and incredibly close. It could also get ugly. Allen is attempting to rebuild his political reputation amid accusations from Democrats that he's too conservative for the state; his Republican primary challenger, meanwhile, has blasted him for not being conservative enough. And Kaine faces troubles of his own. As the former head of the Democratic National Committee (DNC), he is working to distance himself from President Obama and the Democratic establishment.

Top ten Senate races to watch in 2012

MAINE

When longtime Maine Sen. Olympia Snowe announced her retirement from the Senate earlier this year, she sent shock waves through the political world. In deciding to abdicate her seat just two weeks before the race's filing deadline, the moderate Republican left both Republicans and Democrats scrambling to find competitive candidates to put forth.

Democrats have said they're ready to seize on the opportunity to pick up a Senate seat in the state, but so far Maine voters seem inclined to honor their nonpartisan streak: former two-term Gov. Angus King - an independent - currently seems to have an edge over his Democratic and Republican rivals. Meanwhile, neither Republicans nor Democrats have coalesced behind a candidate. It's unclear if King will caucus with Democrats, Republicans or neither if elected -- he's said he wants to keep both parties "guessing." But there are signs he'd favor Democrats: He's supporting President Obama's re-election campaign and has decried the Republican budget proposal.

Top ten Senate races to watch in 2012

MISSOURI

Incumbent Democratic Sen. Claire McCaskill, of Missouri, has faced an uphill battle in her bid for re-election pretty much from the get-go. The moderate Democrat was elected by a hair in 2006, and the state has only grown more conservative since then. McCaskill's popularity also took a hit during her first term after it was revealed that she had billed taxpayers for a trip in her private airplane to a Democratic political event.

Regardless, as an incumbent, McCaskill enjoys an automatic advantage when it comes to name recognition. And her Republican rivals are still fighting it out for the GOP nomination. So far, there are four candidates vying to take McCaskill on: Businessmen John Brunner and Mark Memoly, Rep. Todd Akin and former state Treasurer Sarah Steelman. Steelman has picked up a handful of crucial endorsements in her bid so far, including that of the Tea Party Express and a number of state lawmakers, but a clear-cut challenger has yet to emerge.

Top ten Senate races to watch in 2012

NEVADA

The Nevada Senate race is one of several contests in the Southwest that could be crucial to determining which party controls the Senate next year. Republican incumbent Dean Heller (who was appointed in May 2011 after former Sen. John Ensign resigned amid-scandal) is facing a tough challenge from Democratic Rep. Shelley Berkley.

Both candidates will need to do well in the crucial Washoe County - which includes Reno - to win, but they each face major challenges in achieving that goal. Neither candidate is likely to suffer from a lack of funding in this contest, however, which will surely attract donors from outside the state. Berkley will also benefit from the Obama campaign's strong re-election network there. Either way, the race is expected to come down to the wire.

Top ten Senate races to watch in 2012

WISCONSIN

Wisconsin has been at the foreground of a handful of high-profile political battles in the last year - and it doesn't look like the attention is about to recede. The Senate contest between Democratic Rep. Tammy Baldwin and her Republican rivals, which will come just a couple months after the recall election for GOP Gov. Scott Walker, is sure to heat up as the election nears.

Former Governor Tommy Thompson is vying for the GOP nomination, but he is being challenged by state Speaker Jeff Fitzgerald and former Rep. Mark Neumman, who has the backing of both the conservative Club For Growth and Tea Party favorite Jim DeMint.

Baldwin, who would be the nation's first openly gay senator, announced a $2 million fundraising haul in the first quarter of 2012 - but the state's politics have proved relatively unpredictable over the last few years. In 2008, Wisconsin voted for President Obama.  Just two years later, voters ousted longtime Democratic Senator Russ Feingold from office. Given the recent state of political play out in the Badger State, it's anyone's guess as to who will emerge victorious this November.

Top ten Senate races to watch in 2012

MONTANA

Montana represents one of the GOP's best chances to pick up a Senate seat this election cycle. Democratic incumbent Senator Jon Tester is running neck-and-neck against his Republican challenger, the well-known Rep. Danny Rehberg, and outside groups have already invested millions of dollars into the race.

Tester, a first-term senator and organic farmer, was elected with just a 1-point lead in the Democratic wave of 2006 - and his success this time around depends at least in part on his ability to distance himself from President Obama on the campaign trail in solidly red Montana. Rehberg, meanwhile, will have to make up ground in the fundraising department, where Tester has so far shown an advantage.

Top ten Senate races to watch in 2012

NEBRASKA

Updated: May 18, 4:27 p.m.

When Nebraska's Democratic Senator Ben Nelson announced his decision not to seek re-election late last year, most political observers assumed his seat would be a sure bet for Republicans in 2012: Nelson was a conservative Democrat in a red state, and was himself expected to come up against serious challenges in a bid for re-election. But the entrance of prominent Nebraska Democrat Bob Kerrey into the race has given Democrats new hope. Kerrey, who has served both as the state's governor and senator, is a popular, well-known figure there.

Nevertheless, it will be an uphill battle for Kerrey, who hasn't run for political office since the 1990s. Nebraska state Senator Deb Fischer, who won the Republican nomination in an upset victory against establishment favorite Jon Bruning, is likely to receive a flood of money from outside groups. Even if Fischer pulls off a win in Nebraska, Kerrey will do his best to make sure she and her allies spend plenty of time and money in the process.

Top ten Senate races to watch in 2012

NEW MEXICO

With polls showing an exceedingly tight race to fill the seat being vacated by retiring Democrat Jeff Bingaman, some big names have already stepped in to exert their influence. Lisa Murkowski, Alaska's moderate Republican senator, flew out to New Mexico earlier this month for $1,000-a-head fundraiser on behalf of the race's likely Republican candidate, former Rep. Heather Wilson, also a moderate.

Meanwhile, Rep. Martin Heinrich is the clear favorite against state auditor Hector Balderas for the Democratic nomination. Heinrich announced in April that he raised nearly $500,000 in the year's first quarter. With the two candidates running neck-and-neck in the polls, it's looking like a long road to November for them both.

Top ten Senate races to watch in 2012

NORTH DAKOTA

North Dakota is another good bet for a possible Republican Senate pickup in 2012 following the retirement of Democratic Senator Kent Conrad. In the Republican-friendly state, the likely GOP candidate, Republican congressman Rick Berg, would seem to have an advantage - and he's already spent heavily in to increase his odds. But Democrats are taking their best shot at holding onto the seat with Heidi Heitkamp, the state's former attorney general.