Democrats hold onto Senate majority WASHINGTON - Democrats secured control of the Senate on Tuesday after holding at least a dozen seats and picking up two seats held by Republicans, although key races remain undecided. With contests still too close to call in Montana, Nevada and North Dakota, it was impossible for the GOP to gain enough seats to take control of the chamber. Republicans went into the 2012 campaign aiming to gain the four seats they needed for a majority. Earlier in the year, that goal had seemed possible - even likely - especially since Democrats were facing a tough year with 23 seats to defend. But a combination of factors thwarted the GOP's efforts. Democratic Senatorial Campaign Committee Chairwoman Sen. Patty Murray cheered her party for overcoming long odds. "When we started this campaign, no one, and I mean no one, gave us a chance," the Washington Democrat said in a statement. But we went on and built the best Senate campaigns in the history of the country. Democrats picked up seats in Massachusetts and Indiana, and held on in tight races in Missouri and Wisconsin. Republicans also lost a seat to an independent candidate in Maine, Angus King, who is expected to caucus with Democrats but has not explicitly said he will do so. That seat was being vacated by Republican Sen. Olympia Snowe, whose retirement caught her party by surprise. Connecticut Democratic Rep. Chris Murphy overcame former wrestling executive Linda McMahon, a Republican, to win the seat formerly held by Sen. Joe Lieberman, the independent who had caucused with Democrats but was known for bucking the party on key issues. Republican Deb Fischer, a state legislator, picked up the seat that is being vacated by conservative Democratic Sen. Ben Nelson, but the GOP failed to make gains in the other must-win contests. Staff writer Lisa Mascaro contributed to this report. Twitter: @kimgeiger