(CBS News) Ever since the 2010 midterm elections, when Republicans regained control of the House of Representatives and picked up six seats in the Senate, the battle for control of the Senate has promised to be competitive: The two parties are separated by effectively six seats in the chamber, and both sides are pouring millions of dollars into the effort to seize control.
Democrats have a particularly robust roster of candidates to defend this cycle: Of the 33 seats up for grabs in 2012, 23 are held by Democrats. Republicans, meanwhile, need to flip just four seats - three if Mitt Romney wins and his vice president can serve as a tiebreaker - in order to gain the advantage.
Thanks to several unexpected and potentially game-changing developments, however, control of the Senate is still very much in play.
"Until the end of February I gave Republicans a 65-70 percent chance of taking the majority because it was all a numbers game," said Jennifer Duffy, who analyzes Senate and governor races for the The Cook Political Report. Now, Duffy gives the Republicans a 45 percent chance of taking the chamber.
"There's still a path to the majority for Republicans," she said. "But it's a lot harder."
One of the most-watched races of the cycle is taking place in Massachusetts - where first-time candidate Elizabeth Warren is taking on Republican incumbent Scott Brown, who was elected with Tea Party support following the death of Sen. Ted Kennedy, who held the seat for almost 50 years. Warren, a consumer advocate tapped by President Obama to head up the Consumer Financial Protection Bureau (CFPB), was seen as a promising leader whose voice could easily find a place in the solidly liberal Massachusetts. But Warren consistently struggled to secure a definitive advantage over Brown in the polls, and she faced setbacks over a controversy about whether or not she improperly identified herself as Native American to further her career.
Two new surveys, however, show Warren opening up an apparent lead against the Republican incumbent - suggesting that the significant Democratic investments into her candidacy may finally be paying dividends.
A new poll out Tuesday by the Suffolk University Political Research Center shows Warren leading Brown 48 percent to 44 percent, while a survey released Monday by Western New England University's Polling Institute (WNEU) shows Warren leading by 6 points, at 50 percent to Brown's 44 percent. A previous WNEU poll from June showed Warren with a slimmer 2-point edge over Brown.