Democrats currently hold a 53 to 47 majority in the Senate, including two independents that caucus with them. After gaining six seats in the 2010 midterm elections, Republicans need to net an additional four this year to reach a majority--or only three if Mitt Romney is elected president, since Paul Ryan would cast the tie-breaking Senate vote as vice president.
This year, Democrats have 23 seats up for reelection (including the two independents that caucus with them) while Republicans have only 10 seats up for reelection. According to the final HuffPost Pollster estimates, the combination of returning senators and candidates leading in the 2012 contests gives Democrats 50 seats and Republicans 45 seats. An independent candidate from Maine, who is expected to caucus with Democrats, continues to lead in the polls and there are four races rated as "toss-ups."
To win a majority, Republicans not only need to win those four "toss-up" races but also make at least one inroad into a race that currently "leans Democratic." Meanwhile, if Democrats win a majority of the "toss-up" races, they have an opportunity to expand their current majority.