Sen. Bob Casey, the Democratic incumbent from Pennsylvania who has led comfortably for most of the race, is trying to stave off a late surge from his opponent, business executive Tom Smith.
Republicans point to heavy spending by Democrats in blue sates like Pennsylvania, Connecticut, and Maine as evidence they have a chance to win enough seats to win the majority. Democrats point to heavy spending by Republicans in red states like Indiana and Montana as evidence they can't.
Operatives from both parties agree that many of these races could be determined by which presidential candidate carries the state, particularly in key battleground states like Florida, Virginia, Pennsylvania, Wisconsin, and Nevada.
As a sign of the high stakes over control of the Senate, a report out this week by the Sunlight Foundation says that outside groups spent a whopping $190 million on Senate races as of October 23.
Key senate race snapshots
Compiled by Adam Levy and Robert Yoon, CNN Political Research
Arizona: Rep. Jeff Flake (R) vs. Richard Carmona (D)
Open seat -- Sen. Jon Kyl (R) is retiring
This race has turned more competitive than originally expected. Democratic nominee Dr. Richard Carmona, a Vietnam veteran and a former U.S. Surgeon General under President George W. Bush, has proven a formidable opponent to six-term Rep. Jeff Flake, whose tough August primary for the GOP nomination left him bruised as he began the general election.
Tightening polls caused both campaigns to go negative with Flake accusing Carmona of having anger issues over an incident in which a former HHS official accused him of banging on her door in the middle of the night and scaring her family (Carmona denies the incident ever occurred). Carmona accuses Flake of not supporting veterans as a congressman (Flake says Carmona is cherry-picking votes and not looking at his entire record).
Democrats think their candidate's strengths and the state's growing Hispanic population will lead to their party's first successful Senate election since 1988. But Republicans point to no significant changes in Hispanic voting records, Flake's endorsements from Sens. Kyl and John McCain, and the state's traditional GOP support as reasons for a Flake victory.
California: Sen. Dianne Feinstein (D) vs. Elizabeth Emken (R)
Democratic incumbent Dianne Feinstein should easily win a fourth full term this November. Her GOP opponent is Elizabeth Emken, a former vice president for government relations at Autism Speaks.
Connecticut: Rep. Chris Murphy (D) vs. Linda McMahon (R)
Open seat -- Sen. Joseph Lieberman (I) is retiring
After waging a competitive but ultimately unsuccessful bid for U.S. Senate in 2010, former wrestling executive Linda McMahon is making a strong comeback this year. Buoyed by the millions of her own money she invested in the campaign, McMahon's made an effort to soften her image with ads about her personal life and combat attacks from third-term Rep. Chris Murphy.
Despite McMahon's significant financial advantage, Murphy is polling even or ahead of his opponent. In a state where President Barack Obama won by more than 20 points in 2008, a tie in the polls going into Election Day could mean a Murphy win on the president's coattails.
This is a tough state for Republicans in federal office. The last Connecticut Republican to hold a U.S. Senate seat left office in 1989.
Delaware: Sen. Tom Carper (D) vs. Kevin Wade (R)
Democrat Tom Carper has a clear path to winning a third term in November. He faces Republican businessman Kevin Wade.
Florida: Sen. Bill Nelson (D) vs. Rep. Connie Mack IV (R)
Democratic Sen. Bill Nelson's bid for a third term hasn't generated the competitive buzz originally expected in one of the most important presidential battleground states. Nelson's lead over four-term Rep. Connie Mack IV held steady throughout most of the campaign, though recent polls show Mack closing the gap.
TV ads from the candidates and outside groups have driven the race heavily, with less campaigning than usually seen in a U.S. Senate race in the Sunshine State on both sides. There was just one hour-long debate. Mack's poll numbers have improved as Romney's numbers have gone up in the state.
Hawaii:Rep. Mazie Hirono (D) vs. Former Gov. Linda Lingle (R)
Open seat -- Sen. Daniel Akaka (D) is retiring