Two years ago, Republicans had every reason to believe they could take back the Senate this year, after major midterm election gains. But heading into Tuesday, Democrats appeared well-positioned to retain their slim majority, and with it, the ability to influence much of the Washington agenda during the next two years.

Democrats were victorious in West Virginia, with incumbent Sen. Joe Manchin's win, according to CNN's projection. Independent Bernie Sanders, who votes with the Democratic party, is also projected to win in Vermont.

Maine voters handed a victory to the independent candidate, former Gov. Angus King, to fill the seat of retired Sen. Olympia Snowe, a moderate Republican, CNN projects. Considered a shoo-in for re-election, Snowe shocked her colleagues earlier this year when she suddenly announced that she would leave the Senate, which she described as hopelessly partisan.

There are 33 Senate seats at play on Tuesday, with polls showing a number of key races neck-and-neck.

Republicans would need four seats to win the chamber outright; just three if a Republican were to win the White House and a GOP vice president could break tie votes.

Republicans are protecting only 10 seats, while Democrats are defending 23, many in narrowly divided swing states. In addition, several veteran Democratic incumbents, mostly moderates, announced they would retire, making it potentially even easier for Republicans to win those seats.

Republicans have encountered their own problems.

In August, the campaign of Rep. Todd Akin nearly collapsed after the Missouri Republican's comments about "legitimate rape" and his suggestion that women could biologically prevent pregnancy if they are raped. Until then, Republicans believed Akin would defeat Democratic Sen. Claire McCaskill, who polls showed was not very popular after just one term.

Now, Republicans are grappling with the controversy over Richard Mourdock of Indiana, the tea party-backed state treasurer. During a televised debate with his opponent last month, conservative Democratic Rep. Joe Donnelly, Mourdock defended his opposition to abortion even in the case of rape because, "I think even when life begins in that horrible situation of rape, that it is something God intended to happen."

Democrats pounced on his remarks and several key Republicans denounced them, too. Romney, who had just appeared in a TV ad for Mourdock, said he disagreed with the comment but didn't pull his endorsement.

Strong Democratic candidates also have put several GOP seats into play. And polls show tight races in three of the states where moderate Democratic incumbents retired this year and Republicans were early favorites to win.

But Republicans point to their own prospects, including seats they hope to pick up across the aisle.

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.

But Democratic and Republican political operatives caution that there are more toss-up races this year than in recent memory, so it's very hard to predict the outcome.

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.

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.