In the lead-up to November 2, Senate Democrats don't have nearly as much to worry about as their counterparts in the House. Alhough there remains a small chance that Republicans could gain control of the upper chamber, the party would need just about everything to go their way on election night. Here are four tight races that Republicans will certainly need to carry in order to have a chance of taking the Senate: 

1. Illinois — Giannoulias vs. Kirk
The battle for Barack Obama's former Senate seat has been "one of the nastiest and most bitterly fought races around" for months, says The New York Times. GOP candidate Mark Kirk has led narrowly all the latest polls, and statistical wunderkind Nate Silver gives him a 66.7 chance of winning — with the caveat that "the number of undecided voters in Illinois remains high." Democrat Alexi Giannoulias is counting on personal testimonials from the Obamas to help nudge him over the top on Tuesday.

2. Colorado — Buck vs. Bennet
In this extremely close contest, money from outside groups is pouring in for both Republican Ken Buck and Democrat Michael Bennet — with the total nearing $25 million. Buck has been ahead or tied in all recent polls, but, as Eric Lach at Talking Points Memo notes, "four separate pollsters have all shown [his] lead shrinking." Buck's cause probably has not been helpled by recent comments he made equating gay people to alcoholics, but Bennet has his own issues: As RealClearPolitics notes, he "suffers from a lack of charisma."

3. Nevada — Reid vs. Angle
Recent polls show Republican Sharron Angle with a consistent 4-point lead over Harry Reid, the Senate Majority Leader, and Nate Silver gives her a better than three-in-four chance of winning. The race is seen as "a referendum on Reid and Obama," says political analyst Larry Sabato, and that fact is not encouraging for Reid — he and his boss are both distinctly unpopular in Nevada.

4. Alaska — Miller vs. Murkowski vs. McAdams
In perhaps the oddest, least predictable race in the country, incumbent Lisa Murkowski is hoping to win as a write-in candidate after losing the Republican primary to Sarah Palin-endorsed Joe Miller. Alaskan polls are notoriously unreliable, though most show the race coming down to the wire, thanks in part to a wariness among voters — even Republicans — of the "disturbing level of secrecy" surrounding his campaign. Democrat Scott McAdams — still a distinct underdog — has managed to "double his support" in the past month.

Sources: Chicago Sun-Times, The New York Times, The Washington Post, Talking Points Memo, FiveThirtyEight, Las Vegas Review Journal, Mother Jones, Politico, Fox News