These are the 10 Senate races, out of 37 contests today, that are most likely to determine which party --- if any --- controls the upper house of the US Congress up to the 2012 Presidential contest.
With the current balance at 59 Democrats/Democratic-allied Independents and 41 Republicans, the GOP needs to win all 10 of these battles to take a slim majority. If the Republicans win nine, the Democrats maintain nominal supremacy through the casting vote of the presiding officer, Joe Biden.
Other high-profile races that are unlikely to affect the overall picture:
Delaware: Christine O'Donnell, the Tea Party candidate who has livened up this campaign (especially on the pages of EA USA), is now trailing significantly to Democrat Christopher Coons.
Kentucky: Rand Paul, the first Tea Party insurgent to topple a Republican in a primary, is odds-on favourite to hold the seat for the GOP against Democrat Jack Conway.
Florida: In a three-way contest, upstart conservative Marco Rubio will probably win for the Republicans against Governor Charlie Crist, who left the Republicans in April to run as an independent, and Democrat Kendrick B. Meek.
Alaska: Another three-way battle, with Tea Party and Sarah Palin favourite Joe Miller likely to triumph over Lisa Murkowski, the Governor whom he defeated in the Republican primary, and Democrat Scott McAdams.
Now the key contests. Latest polls and prospects of a Republican victory are taken from the FiveThirtyEight website. An asterisk indicates the candidate is currently in the US Senate:
NEVADA
Republican: Sharron Angle (50%)
*Democrat: Harry Reid (47%)
Republican Chance of Gain: 82%
The top of the ticket race. Reid, the Senator Majority Leader, was close to dead in the water against Angle, the Tea Party insurgent. He has recovered some ground in recent weeks but is still the underdog.
PENNSYLVANIA
Republican: Pat Toomey (52%)
Democrat: Joe Sestak (48%)
Chance of Republican Gain: 97%
Perhaps the touchstone race. Sestak upset incumbent Arlen Spector in the Democratic primary. He represents the "liberal" wing of the Democrats vs. the very conservative Toomey. A Sestak upset here would signal that the Republican resurgence has been limited.
WISCONSIN
Republican: Ron Johnson (51%)
*Democrat: Russ Feingold (46%)
Chance of Republican Gain: 96%
The race that will probably best represent the shift in US politics in two years. Feingold is a well-respected progressive Democrat who has fought diligently on social issues and campaign reform; Johnson is a businessman, backed by the Tea Party, capitalising on disillusionment with politics in Washington.
COLORADO
Republican: Ken Buck (49%)
Democrat: Michael Bennet (48%)
Chance of Republican Gain: 65%
Tea Party-backed Buck upset a mainstream Republican, to face Bennet, a surprise appointment to fill a Senate vacancy in 2009 who survived a tough primary challenge.
WASHINGTON
Republican: Dino Rossi (49%)
*Democrat: Patty Murray (51%)
Chance of Republican Gain: 21%
Murray, a three-term Senator, has risen through the Democratic ranks but has now been tarnished by her association with Majority Leader Reid. Rossi has twice lost contests for Governor.
ARKANSAS
Republican: John Boozman (58%)
*Democrat: Blanche Lincoln (39%)
Chance of Republican Gain: 100%
Senator Lincoln has been targeted by the Republicans as a "liberal" in a Southern state and, after a brusing battle in the Democratic primary, she should be put out of her misery by Representative Boozman.
ILLINOIS
Republican: Mark Kirk (49%)
Democrat: Alexi Giannoulias (48%)
Chance of Republican Gain: 69%
Kirk, a five-term congressman from Chicago’s suburbs, and Giannoulias, the state treasurer, have been locked in a bitter race following the corruption scandal that took down Illinois Governor Rod Blagojevich. Accusations of lying and financial impropriety have muddied a campaign for the seat that was held by Barack Obama before he became President.
WEST VIRGINIA
Republican: John Raese (47%)
Democrat: Joe Manchin (51%)
Chance of Republican Gain: 11%
INDIANA
Republican: Dan Coats (58%)
Democrat: Brad Ellsworth (39%)
Chance of Republican Gain: 100%
Former Senator Coats, now a lobbyist, will return to the US Capitol with victory over Representative Ellsworth.
NORTH DAKOTA
Republican: John Hoeven (70%)
Democrat: Tracy Potter (30%)
Chance of Republican Gain: 100%
Governor Hoeven is a shoo-in to take the seat left by the retiring "progressive" Democrat Byron Dorgan against state senator Tracy Potter.