Party spending, independent groups and a multitude of polling indicate races in Colorado, Illinois, Nevada, Pennsylvania, Washington and West Virginia are the key battlegrounds heading toward Nov. 2, The Washington Post reported.
Any crystal ball-gazing in which Republicans retake the Senate majority -- and some pundits say the Republicans could achieve a net gain of nine for a 50-50 split -- depends on sweeping those six seats now held by Democrats.
In Colorado, Sen. Michael Bennet, appointed by President Obama when Ken Salazar became interior secretary, and Weld County prosecutor Ken Buck have engaged in a nasty back-and-forth throughout the fall. Democrats say Buck's too extreme; Republicans say Bennet is tied to Obama's hip.
The race between Republican Rep. Mark Kirk and Democratic state Treasurer Alexi Giannoulias to take over Obama's seat as the junior senator from Illinois has been another bloody race, with both men giving and taking body blows over records and integrity.
The contest between Senate Majority Leader Harry Reid and former Nevada Assemblywoman Sharron Angle is one of the closest in the country with polls indicating it was a dead heat.
In Pennsylvania, Democratic U.S. Rep. Joe Sestak has been slowly gaining on former Rep. Pat Toomey. But Sestak still has to overcome Obama's lack of popularity in the Keystone State beyond Philadelphia's borders, the Post said.
In Washington, incumbent Patty Murray is ahead or Republican challenger state Sen. Dino Rossi has the lead, depending on which party's talking. Independent polling is all over the place as well.
Both national parties are pouring money into West Virginia in the race pitting the popular Gov. Joe Manchin against Republican businessman John Raese to finish out the unexpired term of Sen. Robert Byrd. Despite his popularity, Manchin also is weighted down by Obama's unpopularity. Analysts told the Post this race will be a harbinger of the Senate Democrats' fortunes.
Politico lists Wisconsin as another key race. Sen. Russ Feingold faces a tough challenger in plastics manufacturer, self-funding neophyte Ron Johnson, whom some polls show ahead of the congressional veteran.
No matter what happens, one political analyst says Reid's days as the party's leader are numbered.
"He's showing up in (GOP) ads all over the country," said Steven Schier, commentator and political science professor at Carleton College in Northfield, Minn.
If he does win re-election and Democrats retain their majority, Schier said, "there may be a challenge" for majority leader, likely from Sens. Charles Schumer of New York or Richard Durbin of Illinois.
"If the Democrats lose the majority, Reid won't be minority leader," he said.
The one person who could be sitting in the catbird's seat Nov. 3 is Sen. Joe Lieberman, Ind-Conn., who could have the power to decide who controls the upper chamber if Republicans pick up nine seats, splitting the parties evenly.
A nine-seat gain for the GOP would give Democrats a caucus of 50, made up of 48 Democrats and two independents, Lieberman and Sen. Bernie Sanders of Vermont, The Hill reported. Vice President Joe Biden would become the crucial tie-breaking vote in the Democrats' favor.
But that skinny one-vote majority hinges on Lieberman still caucusing with Democrats -- and his relationship with the party has been strained at times. Plus, Lieberman faces a tough re-election fight in 2012 and could be lured to caucus with the Republicans -- or switch parties -- during the next Congress.
Lieberman has stuck to his mantra of enjoying his independence, the Hill said.
Asked about the senator's political affiliation, Lieberman spokesman Marshall Wittmann said, "Senator Lieberman is happy where he is in the Senate and has no other plans."
"It is an interesting hypothetical," Quinnipiac University polling chief Douglas Schwartz said. "Lieberman has a 2012 campaign, and after the election, people here in Connecticut will start thinking about Lieberman much more."
White House senior adviser David Axelrod has repeatedly described the midterm election as "tough," but says it's a choice between the two parties, not a referendum on Obama, the Post said.
"We've come through two very difficult years," Axelrod said, later adding, "Two years ago I could have told you this was going to be a tough year."
The White House has long insisted Election Day is about choosing between a vision for moving the country forward and a view that would take the country in the opposite direction, Axelrod said.
"It is natural and easy in a midterm election particularly in difficult times to treat the election as a referendum on current conditions," Axelrod said. "That's not what it is. This is choice between two fundamentally different approaches."