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(CNN) -- With a week to go before voters cast their midterm election ballots, four key Senate races remain statistical dead heats, and Republican Sen. Mike DeWine faces uphill sledding in his re-election battle in Ohio, according to new CNN polls released Tuesday.
The polls were conducted for CNN by Opinion Research Corporation in Missouri, New Jersey, Ohio, Tennessee and Virginia, all of which have hard-fought Senate races that could determine whether Democrats manage to wrest control of the upper chamber from Republicans.
Among likely voters in Missouri, Republican Sen. Jim Talent and his Democratic challenger, State Auditor Claire McCaskill, were at a dead heat, each with 49 percent support. However, among the larger pool of registered voters, McCaskill led 51-43 percent. (Missouri race a dead heat -- 1:08 )
The sampling error for likely voters was plus or minus 4 percentage points; for registered voters, plus or minus 3 percentage points. (Read the complete poll results -- PDF)
Meanwhile, in New Jersey, Democratic Sen. Bob Menendez held a 51-44 percent edge among likely voters over his GOP challenger, state Sen. Tom Kean Jr. However, that gap was within the sampling error.
In Tennessee, Democratic Rep. Harold Ford Jr. -- hoping to make history as the first black candidate ever elected by Southern voters to a Senate seat -- trailed his Republican opponent, Chattanooga Mayor Bob Corker, by a margin of 52-44 percent among likely voters. (What how Corker is beginning to pull ahead in a tough race -- 1:25 )
However, among registered voters, Corker's lead was just 47-45 percent, and the results for both likely and registered voters were within the sampling error.
Ford and Corker are battling for an open seat now held by Senate Majority Leader Bill Frist, who did not seek re-election.
In Virginia, Republican Sen. George Allen -- who has watched his once considerable lead vanish after a series of gaffes and controversies -- trailed his Democratic challenger, former Navy Secretary Jim Webb, by 50-46 percent among likely voters, which was also within the sampling error. (Allen's gaffes have made Virginia a tight race -- 2:07 )
The GOP incumbent who appears to be in the most trouble in the states polled by CNN was Sen. Mike DeWine of Ohio, who trailed his Democratic rival, Rep. Sherrod Brown, by a margin of 54-43 percent among likely voters.
Democrats need to make a net pickup of six seats next Tuesday in order to win control of the Senate. To get there, they are trying to win the open GOP seat in Tennessee and targeting six vulnerable Republican incumbents, including DeWine, Talent, Allen and Sens. Lincoln Chafee of Rhode Island, Rick Santorum of Pennsylvania and Conrad Burns of Montana.
Republicans are hoping to make that task more difficult by challenging Menendez and trying to pick up two open Democratic seats in Maryland and Minnesota.
The new CNN polls also showed Democrats poised to gain a governorship in Ohio and keep one in Republican-leaning Tennessee.
In the Buckeye State, Democratic Rep. Ted Strickland led Secretary of State Ken Blackwell by a margin of 59 percent to 36 percent among likely voters in a race to succeed outgoing Republican Gov. Bob Taft.
Meanwhile, down in the Volunteer State, Democratic Gov. Phil Bredesen, seeking a second term, held a lead of 59-37 percent among likely voters over his Republican opponent, state Sen. Jim Bryson.
Close races in Missouri, Ohio, Tennessee and Virginia could decide which party controls the Senate next year.