What follows is an NBC analysis:
"Five weeks out from the midterm elections, MSNBC/McClatchy polls, conducted by Mason-Dixon in eight states, show Democrats are in striking distance of taking control of the Senate. The Democrats are very likely to gain several Senate seats with some races still rated as toss-ups.
In the Senate, Democrats need to gain six seats to regain control of the chamber. Our polls show that this is certainly possible as five races are toss-ups, one now narrowly shows a gain for Democrats -- Pennsylvania -- and the party maintains control of Sen. Maria Cantwell's Senate seat in Washington. In addition, other Mason-Dixon polls released Sunday indicate trouble for Republicans in three other GOP-held seats.
In Pennsylvania, incumbent Sen. Rick Santorum is well behind his challenger Bob Casey, with Casey currently ahead by 9 percentage points, 49 percent to 40 percent, with 10percent undecided.
In Rhode Island, incumbent Republican Sen. Lincoln Chafee and Democrat Sheldon Whitehouse are in a virtual tie with Whitehouse supported by 42 percent of likely voters compared to Chafee's 41 percent. But there are still 1 7percent undecided.
In Virginia, incumbent Republican Sen. George Allen, whose campaign has recently been plagued with public gaffes and charges of the candidate as racist, and his Democratic opponent, Jim Webb, are tied with 43 percent each and 12 percent undecided.
In Missouri, incumbent Republican Sen. Jim Talent and Democrat Claire McCaskill are tied with 43 percent each and 13 percent still undecided.
In New Jersey, incumbent Democratic Sen. Bob Menendez is in a virtual tie with his Republican challenger, Tom Kean, Jr., with 44 percent of likely voters supporting Menendez and 41 percent supporting Kean. There are still 13 percent undecided.
In Washington, incumbent Democratic Sen. Maria Cantwell leads Republican challenger Mike McGavick by 10 percentage points, 50 percent to 40 percent, with 9percent undecided.
In Maryland, Democratic candidate Ben Cardin is leading his Republican opponent, Michael Steele, 47 percent to 41 percent, with 12 percent of voters still undecided.
And in other Mason-Dixon polls, Democrats seem well positioned to gain seats:
In Ohio, a Plain Dealer poll shows incumbent Republican Sen. Mike DeWine and Democrat Sherrod Brown in a virtual tie, 43 percent for DeWine to 45 percent for Brown. There are 10 percent undecided in this race.
In Montana, a Lee Newspaper poll shows incumbent Republican Sen. Conrad Burns trailing Democratic challenger Jon Tester by a 40 percent to 47 percent margin with 10 percent undecided.
In Tennessee, a poll conducted for the Memphis Commercial Appeal Chattanooga Free Press shows Harold Ford, Jr. and Bob Corker in a virtual tie, 43 percent for Ford to 42 percent for Corker with 14 percent still undecided.
In all, these key Senate races show the following:
Two Republican incumbents in very serious trouble, Burns and Santorum.
Four Republican incumbents tied with their challengers, Chaffee, Allen, Talent, and DeWine.
One Democratic incumbent tied with his challenger, Menendez.
One Democratic incumbent with a real lead, Cantwell.
One Democratic open seat with a Democrat in the lead, Cardin in Maryland.
One Republican open seat with a tie, Tennessee.
The results show that the Democrats have a real chance of gaining control of the Senate. However, as the election approaches, Democrats may have to lead by significant amounts to counteract the well-funded Republican get-out-the-vote effort. And almost every toss up seat needs to break for the Democrats for them to gain the six seats that they need."