Democratic Senate contenders hold clear leads in two states their party is defending, but face tighter contests in two others Republicans are hoping to flip, a new round of polls released Thursday shows. 

In Ohio and Florida, Democratic incumbents lead by double-digits, while in Virginia and Wisconsin -- both open races to replace retiring Democrats -- the margins are close. Finally, a poll in Nevada, where Sen. Dean Heller (R) is trying to hold his seat, shows a close race that tilts slight toward the Republican. Here’s a complete rundown of Thursday’s Senate polls: 

* Ohio: A new NBC News/Wall Street Journal/Marist College survey shows Sen. Sherrod Brown (D) leading Josh Mandel 52 percent to 41 percent, a margin largely unmoved from Brown’s 50 percent to 41 percent lead in the previous poll. GOP groups have spent more than $20 million trying to dislodge Brown, but given the current state of this race, the effort could turn out to be very expensive loss for Republicans.

* Virginia: Two different polls tell two different stories. The NBC/WSJ/Marist survey shows former governor and DNC chairman Tim Kaine and former governor and senator George Allen (R) running about even, with Kaine at 47 percent and Allen at 46 percent. Meanwhile, Kaine leads Allen 51 percent to 44 percent in a Quinnipiac University/New York Times/CBS News survey.

* Florida: Sen. Bill Nelson (D) leads Rep. Connie Mack (R) 52 percent to 39 percent in the NBC/WSJ/Marist poll. Nelson runs four points ahead of President Obama, who clocked in at 48 percent support among those likeliest to vote.

* Wisconsin: The race between former governor Tommy Thompson (R) and Rep. Tammy Baldwin (D) is about even, with Baldwin at 48 percent and Thompson at 46 percent in the Quinnipiac poll. Democrats hammered Thompson over the airwaves in the immediate aftermath of the mid-August primary, a bruising contest which left his campaign account depleted, forcing him to focus on raising more money. With reinforcements from GOP groups, Thompson has pulled back to about even in this hotly contested race. 

* Nevada: Sen. Dean Heller (R) holds a small, 40 percent to 37 percent lead over Rep. Shelley Berkley (D) in a new Suffolk University survey. Heller’s doing comparably better in his matchup than Mitt Romney, who trails Obama 45 percent to 47 percent in the Nevada poll.