Sens. Bob Casey Jr. (D-Pa.) and Sherrod Brown (D-Ohio) are in strong position for reelection, but Sen. Bill Nelson's (D-Fla.) race against Rep. Connie Mack (R-Fla.) has tightened considerably, according to a series of new polls from Quinnipiac University.

Nelson leads Mack by 41 percent to 40, a closer margin than most polls have shown and a sign that the race might be tightening as Mack becomes better-known. President Obama also led Mitt Romney by 4 percentage points in the poll.

Both Casey and Brown hold double-digit leads over their opponents, according to the polls. The number is good news for Brown — most other recent polls have shown the race between him and Ohio state Treasurer Josh Mandel (R) within single digits, and while Mandel has had a few weeks of rough headlines, outside groups have already spent $8 million on his behalf, which had kept the race relatively close. Brown leads Mandel by 50 to 34 percent in the state, wider than Obama's 9-point lead in the poll.

Casey has maintained a strong lead in nearly every public poll of his race, and this is no exception: He leads Republican Tom Smith by 49 percent to 32, while Obama leads Romney by 45 to 39 in the same poll.

The Hill rates the Ohio and Florida races "lean Democratic" and the Pennsylvania race "likely Democratic."

All three polls were of more than 1,200 registered voters, were conducted from June 19-25 and had margins of error of 2.8 percentage points.

This post has been updated.