In Florida, where Obama led by 6 percentage points in the poll, conducted July 24-30, Democratic Sen. Bill Nelson
leads Rep. Connie Mack
by 7 points, 47 percent to 40 percent. Here, the fortunes of each Senate candidate appear to be closely linked to their party's standard-bearer. Among Obama supporters, Nelson leads, 79 percent to 7 percent for Mack. Mack leads among Romney voters by a similar margin, 80 percent to 11 percent.
But in Ohio, Brown slightly outperformed Obama in the poll, conducted concurrently to the Florida survey. Obama led Romney by 6 percentage points, but Brown's margin over GOP state Treasurer Josh Mandel
was twice that, 51 percent to 39 percent. Brown wins a commanding 87 percent of Obama supporters, but Mandel holds only 79 percent of Romney voters. Outside spending may help Mandel close the gap, but with Obama edging ahead in Ohio, the incumbent is well-positioned at this stage.
With two established candidates on the ballot in Virginia -- Democratic former Gov. Tim Kaine
and Republican former Sen. George Allen
-- it is not surprising how closely the vote tracks to the presidential ballot. Kaine leads among Obama voters, 85 percent to 9 percent, while Allen dominates among Romney supporters, 86 percent to 8 percent. Overall, the July 31-Aug. 6 poll shows Obama leading by 4 percentage points, while Kaine leads by 2; the difference between their advantages is not statistically significant.
The Senate race in Wisconsin is not yet settled; Republicans will pick their nominee in Tuesday's primary. A Marquette Law School poll released last week showed Thompson holding a tenuous lead
in a wide-open primary field, while the Quinnipiac University/CBS News/New York Times
poll showed Thompson as the strongest GOP candidate in a general election. Thompson's 5-point lead over Democratic Rep. Tammy Baldiwn
in a state in which Obama leads by 6 points is a result of slightly better crossover appeal: Thompson dominates among Romney voters, 87 percent to 7 percent, while also stealing 12 percent of Obama supporters, to 83 percent for Baldwin. The other two main GOP contenders -- businessman Eric Hovde
and former Rep. Mark Neumann
-- do not have the same crossover appeal. Hovde and Neumann only win 6 percent of Obama voters and trail Baldwin narrowly overall, according to the Quinnipiac poll. (The Wisconsin poll was also conducted July 31-Aug. 6, prior to Romney's announcement of Rep. Paul Ryan
, R-Wis., as his running mate.)
These poll results underscore the growing tendency of voters to choose their Senate candidate based on the race for president or, in midterm elections, their opinion of the current president's job performance. A National Journal
analysis this spring found that in virtually every competitive Senate race in 2004, 2006 and 2010, at least 70 percent of voters who approved of how George W. Bush
or Obama was handling his job as president voted with the president's party, and at least 70 percent of those who disapproved voted for the other party. (National Journal
subscribers can read more on this phenomenon here
A note on methodology: The polls were released on Aug. 1 (Florida and Ohio
) and Aug. 8 (Virginia and Wisconsin
). Quinnipiac also conducted polls in Colorado, which has no Senate race this year, and Pennsylvania, where Sen. Bob Casey
, D-Pa., is a heavy favorite for reelection.
Ronald Brownstein contributed reporting.