For months it has been clear that Democrats have a red-state problem in the Senate midterm elections: Seven Democratic seats are up for grabs in states Mitt Romneycarried in 2012, three of them opened by retirements.
Republican Congressman Cory Gardner's decision on March 1 to run against Democratic Sen. Mark Udall in Colorado—a state President Obama carried in 2008 and 2012—indicates that Democrats have a blue-state problem too.
The problem stems from President Obama's poor approval rating, compounded by able Republican candidates like Mr. Gardner, a rising star in the House.
Mr. Obama's average Gallup approval rating in Colorado last year was 42.3%, compared with his 2013 national average of 46.5%. The state is also one of 44 where Mr. Obama received a smaller margin in 2012 (5.4%) than 2008 (9%). A February Quinnipiac poll put Sen. Udall a mere three points ahead of 2010 GOP nominee Ken Buck, who was widely seen as a flawed candidate.
Sen. Udall is not the only Democratic incumbent so situated. If former Massachusetts Sen. Scott Brown challenges New Hampshire Sen. Jeanne Shaheen, she will have to contend with Mr. Obama's low (44.6%) approval rating in the Granite State.
Sens. Tom Udall (N.M.), Jeff Merkley (Ore.), and Mark Warner (Va.) also face headwinds. Mr. Obama's 2013 approval rating in each of their states was below his national average last year—45%, 45.4% and 46.4%, respectively.
Sen. Warner's likely Republican opponent, former Republican National Committee chairman and White House official Ed Gillespie (and a close friend), is off to a strong financial start and drawing large numbers of young people and first-time volunteers. Sens. Udall and Merkley could face determined challengers as well—Mr. Udall from Marine veteran Allen Weh and Mr. Merkley from pediatric neurosurgeon Monica Wehby.
Though Iowa Republicans won't settle on a nominee until at least June 14, that state's open Democratic Senate seat could also be in play: Mr. Obama's approval rating in Iowa last year was 42.4%, well below his national average.
In the contest for the open Michigan Democratic Senate seat, Republican former Secretary of State Terri Lynn Land is winning the money race and has led in all four public polls conducted since Dec. 1. Mr. Obama's approval rating in the state last year was 47.8%, just above his national average.
Even Minnesota's Sen. Al Franken could be in for a tough run: Mr. Obama's approval in his state last year was 48.2%. The Republican primary won't be settled until Aug. 12. But if Mr. Obama is below 50% in October and Republicans nominate a strong candidate, they could have the last laugh on the former comedian's fate.
In the six blue states with Democratic senators up this fall, Mr. Obama's approval rating last year was below his 2013 national average in four and in two it was just above it. If the president's ratings in these states have moved in tandem with his national number, he's less popular in them now, since the president's Gallup approval was down to 42% on Wednesday. History says it will be hard for Democrats to run much ahead of the president's October approval number—and odds are that number will be lower than it is now.
The GOP's 2010 experience is instructive. Republicans picked up four Democratic Senate seats in states that Mr. Obama carried in 2008, defeating an incumbent in Wisconsin (won by Mr. Obama by 14 percentage points) and grabbing three Democratic open seats in Illinois (25 points), Pennsylvania (10 points) and Indiana (one point).
These were paired with 2010 red-state victories, with the GOP winning a Democratic open seat in North Dakota (which Mr. Obama lost by nine points in 2008) and knocking off a Democratic incumbent in Arkansas (which Mr. Obama lost by 20 points). In addition, the GOP retained three open seats in states Mr. Obama carried in 2008: New Hampshire (which Mr. Obama won by 10 points), Florida (three) and North Carolina (one).
That as many as 15 Democratic seats might be in play might offset any potential GOP losses in Kentucky (Mitch McConnell), Maine (Susan Collins) and Georgia (retiring Saxby Chambliss). Sen. McConnell in particular will face an all-out assault from every union and liberal pressure group that fears his effective leadership.
At this moment, Republicans look likely to sweep the open red-state Democratic seats in West Virginia, South Dakota and Montana, and are poised to take most, perhaps all, of the four races involving red states with Democratic incumbents. That eight Democratic blue-state seats could also be in play dramatically increases chances Republicans pick up the net of six seats needed for a Senate majority, a result with big implications for Mr. Obama's final two years as president.
A version of this article appeared March 6, 2014, in the U.S. edition of The Wall Street Journal, with the headline Now Blue-State Senate Democrats Feel The Heat and online at WSJ.com.