For months, the key question about the upcoming midterm congressional elections has been whether Republicans can overcome their six-seat deficit to become the majority party in the US Senate.

Now the question has changed somewhat — to whether the surprising strength of Republican candidates in states once considered long shots for the GOP portends disaster for Democrats hobbled by ObamaCare and the president’s weak approval ratings.

No one doubted the GOP would come close or better than close to a Senate takeover in November. But every day, the map looks more favorable for the Republicans.

First, four races feature wounded Democrats in states that went against President Obama in 2012: Mark Pryor (Arkansas), Mary Landrieu (Lousiana), Kay Hagan (North Carolina) and Mark Begich (Alaska).

The GOP challenger in Arkansas, the extremely impressive Rep. Tom Cotton, may already be running away with his race; he’s ahead anywhere from four to seven points over Pryor. Every potential Republican challenger to Hagan is beating her in head-to-head polls. In Alaska, Begich is losing to both possible GOP candidates and has a job-approval rating around 40 percent, which is basically death for an incumbent. Landrieu is essentially tied with her leading challenger, Rep. Bill Cassidy; that’s better than her colleagues, but not where you want to be when you’ve served as a senator for 18 years.

Among seats held by retiring Democrats, the likely Republican candidate in West Virginia, Shelley Moore Capito, is up by 14 points in the latest poll. In South Dakota, a popular Republican ex-governor, Mike Rounds, is favored to win the seat being vacated by Democrat Tim Johnson.

In Montana, Max Baucus announced his retirement and then left the Senate early to become ambassador to China — solely, it seems, so that the Democratic governor could appoint this year’s candidate, Jim Walsh, to the Senate to give him incumbent status in the race. But Walsh is down double-digits to the two Republican contenders even so, and this one will almost surely go GOP.

Right there are the seats Republicans need to take over. The best hope of a Democratic takeaway might be in Georgia, where the Democrats have fielded a potential star in Michele Nunn — and where it appears most likely the GOP might field an unacceptable nutball in the mode of the disastrous candidates (Christine O’Donnell, Sharron Angle, Todd Akin, Richard Mourdock) whose bids in 2010 and 2012 made a Senate takeover impossible then.

And in Colorado, where there’s another first-term incumbent named Mark Udall, something fascinating just happened. The 2010 nutball who lost that Senate race for the GOP and seemed poised to run and lose again dropped out in favor of the strongest potential rival, Rep. Cory Gardner. That’s now winnable for the GOP.

And this is where it gets really interesting. Republicans are coming on strong in other races no one expected. In Michigan, where another Democratic incumbent is retiring, Republican Terri Lynn Land has shown strength for months, leading her likely Democratic rival, Gary Peters, by a few points in a state Obama carried by eight points in 2012. In New Hampshire, the one-term ex-Gov. Jeanne Shaheen seems to have a formidable foe in Scott Brown, who won the surprising January 2010 special election for Ted Kennedy’s seat in Massachusetts that portended the huge GOP wave later that year.

Brown, who lost to Elizabeth Warren in 2012, surely wouldn’t be running again in the state next door so soon after a painful defeat if he didn’t see a serious path to victory. The same is surely true of Ed Gillespie, the former chairman of the Republican National Committee, who has decided to take on first-term Sen. Mark Warner in Virginia.

And I haven’t even mentioned Al Franken in Minnesota, who won his race by a mere few hundred votes in 2008. Although the state seems more Democratic than ever, Franken only has a 39 percent approval rating, and if the GOP gets itself a solid candidate, he could be in real trouble.

Republicans have “expanded the playing field,” as they say. Democrats are on defense. It’s always possible GOP primary voters will knock off a solid sitting senator (like Thad Cochran in Mississippi) for a crackpot and make it more difficult for the party to triumph. But if those voters keep a cool head, Nov. 6 could be a humbling day for Obama Democrats.