Last week Sen. Max Baucus (D. Mont.) announced that he would not seek a seventh term. His retirement makes it even more likely the GOP will make gains in the U.S. Senate next year.
Of the 35 Senate seats up for grabs next year, 21 are held by Democrats, 14 by Republicans. Six Democratic seats are in states (W. Va., Ark., S.D., Louisiana, Alaska and Mont.) that Mitt Romney won by at least 10%. Only one Republican seat is in a state (Maine) that President Obama won by more than 10%.
A GOP pickup of even three or four Senate seats would produce big changes. The ratio between Democrats and Republicans on committees would shift, and Republicans would be more likely to cobble together a majority on issues like spending and defense policy. Not every Democrat wants to go over the liberal cliff with Mr. Obama.
To take control of the Senate, however, Republicans must win a net of six seats. They won that many seats in 2010 but lost two seats in 2012, leaving the Democrats with a 53-45 margin today (independents Bernie Sanders in Vermont and Angus King of Maine caucus with the Democrats.)
Eleven Republican seats up next year are considered safe; three need watching. Maine's Susan Collins is the only GOP incumbent running in an Obama state, but she's personally popular and an effective campaigner. Mitch McConnell understands that while Kentucky is a Republican presidential stronghold, every state capital office but one is occupied by a Democrat. That's why he's raised more money than any other senator up in 2014. Avoiding serious primaries is important if the GOP is to retain Maine and Kentucky. Georgia will have a big primary but should remain Republican unless a candidate ill-suited for the general election sneaks through the primary.
Open seats are more likely to flip, and so far only two Republicans are retiring (Georgia's Saxby Chambliss and Nebraska's Mike Johanns) while six Democrats are, including three in states handily carried by Mr. Romney (West Virginia, South Dakota and Montana). Republican Rep. Shelley Moore Capito, a terrific campaigner, is running in West Virginia while former Gov. Mike Rounds is running in South Dakota. If Mr. Rounds faces a first-tier Democratic opponent, he'll have to raise more money and mount a stronger campaign than in his gubernatorial races.
Republicans will be competitive in Democratic open seats in Iowa, Michigan and Montana if they recruit top-quality candidates like Rep. Mike Rogers, who's considering a run for Michigan's open seat.
Democratic incumbents are at risk in four states carried by Mr. Romney. In Arkansas, which Mr. Romney carried by 23.7%, Mark Pryor would be hard-pressed if freshman Rep. Tom Cotton, an Iraq and Afghan war vet, runs.
In Alaska, which Mr. Romney won by 14%, Sen. Mark Begich won a narrow, 4,000-vote victory last time. A GOP candidate such as Gov. Sean Parnell, Lt. Gov. Mead Treadwell or Natural Resources Commissioner Dan Sullivan could erase that margin.
Louisiana Sen. Mary Landrieu hails from a storied political family, but her state's voters chose Mr. Romney by 17.2 points. Republicans are coalescing behind Rep. Bill Cassidy, who is off to an impressive start. The picture is less clear in North Carolina. Several Republicans, including the speaker of the state house and the state senate majority leader, could run against freshman Sen. Kay Hagan. North Carolinians elected a GOP governor and big GOP legislative majorities last year.
Elsewhere, competitive races could yet develop such as in New Hampshire, where former Massachusetts Sen. Scott Brown may challenge freshman Democrat Jeanne Shaheen. He's a ninth generation Granite Stater; she was born in Missouri.
Republican success will depend on having quality Senate candidates. Todd Akin and Richard Mourdock self-destructed last fall, and other candidates squandered important opportunities.
Fundraising is important. Last year, Democratic Senate candidates outraised Republicans by $60 million (not including the Connecticut and Pennsylvania races with GOP self-funders). The Democratic Senatorial Campaign Committee outraised its GOP counterpart by an additional $20 million. Republicans won't make big pickups if there's a disparity like this in 2014.
The quality of GOP campaigns will matter as well. Republicans must go toe-to-toe with Democrats on ObamaCare, spending, deficits, the president's social agenda and, where appropriate, their opponent's character. But even done effectively, this won't be enough.
The GOP has an outside chance of a Senate majority. To win, Republicans must also offer a compelling and substantive agenda for America's economy, jobs, health care and fiscal situation that attract discerning independents (and the occasional disgruntled Democrats) on whom victory will depend. 2014 represents a great opportunity for Republicans: they'd better not let it get away.
A version of this article appeared May 2, 2013, in the U.S. edition of The Wall Street Journal, with the headline: The GOP Sets Its Sights on the Senate in 2014 and online at WSJ.com.