But Republican operatives are working feverishly to ensure that the bitter split between the party establishment and insurgent tea party conservatives does not lead to a squandered opportunity and GOP disappointment for the third set of elections in a row.
Democrats must defend 21 seats in 2014 , including six in states that President Barack Obama lost in 2012 by double-digit margins, while Republicans are defending 14. Add to that the retirement of a series of veteran Democrats - John Rockefeller IV of West Virginia, Carl Levin of Michigan and Tom Harkin of Iowa - whose appeal at the ballot box will be hard for their party to replace.
Republicans need to pick up six seats to retake the majority they lost in 2006, and recent history suggests that the task is doable, if difficult. More concerning for Republicans, however, is whether they will again have to endure nasty primaries that produce either triumphant insurgents with limited appeal or establishment survivors who underperform with conservative voters in the general election.
There are already declared and potential candidates that the GOP establishment sees as less than desirable, including Reps. Paul Broun and Phil Gingrey of Georgia, Rep. Steve King of Iowa, Rep. Justin Amash of Michigan and former congressman Jeffrey Landry in Louisiana.
Senate Republicans have conducted an internal review of their recent failures to better prepare themselves for the pitfalls ahead, both in defending incumbents and in playing offense in winnable races. The review is similar to the "autopsy" that the Republican National Committee performed after Mitt Romney's disappointing loss to Obama last year.
The National Republican Senatorial Committee (NRSC) surveyed candidates, staff and consultants involved in 20 Senate races over the past three elections. The single biggest problem identified was poor communication support in dealing with the fast pace of modern campaigns, GOP strategists said.
For example, when candidates made gaffes in debates or local interviews, it would sometimes take days to repair the damage. The NRSC dispatched senior media advisers from Washington to stumbling campaigns in places such as North Dakota and Wisconsin in 2012 and Nevada and Kentucky in 2010. They lost three of those races, and the NRSC postmortem determined that by the time the top advisers got to the states, it was too late to fix the problems. In 2014, they hope to avoid falling behind and to use the disparity in the number of seats up for grabs to their advantage.
But for Democrats, playing defense will be nothing new.
In November, they had to defend 23 of the 33 Senate seats up for grabs in 2012. They could afford to lose only four seats to maintain a majority, and several of their veterans retired after long runs representing conservative states.
But instead of losing ground, Democrats gained seats -- which seemed unthinkable in March 2011 -- by holding all 23 and picking off two from Republicans.
Moreover, in the past four elections, just two Democrats lost re-election bids, while 12 GOP incumbents fell.
The NRSC is planning at least eight boot camps for staff in states with competitive races. Kevin McLaughlin, a veteran operative hired to work with campaign staff around the country, will run the communications schools with an eye toward preventing mishaps. He will also be in charge of a kind of campaign fire department, charged with quickly putting out blazes once mistakes are made.
NRSC officials say they are also taking a new approach to contested primaries.
In 2010, the committee endorsed preferred candidates, only to see five of them defeated. In 2012, the NRSC chose to make no endorsements and provided only behind-the-scenes guidance to its preferred candidates.
This time, the committee intends to stay neutral unless a particularly undesirable nominee begins to emerge. In addition, if Democratic groups wade into GOP primaries to help a candidate they deem weaker and easier to beat, the NRSC promises to fight back.
This neutrality approached will be vigorously tested in several places. Already, former South Dakota governor Mike Rounds and Rep. Shelley Moore Capito of West Virginia - early establishment front-runners to claim seats being vacated by retiring Democrats - are being targeted by GOP outside groups that say they are not conservative enough despite having no declared opponents. The Club for Growth has made it known that it doesn't like Capito, and the Senate Conservatives Fund announced Tuesday that it will seek an alternative to Rounds.
At the same time, conservatives recoiled at the news that legendary GOP strategist Karl Rove was starting a group to challenge their influence and to nominate more "electable" candidates.
In 2012, Republicans needed just three seats and a Romney victory to take control of the Senate but came up short. Instead, Democrats got a chance to boost their margin going into 2014 and are eyeing 2016, when they think the election map will finally tilt in their favor; Republicans will be defending 24 seats to the Democrats' 20.
For Mitch McConnell, the Senate Republican leader, 2014 may be his chance to realize a 50-year-old dream of becoming majority leader, and one of the key races that will decide that question is his re-election contest in Kentucky.
Should McConnell, 71, cruise to a sixth term without a real primary challenge from the right, it could signal that Republicans have broken the spell of their damaging internal feuds and are positioned for the majority.
Democrats, however, are betting that McConnell is vulnerable to the kind of conservative primary challenge that has hobbled the GOP in recent elections.
This week, the much anticipated Democratic candidacy of actress Ashley Judd fizzled as she abandoned the race before it started. With Judd out, Democrats are turning to Kentucky Secretary of State Alison Lundergan Grimes, 34, whose father was state Democratic chairman and a friend of former president Bill Clinton.
Still the most important Kentucky politician of his generation, McConnell is already running hard against whoever eventually emerges to challenge him.
In 2011 and 2012, when he wasn't even up for re-election, McConnell raised more than $7.5 million - as much money as Kentucky Republican colleague Rand Paul spent in his entire 2010 Senate race. That Paul win was a tea party triumph that signaled its ascendancy against the establishment. McConnell, for his 2014 race, has hired Paul's campaign manager, Jesse Benton, to help navigate the tea party waters during his re-election effort. As a whole, the party has decided it needs to take similar action beyond Kentucky if it is to press its numbers advantage over Democrats.
"We'd rather be us than them, but ... it's early, and it's incumbent upon us to capitalize on the opportunity," said Brad Dayspring, a spokesman for the NRSC.
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