Americans for Prosperity "wants to hold accountable" Democrats who support Obamacare. But how? The group's president, Tim Phillips explains. (Jeff Simon/The Washington Post)

At the headquarters where Republicans are plotting their takeover of the Senate, camouflage netting hangs from the ceiling and walls. Military surplus sandbags are piled up around operatives’ desks. And an ex-Marine named Ward Baker rattles off statistics that add up to trouble for Democrats.

“Our mentality is that we are at war every day,” said Baker, who as political director of the National Republican Senatorial Committee is helping command the 2014 midterm campaigns. “We’re here for one reason: to win the majority. Anything else is a failure.”

Republicans are increasingly confident that they will gain control of the Senate for the first time in eight years, buoyed by President Obama’s unpopularity and the historic midterm challenges confronting his party in the sixth year of his presidency.

To win the majority, Republicans need to hold onto their seats and gain six more — leaving little room for error. But the most competitive Senate races are happening in predominantly conservative states such as Arkansas and Louisiana, where many voters are hostile to Obama and his signature health-care law.

Both parties consider at least 11 races competitive, with several others possibly coming into play, but their assessments of the state-by-state details vary.

Inside the 36 Senate contests of 2014

At stake for Democrats is their power center on Capitol Hill. A GOP takeover could effectively paralyze Obama’s agenda during his final two years in office and jeopardize his executive and judicial nominees. The 2014 contests also could be the Democratic Party’s last stand in the South, where a dramatic shift since the 1990s has left Republicans with most statewide offices.

Across the Democratic firmament, a sense of urgency is taking hold. At the White House, top officials are stepping up their efforts to coordinate strategy with Senate leaders. Democrats are particularly alarmed that Americans for Prosperity, a super PAC funded by the industrialist Koch brothers, has been pummeling vulnerable Democrats with $30 million in attack ads, most of them regarding the health-care law.

“You’re seeing wealthy special interests try and come buy these Senate races,” said Jim Messina, Obama’s 2012 campaign manager. “But in the end, as we learned in 2012, what’s more important are the candidates and their focus on the issues.”

But for the GOP, there’s also a cautionary tale: In 2010 and 2012, gaffe-prone candidates lost winnable Senate races by alienating mainstream voters. Again this year, hard-fought primaries could yield weak nominees in Alaska, Georgia, Iowa or North Carolina. Sen. John McCain (R-Ariz.) said that he is “guardedly optimistic” but that Republicans must “nominate candidates that can win.”

Democratic strategists believe they can overcome a bad political environment on the strength of their incumbents, some of whom have nurtured deep local connections and independent bona fides. Two centrist women are mounting credible challenges for Republican-held seats in Georgia and Kentucky.

Democrats believe the key to preserving their majority is a $60 million program to expand the off-year electorate to include more blacks and women who traditionally vote only in presidential elections. If the 2014 electorate mirrors the 2010 midterm electorate, on the other hand, Democratic officials concede they could lose the Senate.

Both parties fundamentally disagree about the issues that will motivate voters. Republicans see the disastrous rollout of the health-care legislation, which most Democratic incumbents voted for in 2010, as a way to link them to other Obama administration pratfalls.

“Obamacare is one vehicle to drive home a much larger, more tangible credibility problem,” said Brad Dayspring, the NRSC’s communications director. “This electorate is primed to send a message to President Obama and Washington. It will be a referendum on their policies.”

But Democrats believe Republicans are overplaying their hand. They say there is a diminishing return on the money spent attacking the health-care law, pointing to the successful 2012 campaigns of Sens. Jon Tester (D-Mont.) and Heidi Heitkamp (D-N.D.) despite ads likening them to Obama in states where the president was unpopular.

“I don’t believe they can win the majority as a one-trick pony on Obamacare,” said Matt Canter, deputy executive director of the Democratic Senatorial Campaign Committee. “How do you persuade voters over the next eight months running that same ad over and over again?”

Democrats are trying to shift attention to pocketbook concerns of everyday Americans, touting proposals such as expanding the federal minimum wage. They also want to highlight issues such as equal pay to motivate more women, especially those who are unmarried, to vote.

“Economic opportunity and fairness, if we can keep the discussion there, then that will be helpful,” senior White House adviser Dan Pfeiffer said. “If what happened to us in 2013 happens to us in 2014 — where it’s NSA, shutdown debacle, health-care Web site, Syria — that’s a bad issue environment.”

Top White House officials are working with Senate leaders to align the legislative calendar with the administration’s activities to help endangered senators. Katie Beirne Fallon, who has a deep connection to the Senate, has taken over the White House legislative affairs operation and convenes biweekly strategy sessions that include Pfeiffer; David Simas, who runs the White House’s political operation; Guy Cecil, the DSCC’s executive director; and David Krone, chief of staff to Senate Majority Leader Harry M. Reid (D-Nev.).

Money could be more decisive in 2014 than in past years because most of the toughest Senate campaigns are in small states with relatively cheap media markets, making an avalanche of attack ads all the more potent.

Consider Arkansas, where Americans for Prosperity has been attacking Sen. Mark Pryor (D) over the past month. Democratic super PACs have not kept pace, and some major party donors are distracted by preparations for the 2016 presidential campaign.

For the first time, Obama will openly court super PAC donors, attending one fundraiser each for the Senate Majority PAC and the House Majority PAC. Obama also has pledged to headline six fundraisers each for the DSCC and the Democratic Congressional Campaign Committee.

Obama is unlikely to do much campaigning in the most competitive conservative states, where some senators say he would not be welcome.

“We’ll play a role where we can and we’ll also give these candidates space,” Pfeiffer said. “They’ve won election by being strong advocates for their states, and that will always mean differing from the administration on something. We respect that.”

Both parties agree that Republicans have the advantage in Montana, South Dakota and West Virginia — three states where Democrats are retiring.

Control of the Senate could be determined in four states Obama lost in 2012 and where Democrats are up for reelection: Alaska, Arkansas, Louisiana and North Carolina. Of the four, North Carolina is considered the friendliest terrain for Democrats, and party strategists said Sen. Kay Hagan has built one of the best campaign organizations in the nation.

In Arkansas, Pryor is attempting to hold onto his seat in a state that Obama lost by 24 percentage points in 2012. Pryor’s challenger, Rep. Tom Cotton, is considered the biggest star in a strong class of GOP Senate candidates.

But Democratic strategists say Pryor has withstood attacks and is revered in the state. They said they are more concerned about Sen. Mary Landrieu in Louisiana.

“I’m not trying to be Pollyannaish, but the idea floating out there that, ‘Gee, these incumbents are in huge difficulty and the election was last week’ — people are losing perspective,” said Jim Margolis, a longtime media consultant and close adviser to Reid.

If Democrats hold their Senate majority, it could be on the strength of women in the South, including Georgia, where the retirement of Sen. Saxby Chambliss (R) leaves an open seat, and in Kentucky, where Senate Minority Leader Mitch McConnell (R) faces a primary challenge and has low favorability ratings.

In Georgia, Republicans could jeopardize an otherwise safe seat by nominating a far-right candidate. Democrats are bullish about their likely nominee, Michelle Nunn, daughter of former Georgia senator Sam Nunn.

In Kentucky, the two parties couldn’t disagree more about McConnell’s challenger, Democrat Alison Lundergan Grimes. To Republicans, she’s a novice who ducks tough questions, while McConnell is a seasoned and wily campaigner. But to Democrats, she’s the ideal antidote to a 30-year incumbent during a time of deep public loathing of Congress.

Democrats are fearful of other states coming into play, including Iowa and Michigan, where longtime incumbents Tom Harkin and Carl Levin, respectively, are retiring. If Democrats have to devote resources to defending any of these seats, that will take away from the key battlegrounds.

Republicans, meanwhile, would like to expand the map even further by making Democratic seats in Colorado, New Hampshire, Oregon and Virginia competitive. But, at least for now, Democrats are favored in those states.