Sen. Mary Landrieu (D-La.) has been near the top of the list of the most vulnerable senators facing reelection since day one of the 2014 election cycle. But notable developments in recent weeks suggest her seat is increasingly at risk of falling into Republican hands.

 Senator Mary Landrieu (D-LA) and Democratic leadership during a press conference to address "the Senate's third day of legislative business without passing a single measure to provide immediate relief to the victims and survivors of Hurricane Katrina" Thursday, September 08, 2005 on Capitol Hill. (Melina Mara/The Washington Post)
Sen. Mary Landrieu (D-La.). (Melina Mara/The Washington Post)

Republican groups have blitzed Landrieu over the airwaves while she has gotten limited cover from Democratic allies; polls show she is locked in a close race against Rep. Bill Cassidy (R); and now, Landrieu is reserving $2.6 million in airtime between April and June, an apparent recognition that the cascade of opposition ads has forced her to consider spending big months before the November election.

Landrieu's air reservation, first reported by Politico on Friday, comes as Americans For Prosperity has been flooding the airwaves with negative ads hitting the Democrat over Obamacare. The advocacy group backed by billionaire industrialists Charles and David Koch has spent more than $3 million in the race, including a new $700,000 ad buy the group just launched.

Landrieu's campaign said her reservation is not set in stone and insisted that it simply reflects early planning to check ad rates and get the best possible ones. But it also appears to be a tacit acknowledgement that the full-court press by GOP groups led by AFP have forced the senator to respond.

Senate Majority PAC, a super PAC focused on helping Democrats win Senate races, has been no match for AFP, spending just under $850,000 in the campaign. The group hit Cassidy and expects to spend at least another $400,000 over the next month. But Republican groups will still probably have spent more a month from now.

AFP has emerged as the dominant GOP group taking on Democratic senators early in the 2014 campaign, raising alarm bells in other vulnerable states like North Carolina and Arkansas. In Louisiana, Republicans like their chances with less than eight months until the all-party primary election. A recent Democratic poll showed Landrieu in a neck-and-neck race against Cassidy (R) and confirmed that President Obama is unpopular in Louisiana. Other surveys have also shown a close race.

Democrats hope Landrieu's ascension to chair of the Senate Energy Committee will pay dividends in the next phase of the race, given how important energy issues are in Louisiana and what that kind of power will mean to many voters. She favors  constructing the Keystone XL pipeline, protecting oil drilling incentives and curbing the scope of federal regulations on emissions.

The question is whether Landrieu will able to turn the race into a hyper-local campaign -- in the mold of Sen. Heidi Heitkamp (D-N.D.) and Jon Tester (D-Mont.) circa 2012 -- that pivots on Louisiana issues or whether the unpopularity of the Obama administration and the health-care law will weigh her down. Landrieu's very first TV ad touted her legislative effort to allow Americans losing their health-care coverage under Obamacare to stay on their plans, a clear sign she recognized how potent the issue promises to be in the campaign.

The Senate landscape has been looking up for Republicans lately. Capable candidates in lower tier pickup opportunities like New Hampshire and Colorado have stepped up to run there, giving the GOP more possible paths to winning the six seats they need to reclaim the majority this fall. Louisiana is one of four states in which elected Democratic incumbents are running for reelection and Mitt Romney won in 2012. North Carolina, Alaska and Arkansas are the other three. If Republicans can win three of those four in addition to their three best pickup opportunities overall (Montana, West Virginia and South Dakota) without surrendering any of their own seats, the majority will be theirs.

That's why what happens in Louisiana is so vital.

Landrieu has money to spend. She had stockpiled some $6.4 million at the end of 2013. So if she wants to go big with a Spring/Summer ad buy, she can afford to do so more than most incumbents. But as the Florida special election showed, candidate fundraising is only part of the battle. There, Democrat Alex Sink raised big bucks and zoomed past her Republican opponent in the money chase. But Republican groups rallied to the side of now-Rep. David Jolly (R), bridging the overall gap in Democratic versus Republican spending and delivering a win.

National Democrats and Democratic groups engaged in the battle for the Senate majority should be worried about the tilt of the outside spending battle in Landrieu's race, especially against the backdrop of an unpopular president and health-care law.

Judging by her strategy, Landrieu is apparently concerned.