Senator Mitch McConnell, Republican from Kentucky. (Andrew Harrer/Bloomberg)

CORRECTION: The original version of this story incorrectly reported the amount of spending against Sen. Kay Hagan. Americans for Prosperity has spent $6.2 million against Hagan on radio, cable and broadcast television.

Voters in Arkansas began hearing from Sen. Mark Pryor in paid advertisements in August, more than a year before the midterm elections. In March, Sen. Mark Begich started making his case to Alaska voters on television. Voters in Kentucky have seen advertising paid for by Senate Minority Leader Mitch McConnell’s campaign for 13 months.

For much of the past year, voters still weary from the record onslaught of advertising during the 2012 presidential election season have been bombarded with new television ads sponsored by senators seeking another term in 2014. Where campaigns once carefully husbanded their advertising cash for late blitzes in the two or three months leading up to Election Day, strategists now say it pays to begin talking to voters earlier than ever.

Since Dec. 30, 11 incumbent senators from both parties have spent a total of more than $6.2 million on television ads, according to public data filed with the stations that broadcast those spots. At the same time, non-incumbents running for Senate seats have spent $7.6 million in early advertising.

The candidates, strategists say, are racing to get their messages on television to have more control over the process. A huge influx of outside spending from conservative and liberal groups is forcing the candidates’ hands sooner. Campaigns realize that allowing an outside group to define a campaign is detrimental to senators and challengers who would rather run on their own terms.

Sen. Mary Landrieu's (D-La.) Senate campaign ad touting her advocacy of the oil and gas industry. (Landrieu for Senate via YouTube)

In some races, outside groups are playing a dominant role. Since the beginning of the year, Americans for Prosperity, a group connected to the conservative billionaires Charles and David Koch, has spent $14.7 million on television advertising. Senate Majority PAC, which runs ads for Democratic candidates, has spent $5.5 million over the same period. A smattering of other groups have combined to spend millions more.

“Early money by outside groups is forcing candidates to rethink their strategies and in some cases go up early,” said Carl Forti, a Republican strategist who advises American Crossroads, the well-funded super PAC.

Also, television viewing habits are changing as viewers increasingly record their favorite shows and watch them later or stream them through any of a number of services. That lets viewers skip annoying advertisements and makes it more difficult for candidates to reach the voters they could once count on sitting in front of a television.

“We are in an era of pay more, get less,” said Peter Fenn, a veteran Democratic operative who has spent decades crafting advertising strategy for his candidates. “You’ve got so few people who are watching shows live and seeing the ads. You’ve got so much content that’s streamed on Apple TV or on iPads.”

Finally, many senators seeking reelection this year hail from relatively small states with inexpensive media markets. The millions of dollars those incumbents have in the bank allows them to purchase significant amounts of television time.

“Why not get out in front and try to define your opponent before they are able to define themselves?” said Lara Brown, program director at George Washington University’s Graduate School of Political Management. “It’s cheap, so it’s easier to do more ads for fewer dollars early, and you’re able to reach more people.”

In Arkansas, Pryor (D) began running his first advertisement the day Rep. Tom Cotton (R) entered the race in August. The minute-long spot ran for less than a week, accusing Cotton, a freshman, of “blind ambition” and spotlighting some of Cotton’s early votes.

This week, Sen. Mary Landrieu (D-La.) began running her first advertisement, a spot highlighting her position as chair of the Senate Energy and Commerce Committee. Landrieu “holds the most powerful position in the Senate for Louisiana,” the narrator says.

Public records show that Landrieu’s campaign will spend $2.7 million on those advertisements, which will run through the end of June.

In the past, a candidate would be worried about saving some money for last-minute blitzes, but Landrieu and other incumbents have stockpiled huge sums of money. Most campaigns follow a rule: Once a campaign begins spending serious money to air advertisements, it continues to run ads until Election Day — in campaign manager-speak, once you go up, you don’t come down. The amount that many incumbents have in the bank means they can begin spending now and continue airing ads through Election Day.

Landrieu reported $7.5 million in her campaign account at the end of March, meaning that if she stopped raising money for the rest of the cycle, she would still have about $5 million with which to run advertisements from July through Election Day, or more than $1 million a month.

Other incumbents are similarly flush with cash. McConnell (R), who faces a conservative challenger in his May 20 primary, reported more than $10 million in the bank at the end of March. Sen. Mark Udall (D-Colo.) had $5.9 million in the bank. Sen. Jeanne Shaheen (D-N.H.) had $4.3 million on hand. Begich (D) reported only $2.8 million in the bank, but advertising is sufficiently inexpensive in Anchorage, Fairbanks and Juneau that he is able to blanket the airwaves for just a few tens of thousands of dollars.

Several Republicans who face possibly competitive primaries have been on the air early. Sen. John Cornyn (R-Tex.), who won his primary by a wide margin in March, has spent $653,000 since the beginning of the year. Sens. Thad Cochran (Miss.), Lindsey O. Graham (S.C.), Pat Roberts (Kan.) and Lamar Alexander (Tenn.) have all spent hundreds of thousands of dollars aimed at fending off conservative challengers.

One candidate stands in contrast to the strategies her fellow incumbents are pursuing. Sen. Kay Hagan (D-N.C.) has been targeted with more advertising by conservative outside groups than any other incumbent; Americans for Prosperity has dumped more than $ 6.2 million into the state. Hagan has a sizable war chest. As of the end of March, her campaign said it had $8.3 million on hand. But she hasn’t run any television advertising yet, despite polling showing her approval ratings in dangerously low territory.

A spokeswoman for Hagan’s campaign declined to discuss strategy. “We will be up when the time is right,” the spokeswoman, Sadie Weiner, said.

Some non-incumbents are spending now because of looming primary contests that they must win to have a chance of becoming senators. North Carolina House Speaker Thom Tillis (R) has bought $1.4 million in ads before next month’s primary, in which he faces two candidates running to his right. In Georgia, Rep. Jack Kingston (R), one of the leading candidates in the race to replace retiring Sen. Saxby Chambliss (R), has spent $1.4 million; his top rivals, Rep. Phil Gingrey (R) and businessman David Perdue (R), have spent more than $700,000 each.

One operative running a Democratic Senate race, who spoke on the condition of anonymity to discuss strategy, said his candidate would spend money early to rebut attacks from Americans for Prosperity and keep the race close. Spending early won’t win a race, the manager said, but it could prevent the Republican opponent from building an insurmountable lead.

“You need to get to October,” the manager said. “Election Day is no longer in November in this super-PAC day and age. You need to be competitive by Labor Day.”

Both parties spin the early spending as signs of strength for their candidates and signs of weakness for their rivals.

“What we’re seeing is that a lot of Democrats are going on television early because their numbers are so poor that they can’t afford not to,” said Brad Dayspring, spokesman for the National Republican Senatorial Committee.

“There is not a one-size-fits-all approach that works to winning the races. The results speak for themselves,” countered Matt Canter, a spokesman for the Democratic Senatorial Campaign Committee. “Despite being outspent in each race, our candidates lead today.”