In the age of super PACs, the Senate’s many vulnerable Democrats are under immense pressure to post large fundraising numbers early in the cycle.

Senate Democratic campaign officials stress that their incumbents must get out of the gate in strong shape to start buying airtime ahead of cash-flush third-party advertisers.

So far, the party’s most vulnerable senators are meeting the challenge. One by one last week, Senate Democrats rolled out beefed-up first-quarter fundraising reports ahead of Monday’s filing deadline.

Nearly every targeted senator announced a three-month haul of well more than $1 million: Sen. Mark Pryor of Arkansas raised more than $1.9 million. Sen. Mary L. Landrieu of Louisiana raised $1.2 million. Sen. Kay Hagan of North Carolina raised $1.6 million. Sen. Mark Begich of Alaska raised $948,000.

“Early fundraising is always important for an incumbent to show strength and dissuade potential opponents, but it’s especially imperative in this era of massive [independent expenditure] spending,” Democratic media consultant Mark Putnam said.

The motivation behind the increased pressure to raise money early is threefold: Candidates can secure the lowest unit rate for TV ads; with their own money, candidates control their own message; and outside groups generally begin ad campaigns months before the traditional fall messaging onslaught.

In the 60-day window before the elections, television stations offer candidates far cheaper airtime rates than super PACs, which began flooding the airwaves in the 2010 cycle. But before that window, incumbents need to have the resources to defend against outside groups, which have historically launched attack ads well before the challengers they were supporting did.

With competitive challenges forthcoming, some of the most vulnerable Democrats are likely to begin putting the money to use this year. One source close to a Democratic senator up in 2014 said it was highly likely the incumbent’s campaign would get on the air in 2013.

Apart from Senate Minority Leader Mitch McConnell in Kentucky and an open seat in Georgia, Republicans are largely the challengers in the top Senate races this cycle. Still, several GOP incumbents not vulnerable in the general election may see a competitive primary challenge and spending against them from issue-advocacy groups.

As helpful as outside spending can be, challengers aided by outside groups will likely still want as much control of the message and ad quality as possible — not to mention the ability to respond to Democrat-aligned outside groups.

“As much as we all want our friends to help, you always worry about the ‘execution’ of outside groups’ advertising,” GOP media strategist Erik Potholm said.

Highlighting the importance of candidate money, nowhere was the disparity between money spent and advertising spots purchased more evident than in the 2012 presidential election.

2012 presidential nominee Mitt Romney’s campaign and aligned GOP outside groups combined spent about 55 percent of all the money expended on broadcast TV advertising, according to TVB, the trade association of the commercial broadcast TV industry. However, President Barack Obama and aligned Democratic groups purchased about 10 percent more broadcast spots with less money.

The difference was that the Obama campaign purchased nearly 90 percent of its broadcast media, while the Romney campaign accounted for less than 40 percent of GOP broadcast TV buying, according to TVB. The disparity was also due in part to the Obama campaign’s spending strategies, including buying advertising time well ahead of when the spots ran.

One Democratic media buyer pegged the total amount of broadcast and cable TV spending on the presidential campaign at nearly $1.1 billion.

In Senate races last cycle, outside groups spent more than $300 million on a dozen of the priciest contests.

According to the Center for Responsive Politics, campaign committees and political action committees from both parties combined to spend $52 million on the top-tier Senate race in Virginia, $46 million in Wisconsin, $39 million in Ohio, $33 million in Indiana and $28 million in Nevada. There was at least $10 million spent by outside groups in seven other states.

“Bottom line, you always want to raise as much as you can to be able to control your own destiny and drive your own messaging,” Potholm said, “especially in the midst of the massive proliferation of outside advertising.”