WASHINGTON -- Democrats appear to be winning the fundraising race at nearly every turn in Senate contests and doing well enough at the House level this year. Their congressionalparty committees and superPACs are on record pace ahead of their Republican counterparts, while the majority of Democratic incumbents in key races boast a financial advantage over their opponents. And yet, it's still likely Democrats will be outspent by wide margins.

The reason can be summed up in three words: the Koch brothers.

The two billionaire industrialists, Charles and David Koch, and their dual networks of undisclosed donors and "dark money" nonprofits, have emerged as the largest independent political force on the Republican side. Already, groups linked to the Kochs have spent more than $25 million on television and radio in crucial 2014 House and Senate races, according to Democratic media-buying sources. Other reports, which also count spending on digital advertising, show a total that is closer to $30 million.

"It is clear we will be outspent in Senate races across the board just like we were outspent in Senate races last cycle," said Justin Barasky, spokesman for the Democratic Senatorial Campaign Committee (DSCC).

Data from Democratic media-buying sources.

In all but one key Senate race so far this year, Koch-linked organizations like Americans for Prosperity, Concerned Veterans of America and the American Energy Alliance have spent more on issue-based ads attacking Democratic candidates than all independent Democratic groups combined have spent on similar ads criticizing Republican candidates.

The outside spending in House races is also dominated by Koch-linked groups. These groups have dropped $5.7 million in 14 districts, three times as much as all Democratic outside groups have spent on non-special election races this year.

Data from Democratic media-buying sources and Center for Responsive Politics.

"There's no such thing as parity on our side," said Matt Thornton, spokesman for House Majority PAC, the main super PAC supporting Democratic House candidates.

Americans for Prosperity spokesman Levi Russel said that the group's spending is "part of a multi-year effort that AFP has been engaged in to put a stop to and repeal Obamacare."

As to why advertising is only targeting contested House and Senate races Russell said, "The hot political races are where the issues of Obamacare and health care reform are being hotly contested. We want to be a part of that conversation."

The uneven spending has already raised alarms in some quarters that big Democratic donors are sitting out the midterm elections to save their money for the 2016 presidential race. DSCC Executive Director Guy Cecil tweeted in February, "To those who are #ReadyforHillary, we've got another job to do before 2016."

Former White House adviser David Axelrod voiced similar concerns that month, tweeting, "With the Senate seriously at risk, and the Koch Brothers spending prodigiously, shouldn't Dem funders be focused on '14 and not '16 races?"

In response, Senate Majority Leader Harry Reid (D-Nev.) and his Democratic colleagues are trying to turn the Koch-linked money against their Republican opponents. The Democrats' strategy seeks to tie GOP Senate candidates to the agenda of Koch groups by highlighting how specific policies advocated by Republicans will aid billionaires and hurt everybody else.

Barasky points to Arkansas Senate candidate Tom Cotton's vote against the farm bill as an example. Rep. Cotton, a Republican, was the only member of the Arkansas congressional delegation to vote against that bill, which Americans for Prosperity openly opposed. The Koch group has spent nearly $2 million on the Arkansas Senate race.

Sen. Kay Hagan (D-N.C.) has been the top target of Koch group spending overall. Both the DSCC and Hagan's campaign hope to tie state House Speaker Thom Tillis, the Republican Party-endorsed candidate for that Senate seat, to the billionaires in voters' minds. Tillis helped pass a state legislative agenda closely following the one put forward by Americans for Prosperity and other Koch-linked groups.

Hagan press secretary Sadie Weiner said, "The Koch brothers are really trying to buy this race, and they're trying to buy this race because they know that Thom Tillis will push their agenda because he did it in Raleigh last year."

Democratic Party organizations and their unaffiliated super PACs are not conceding the fight, of course. The DSCC has planned a targeted ground game to try to bring out their voters who normally sit out midterm elections.

The congressional super PACs, meanwhile, hope to use their less-limited resources in a coordinated manner. "We can provide the most insulation to those Koch-brothers dollars and spend smartly, which we've been pretty good at doing so far," said Thornton, the House Majority PAC spokesman.

Still, a large pool of donors who gave to the pro-Obama super PAC Priorities USA Action in the last election cycle have either not given to Democratic groups so far this time or are not yet matching their previous giving.

Of the 34 Priorities USA donors who gave at least $1 million in 2012, 19 had not made a contribution to a super PAC working on congressional races through the end of 2013. This includes the two top Democratic super PAC donors in 2012: hedge fund executive James Simons and media mogul Fred Eychaner, neither of whom had contributed to any super PAC working on congressional elections by the end of 2013.

As noted, the leading Democratic super PACs have nonetheless been raising record amounts. House Majority PAC pulled in $8.8 million through Feb. 19, 2014, and Senate Majority PAC raised $8.6 million by the end of 2013.

Senate Majority PAC spokesman Ty Matsdorf said the group's supporters "are becoming even more engaged in our fight."

These financial successes have allowed Democratic groups to respond to the barrage of ads from Americans for Prosperity and other Koch-linked organizations, even if they can't match the latter's level of spending. Senate Majority PAC and its affiliated nonprofit Patriot Majority have expended more than $6 million on seven Senate races so far.

"We understand we're going to be outspent, but that means we have to be twice as effective, twice as efficient," Matsdorf said. "And we've been incredibly active in a litany of states, and we're going to continue to fight back against this unprecedented flood of money."