With their grip on the Senate in danger and with few winning issues to run on, Democrats are trying to turn the midterms into a referendum on the Koch brothers.

Charles Koch, 78, and David Koch, 73, who run the second-largest privately held company in the country, are pouring a chunk of their vast fortune into helping Republicans take back the Senate.

The billionaire MIT graduates have been hugely successful in building Koch Industries, a company founded by and named after their father, into a multinational conglomerate with $115 billion in annual revenues and 100,000 employees, 60,000 of whom work in the United States. The brothers have long bankrolled libertarian and conservative candidates and causes. Over the last decade, one of their spokesmen said, the Kochs also have given $1 billion to educational institutions, hospitals, cultural groups and other philanthropic causes.

With Democratic incumbents facing tough odds amid public discontent with Obamacare, the weak economic recovery and doubts about President Obama's leadership overseas, the party hopes to make the Kochs' support for Republican candidates the key issue for voters.

The Left has long railed against the Kochs, accusing them of using their money to roll back progressive policies. Democrats now have gone a step further, pouring millions of dollars into an effort to demonize the brothers ahead of the November voting. They are taking their case to the public with an array of personal attacks that portray the Kochs as plutocrats using their vast wealth to serve their own interests, not the public interest.

The Senate Majority PAC, Senate Democrats’ top outside group, has embarked on a $3 million ad campaign in 5 states — a tally almost equal to the $3.6 million the group has spent on all other races so far this cycle. The blitz is the most concrete attack on the Kochs yet.

Arkansas, Colorado and Louisiana will receive $500,000 each, and $1 million is going to North Carolina -- all states where Democratic incumbents are in danger of losing. Michigan, with an open Senate race, will also receive $500,000 in ad money.

The Democratic Senatorial Campaign Committee is also launching what it calls a “major campaign” to tar Republican candidates who have benefited from Koch spending, labeling them “addicted to Koch.”

A KochAddiction.com website features colorful graphics accusing the Kochs' companies of producing "toxic chemicals, harmful pollutants, carcinogens and greenhouse gases." Another page says the brothers' "ideal America" would feature "lower wages, no Obamacare, no Social Security, tax breaks for billionaires, dirty air and dirty water."

Another outside Democratic group, Patriot Majority, is conducting a “Stop the Greed Agenda” bus tour in key districts. Participants deliver speeches about “the Koch Brothers’ voter suppression agenda, which ensures their friends and government officials are elected into office.”

For their part, Republicans say the Democrats are focusing on the Kochs because they're playing a losing hand. "We love it when they attack the Kochs because it means they don't have anything else to talk about," said a leading GOP strategist and fundraiser. "They can't run on Obamacare. They can't run on the economy."

Political parties have always tried to shine the spotlight on big-money donors backing the other side, with wealthy partisans often providing easy targets. Republicans regularly hammer both the financial impact of labor unions and wealthy Democratic supporters like George Soros, who has donated millions to the party and its candidates.

The Kochs are the primary backers of Americans for Prosperity, a conservative, tax-exempt, political-advocacy group that has spent $30 million since August 2013 on advertising to unseat Democrats, mostly by slamming Obamacare.

“Those ads have been moving numbers,” Jennifer Duffy, senior editor at the Cook Political Report, which rates political races, told the Washington Examiner.

In North Carolina, Democratic Sen. Kay Hagan has plummeted in the polls since the spots began airing. Hagan, who held a 10-point edge, has sunk to a virtual tie against GOP challengers.

The AFP blitz has also pummeled vulnerable Democratic incumbents in Arkansas, Alaska, New Hampshire and Louisiana. And the group is expanding its efforts to Colorado, where Democratic Sen. Mark Udall is slipping in the polls, and to Michigan and Iowa -- states where Republicans could pick up open Senate seats held for decades by Democrats.

With the GOP needing to gain six seats to take the Senate, the success of the AFP offensive has left Democrats doubling down on their anti-Koch strategy.

In Alaska, vulnerable Democratic Sen. Mark Begich has a new ad that tries to portray the Koch brothers as outsiders who shut down an oil refinery in the state and polluted the water supply.

Koch company officials call the ad “misleading and false” and said they are cleaning up pollution from a previous owner and are helping refinery workers find new jobs.

Senate Majority Leader Harry Reid has played a conspicuous role in the Democrats' onslaught. A former boxer, Reid is known for taking swings at his political opponents. With his Democratic majority in danger, he may have the most at stake, and he has taken the rhetorical attacks against the Kochs to new levels.

“The Kochs should stick to what they know,” Reid said in a speech. “The oil business.”

Reid has taken aim at the brothers’ own reputations, describing them as “un-American” and “shadowy billionaires” out to rig the nation’s political system for their own personal benefit.

“This is about two very wealthy individuals who intend to buy their very own Congress,” said Reid. “A Congress beholden to their money and bound to enact their radical philosophy.”

That theme has also been woven into the Democrats’ fundraising strategy, which depends on smaller donations from the party's base. In a fundraising letter, Reid accused the brothers of “buying our elections. Without your support they’ll succeed in kicking Democrats out of the Senate majority this fall.”

But it is not clear if Democrats can succeed in making the Kochs an unattractive household name tethered to the GOP brand, even as Republican candidates stay focused on Obamacare and the economy.

A spokesman for Koch Industries disputed charges by Democrats that the libertarian-minded brothers are motivated by greed, saying they only had a desire to steer the nation’s beaten-down economy back to growth and prosperity.

“They care deeply about America and they are doing this because they feel the country is going in the wrong direction,” Koch Industries spokesman Robert Tappan told the Examiner.

The brothers want to promote “fundamental policies and philosophies that have gone by the wayside,” he said, such as limited government and freedom from burdensome regulations.

Tappan said the Kochs are not involved in the day-to-day operations of AFP, although Democrats charge that the brothers are obscuring their significant influence on the group.

Because AFP operates as a 501(c)(4) organization, it does not have to disclose its donors. Democrats say their ad hominem tactics against the Kochs are justified because the names of donors to AFP are not made public, even though plenty of “dark money” Democratic advocacy groups do not disclose their donors either.

Democrats argue that there is no comparing the Kochs to Soros or Tom Steyer, a former hedge fund manager who plans to raise $100 million to highlight climate change in the 2014 midterms.

“There’s a night-and-day difference,” Democratic strategist Chris Lehane told the Examiner. “The Kochs are spending money to advance their economic self-interest. Steyer, for example, has committed to giving away his wealth for causes he believes in, that benefit everyone.”

Supporters of the Kochs say many Democratic attacks have been proven false and overstate the brothers' clout. One story that went viral but has since been debunked claimed that Koch Industries stands to make a $100 billion profit from constructing the Keystone XL pipeline, which the brothers support.

According to the Center for Responsive Politics, AFP spent $36 million in the 2012 election cycle trying to unseat President Obama and congressional Democrats -- placing it second among 501(c)(4) groups. Karl Rove's American Crossroads/Crossroads GPS ranked first on the list, with $71 million.

Non-disclosing liberal groups spent far less — about $35 million overall.

Koch Industries, according to CRP, has donated more than $18 million to Republican candidates since 1989, ranking only 59th among political contributors during that period.

At the top of the list is ActBlue, a grassroots fundraising group that has donated $97 million to Democrats since 1989, more than four times as much as the Kochs have given directly to Republican candidates. Two union groups round out the top three, with the American Federation of State, County and Municipal Employees at $61 million and the National Education Association at $59 million.

Americans for Prosperity President Tim Phillips called Democrats’ accusations “laughable,” and said the only real difference between the groups is ideological.

“There is an enormous double standard,” Phillips told the Examiner.

“The largest funder in the American public policy area are the unions. They dwarf anything that we do, or what anyone else does,” he said. “But no one on the Left or in the media questions that.”