Washington - While US Democrats face many hurdles as they prepare for this year's battle for control of Congress, few are more daunting than the Koch brothers - conservative business moguls with deep pockets.
The billionaire industrialists are behind extensive, often opaque efforts to get conservative Republicans elected in November.
And President Barack Obama's Democrats are up against the wall in their bid to maintain control of the Senate, without which he will struggle for even the smallest of legislative victories.
Eight months before polling, Democrats are already targeting the media-shy siblings at the helm of Koch Industries, considered the second largest privately-held company in the United States.
They depict Charles and David Koch as mega-rich schemers fuelling fears about Obama's health care reforms and flooding several races with tens of millions of dollars this year.
With Republicans needing just six net victories to win back the Senate, Democrats paint the Kochs as anti-government boogeymen.
They are committed to defeating vulnerable Democrat incumbents such as Senator Kay Hagan of North Carolina.
'Buy our democracy'
Hagan launched a counter-offensive on Monday against Republican opponent Thom Tillis, who has raised $8m already funnelled into his Koch-backed challenge, according to her campaign.
"It's clear they are trying to buy a senator whose strings they can pull in Washington to get their special interest policies passed," Hagan's communications director Sadie Weiner alleged.
Leading the counter-charge is Senate Majority Leader Harry Reid, who took to the floor several times this month to attack the "un-American" Kochs for trying to "buy our democracy".
He accused the brothers of pumping $30m into advertising in competitive Senate races from North Carolina to Alaska as they push for lower taxes on the rich and smaller government.
"Senate Republicans," Reid seethed, "are addicted to Koch".
Several attack ads have been released by Americans For Prosperity, an advocacy group funded by the Kochs and severely critical of Obama's health insurance reforms.
It argues that Democrats, by targeting the Kochs, seek to distract voters from five years of failed White House policies including so-called "Obamacare".
Democrats "are going to drive off a cliff" if they talk exclusively about the Kochs, GOP strategist Ryan Williams argued.
"Complaining about two brothers, who most people have never heard of before, spending money on TV ads - it's just not a winning strategy," he added. "They look small."
But several vulnerable Democrats are not shying away, vilifying the Kochs as outsiders big footing Senate and House races.
"Koch Brothers go home," exhorted one recent ad for incumbent Senate Democrat Mark Begich of Alaska, where Republican fundraising groups have spent heavily on anti-Begich ads.
The "Senate Majority PAC," which aims to keep the chamber in Democratic control, bought $3m is ads last week to counter rival Koch-funded spots in Colorado and four other states.
"I think the American people don't like to hear that two multi-billionaire brothers are having such a huge effect on the election, and I think if it's exposed... it will be an antidote to what they're saying," Senator Chuck Schumer told AFP.
Koch Industries spokesperson Philip Ellender dismissed the attacks as an attempt "to silence private citizens who dare to disagree with the policies of the majority leader and the current administration."
Piles of 'dark money'
With much "super PAC" funding - cash paid to independent political committees not directly tied to official campigns - hard to trace, it is unclear how much the Kochs have donated.
According to the Washington Post, the Koch network raised $407m in 2012, money that was spread through a vast network of groups supporting Republican candidates and causes.
But Democratic strategist Brad Woodhouse of Americans United for Change conceded that bearing down on the brothers is not, on its own, a winning strategy.
"I don't think anyone thinks this election is going to be won targeting the dark money that's being spent on the right," Woodhouse said.
"There's got to be an offense," he argued, urging a full-throated embrace of Obamacare.
That may be difficult for the several Democrats defending seats in Republican-leaning states.
"Obamacare, in almost every key battleground state, is about as popular as a skunk at a garden party," said Williams.