WASHINGTON – Democrats and the Republican-friendly Koch brothers are locked in a vicious, multimillion-dollar slugfest whose battlegrounds extend from the floor of the U.S. Senate to the airwaves in several states that are key to this year’s congressional elections.

The big prize is control of the Senate. Republicans need a net gain of six seats next fall to win a majority. Particularly vulnerable are seven Democratic-held seats in states that 2012 Republican presidential nominee Mitt Romney won: North Carolina, Alaska, Louisiana, Arkansas, West Virginia, South Dakota and Montana.

Democrats have decided that trying to discredit the efforts of Wichita, Kan.-based billionaire businessmen Charles and David Koch (pronounced “coke”) is good politics. And they’ve been unusually aggressive – but not always accurate – in trying to tar them.

Neither have the ads against Democrats engineered by Koch-backed groups been completely accurate.

The latest Democratic counterattack began when Senate Majority Leader Harry Reid, D-Nev., took the extraordinary step of denouncing the brothers last month on the Senate floor, at one point branding the Kochs as un-American and accusing them of trying to “buy America.”

On Thursday, Reid railed against their “radical agenda.”

Reid has said his comments are “about how those two billionaires would use that political system once they’ve bought it, how they would abuse it in order to add zeroes to their bottom line, while hurting middle-class families.”

The Koch brothers are top officials at Koch Industries, a multinational corporation that’s involved in energy, manufacturing and investments. Last year, Forbes magazine ranked it as the second-largest privately held corporation in the United States. Cargill, the agricultural giant, was No. 1.

Koch Industries spokesman Rob Tappan fired back at Reid: “Contrary to Sen. Reid’s obsessive and continued false statements, Koch has a positive and optimistic view of our country.”

Reid’s volley was one of the latest Democratic steps to counter the more than $30 million worth of anti-Democratic candidate advertising financed by the Kochs. Others include:

-Sen. Mark Begich, D-Alaska, ran an ad this week refuting charges against him by the Koch-backed Americans for Prosperity, a nonprofit group that’s classified under tax laws as a social welfare organization and isn’t required to disclose its donors.

-A Democratic website, www.kochaddiction.com, urges contributions to defend candidates against Koch-backed attacks.

-Reid, at his weekly meeting with reporters, began not by discussing the day’s big topics, such as the turmoil in Ukraine or the dispute between the CIA and the Senate Intelligence Committee, but the Kochs.

“These two brothers don’t like government,” Reid said. “What they would do if they had their way is get rid of the EPA.”

The Kochs and their Republican allies have responded. Senate Republican leader Mitch McConnell of Kentucky, himself a candidate for re-election in what might be a very competitive race, went to the Senate floor earlier this month to defend them.

At the recent Conservative Political Action Conference, New Jersey Gov. Chris Christie, a potential Republican 2016 presidential candidate, said: “Harry Reid should get back to work and stop picking on great Americans who are creating great things in our country.”

Koch spokesman Tappan said the brothers were politically active because “they believe in freedom. Free markets made America what it is today. They are concerned about an out-of-control government, rampant cronyism and they oppose all subsidies and mandates, even when they benefit industries in which our company operates.”

Besides Begich’s race in Alaska, Americans for Prosperity has spread Koch money into the Senate battleground states of Arkansas, Louisiana, Michigan, New Hampshire, North Carolina and Iowa. It’s spent an estimated $8.3 million in North Carolina and just under $1 million in Alaska.

The ads focus on three areas of the Democratic candidates’ voting records: health care, government spending and energy policy.

The latest furor erupted this past week in Alaska, where Americans for Prosperity has been after Begich for a while. Last year, it ran an ad that featured a woman criticizing Begich for backing the Affordable Care Act, the health care law that Republicans have repeatedly – and unsuccessfully – tried to repeal. Although Americans for Prosperity didn’t claim otherwise, it turned out that the woman wasn’t from Alaska.

The Koch-backed group also has ads against Begich’s re-election, claiming he’s “on record supporting a carbon tax,” a tax on greenhouse gas emissions. Begich’s campaign says he is not.

But PolitiFact, a nonpartisan political fact-check service operated by the Tampa Bay Times, found that “If you were counting votes in the U.S. Senate for a straightforward carbon tax and looked at Begich’s record for how he might vote, you’d likely put Begich in the ‘maybe’ column.”

It found that Begich had written a letter to Reid that said energy policy should try to put a price on greenhouse gas emissions, though it didn’t say specifically that it should be a carbon tax.

Americans for Prosperity spokesman Levi Russell claimed that was evidence Begich was effectively supporting a carbon tax “or something similar that would have the same impact.”

PolitiFact, though, also found that “Americans for Prosperity exaggerated in saying that Begich is on record supporting a carbon tax. So we rate the claim Mostly False.”

Begich put up his own ad denouncing the Koch-backed airwaves attack, saying the Americans for Prosperity ads “have been called false and not true. Who’s behind the attacks?”

But the organization has pressed on: “Nowhere in the ad does Sen. Begich give a coherent answer on why he pushed for a price on greenhouse gas emissions,” said Heidi Gay, Alaska Americans for Prosperity spokeswoman.

In North Carolina, Americans for Prosperity has spent $8.3 million on ads attacking Sen. Kay Hagan, one of the nation’s most vulnerable Democrats, and the Affordable Care Act, which she supports. A January poll by the Civitas Institute, a conservative research center in Raleigh, found that 52 percent of North Carolina voters surveyed opposed the health care law and 41 percent supported it.

“The Affordable Care Act has always had negative polling in North Carolina,” said Civitas President Francis De Luca.

Polls also show that as Hagan’s name recognition and people’s awareness of her went up in recent months, so did the numbers of people who said they’d vote for someone new. Through last October, about 25 percent to 30 percent of voters surveyed said they’d never heard of her or had no opinion of her. That number had dropped to 16 percent in January.

Also in the January poll, 30 percent said they’d vote to re-elect her and 56 percent said they’d vote for someone else.

Hagan has stood by her support for the health care law, emphasizing that it provides insurance to people who have pre-existing conditions, eliminates a cap on the amount of bills that insurance will cover and makes it possible for people to change jobs without worrying about how they’ll get insurance.

She also has said that “North Carolinians aren’t going to be fooled by the outsiders that are trying to buy this election,” according to campaign spokeswoman Sadie Weiner. “They’re looking for a senator whose strings they can pull. They know that will never be Kay. But they’ve had great luck with Thom Tillis in Raleigh, getting him to push their special-interest agenda.”

Tillis, the speaker of the North Carolina House of Representatives, is one of several Republicans who are hoping to unseat Hagan.

Americans for Prosperity supported the Republican-controlled state legislature’s $500 million budget cut for education, reduction in unemployment insurance, rejection of the federal health care law’s expansion of Medicaid and plan that reduced taxes for the wealthy and raised them for most others.

The health care law has been the most prominent topic in Americans for Prosperity’s ads against Hagan.

Carter Wrenn, a Republican strategist and a former adviser to the late North Carolina Sen. Jesse Helms, said it wasn’t clear how many people hadn’t yet made up their minds about the law. It can take a long time for a campaign to saturate voters with information and reach a point where it’s not worth spending more to keep telling them the same thing, he said.

The Democrats’ campaign against the Koch brothers won’t have much impact, Wrenn said. As in past elections, the real issue is what people think of the president’s policies.

“If you rip off one penny of taxpayer money there will be a revolution,” Wrenn said, “but someone in Kansas wants to spend millions of dollars in North Carolina? That doesn’t upset people. Everybody is spending money in North Carolina right now.”