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 seats held by Democrats 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 zeros 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.“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.”

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