WASHINGTON – Clinging to control of the Senate, a key Republican political committee with ties to Senate Majority Leader Mitch McConnell, R-Ky., is pumping $12 million into television ads to run in six key states in the final days of the campaign.

The wave of ads in Pennsylvania, Indiana, Missouri, North Carolina, Wisconsin and Florida comes on top of $25 million already spent by the Senate Leadership Fund on advertising from Oct. 19 to 25.

It also comes as outside groups not associated with candidates’ campaigns engage in a frenzied, last-minute attempt to sway voters through commercials, online ads, mailings, phone calls and door-to-door canvassing, according to filings with the Federal Election Commission records over the last week.

In addition, a Sunlight Foundation report Friday said $836 million has been spent in the 10 most expensive Senate races — with two-thirds of that coming from independent groups including the leadership fund.

The outside groups’ activities are opposed by campaign finance watchdogs because they can spend unlimited amounts, and many do not have to reveal their donors.

The $156 million poured by independent groups into the Pennsylvania race between Sen. Patrick Toomey and Democrat Katie McGinty appears to make the race the most expensive Senate contest ever, according to the Sunlight Foundation.

The Republican leadership fund’s ads come in response to increased spending by Democrats, labor and other liberal groups in mid-October, said the fund’s spokesman, Ian Prior.

Democratic donors had grown increasingly confident of a victory by their presidential nominee, Hillary Clinton, over Republican Donald Trump, and shifted attention to Congressional races.

Republican donors, in turn, stepped up giving to GOP groups,  allowing for ads to run more frequently, Prior said.

Sadie Weiner, spokeswoman for the Democratic Senatorial Campaign Committee, described it as a last-ditch effort.

“The desperation of this latest ad buy tells you everything you need to know about Republican Senate races — they are struggling and their only chance is a last-ditch bailout to try to save candidates who have been vulnerable all cycle,” she said in a statement.

The Republican leadership fund’s last-minute ads continue to paint Democrats as tied to Washington interests.

The fund — whose executive director, Steven Law, is McConnell’s former chief-of-staff — has had the largest influence of so-called outside groups in the Senate races, having spent $86 million, the Sunlight Foundation said.

The fund and its affiliated nonprofit, One Nation, are part of Karl Rove’s American Crossroads Super PAC. One Nation does not disclose its donors.

It is spending an additional $1.4 million in Indiana on ads playing off recent reports that Democratic candidate Evan Bayh spent much of his last year in the Senate in 2010 looking for jobs and was paid $2 million after being hired by private equity firm Apollo Global Management.

“Coincidence or corruption? You decide,” the ad says.

An extra $1.3 million spent in Pennsylvania highlights Democrat McGinty’s acceptance of a board position with a company that received a state grant while she was Pennsylvania’s top environmental official.

Another $1.9 million in Missouri means TV viewers see more often an ad linking Democrat Jason Kander to House Speaker Nancy Pelosi’s support for an energy tax, Obamacare and Bernie Sanders’ support of amnesty for undocumented immigrants.

Democratic and labor groups also spent millions this week to hit Republican candidates along similar lines — that they are Washington insiders beholden to special interests and Trump.

An ad by the American Federation of State, County and Municipal Employees says about Pennsylvania’s incumbent senator: “Pat Toomey, the banker, he’s not for you.”

The union spent $2.4 million on a television ad and another $700,000 on mailings against Toomey, a former investment banker, according to its disclosures with the Federal Election Commission.

The labor group this week also reported spending $2.2 million on a TV ad and $1.9 million on a radio ad against Missouri Republican Sen. Roy Blunt, and $2.8 million on a TV ad targeting Bayh’s opponent in Indiana, Rep Todd Young.

The Democratic campaign committee reported a $2.9 million media buy on Wednesday against Toomey; a $1.9 million buy against Blunt; and $9.75 million in media buys against Young.

Kery Murakami is the Washington, D.C. reporter for CNHI’s newspapers and websites. Contact him at kmurakami@cnhi.com.