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Experts expect Ohio's 2016 U.S. Senate race to be the most expensive in state history. As of June 30, more than $43 million had already been spent by Republican Rob Portman, Democrat Ted Strickland, and outside groups on both sides.

(Plain Dealer file)

COLUMBUS, Ohio -- With about three months to go before Election Day, Ohio's U.S. Senate race is shaping up to be the most expensive in the state's history.

More than $43 million has been spent by Republican Rob Portman, Democrat Ted Strickland, and outside groups on both sides as of June 30, the most recent date for which data is available.

About three-quarters of that money -- more than $31.7 million -- has been spent by Portman and his allies, according to a cleveland.com analysis of the data from the Federal Election Commission, media reports, and the candidates' campaigns.

That doesn't include more than $2.6 million spent by the campaign and leadership PAC of Cincinnati City Councilman PG Sittenfeld, Strickland's primary opponent. Nor does it factor in millions more spent in the last month and tens of millions of dollars worth of advertising that campaigns and independent groups have reserved between now and Election Day.

It also doesn't account for all of the "dark money" being dropped into Ohio by political non-profit groups, which don't have to disclose their donors or fully detail their spending until after the election.

As of the end of June, pro-Portman organizations spent more than $24.2 million. Portman's campaign reported spending about $7.5 million, according to FEC records.

The lion's share of that cash was spent on television, radio, and digital ads, the data showed.

More than $11.4 million was spent by Strickland's campaign and allies as of June 30. That includes millions of dollars in advertising -- all from outside groups, as Strickland's comparatively cash-strapped campaign has yet to spend a dime on advertising.

The largest outside spenders in the race are each helping Portman. One Nation, a group linked to strategist Karl Rove, spent more than $6.5 million as of June 30. Two organizations supported by the Koch brothers, Americans for Prosperity and Freedom Partners Action Fund, have, combined, dropped nearly $8 million in the race, data showed. Together, those three organizations spent more than Strickland and his allies combined.

The largest pro-Strickland outside spender is Senate Majority PAC, a left-leaning group affiliated with outgoing U.S. Senate Minority Leader Harry Reid. As of the end of June, it spent approximately $5.3 million on Ohio's senate race, FEC records state.

Both sides are preparing to spend a lot more money as well.

Senate Majority PAC says it has booked $9.5 million worth of advertising this fall; Strickland's campaign has reserved an additional $8 million in ad time. Portman's campaign, meanwhile, has reserved $14.9 million worth of TV, radio, and digital ads.

Political scientists in Ohio said they weren't surprised that the Portman-Strickland race is currently the most expensive in the nation. And they predicted this year's contest will be the costliest Senate race in state history, surpassing the $84 million spent on the 2012 Senate campaign won by Democrat Sherrod Brown over Republican Josh Mandel.

One reason for all the money is that the winner of Ohio's Senate race may determine whether Democrats take control of the Senate from the Republicans, said University of Akron political scientist John Green. Republicans currently hold 54 Senate seats, and Portman is one of several GOP incumbents considered vulnerable.

In addition, Ohio is a particularly expensive state in which to run a statewide ad campaign, as it has 12 separate media markets - more than any other state with a competitive Senate race this year. And the large amount of outside money in the race is a result of the U.S. Supreme Court's Citizens United ruling in 2010 which allowed unlimited corporate and union spending in elections.

Green also said he thinks there has been a lot of early spending because outside groups hope to influence the race before the presidential campaign ramps up and air time gets more expensive.

"I think a lot of this early spending tries to play the odds," Green said. "To spend in the race this early may tip the odds toward Portman or Strickland."

Strickland campaign spokeswoman Liz Margolis said in a statement that the Columbus Democrat supports increased campaign-finance transparency and overturning the U.S. Supreme Court's Citizens United ruling that overturned limits on spending by corporations and labor unions.

"Senator Portman's rich and powerful friends are spending millions to prop him up because they want him to keep pushing their agenda at the expense of Ohio's working people," Margolis stated.

Portman's campaign, meanwhile, noted that while Strickland has campaigned against "dark money" in politics, he's received millions of dollars worth of support from outside groups to prop up his campaign.

"There is not a Senate candidate more reliant on out-of-state Super PAC money than Ted Strickland," said Portman campaign spokeswoman Michawn Rich in a statement.