By Lisa Mascaro, Tribune Washington Bureau

WASHINGTON — Not that long ago, Karl Rove emerged as a mastermind of a new type of campaign spending, leading a wave of outside groups whose ability to pour money into the 2010 midterm election helped Republicans gain control of the House and bolster GOP ranks in the Senate.

The former George W. Bush adviser’s reputation as a strategist soared — as did the often oversized caricature of him as a Republican boogeyman who could always be counted on to rattle the political left.

This midterm election cycle, though, is shaping up differently. Rove’s two funding groups, American Crossroads, a super PAC that discloses its spending, and Crossroads GPS, a nonprofit that does not, are playing more of a supporting role.

The new stars of the dark money circuit are the billionaire Koch brothers, whose Americans for Prosperity has funneled a jaw-dropping $35 million so far into the midterm election cycle, much of it spent even before the first primary ballots were cast this spring.

Rove’s activity has paled by comparison. American Crossroads reported raising just $6 million in the first quarter of 2014 and both of Rove’s groups recently announced plans to spend $9.3 million on summer TV ads in four battleground states: Arkansas, Colorado, North Carolina and Alaska.

That’s a fraction of the $175 million his groups spent in the 2012 election, or the $72 million they spent in the 2010 midterm. (Verifying expenditures is difficult because much of the money does not have to be disclosed in a timely way, leaving watchdog groups to cobble together estimates from various sources and estimates, including TV ad buys.)

While other national groups are now rushing to influence Mississippi’s Senate GOP runoff between incumbent Senator Thad Cochran and Tea Party challenger Chris McDaniel, Crossroads’ spokesman Paul Lindsay said last week the group would skip that Republican standoff because it’s “not our fight.”

Part of the change may stem from a drop in contributions. Many conservative donors felt angry or disappointed over Rove’s dismal performance in the last presidential race, when Mitt Romney lost and almost none of the congressional races went Rove’s way.

The Sunlight Foundation, a campaign watchdog, suggested donors’ “return on investment” from money they gave to Rove’s groups was among the worst of 2012. In 13 congressional races that American Crossroads tried to sway, it succeeded in just two, the group found.

“There was certainly a period in 2013 when donors were trying to get over 2012,” said one GOP strategist familiar with the Crossroads groups, granted anonymity to discuss the private operations. But he said donors were beginning to return because the political climate was “driving them back.”

Rove’s lower profile comes at a time when the stakes are particularly high for Republicans. The party is poised to retain their majority in the House and try, for the third election cycle in a row, to wrest control of the Senate from Democrats. Republicans need to net six Senate seats in November to gain the majority.

Although they may not be spending as much, the Crossroads groups are marshaling their resources more strategically in key states where operatives believe they can achieve the most bang for their buck. And they are getting results.

For example, American Crossroads swooped into the GOP primary election for Senate in North Carolina during the final stretch of the campaign to hoist Thom Tillis, the GOP establishment-preferred candidate, over a Tea Party-backed alternative.

While the Koch-backed Americans for Prosperity has spent more than $7 million since last fall on television ads railing against Obamacare and North Carolina’s vulnerable Democratic senator, Kay Hagan, American Crossroads’ smaller $1.6-million expenditure proved to be a smart investment. It helped the Tillis campaign avoid a costly GOP runoff so he could pivot immediately to the general election showdown with Hagan.

The race put Rove back on the national stage. But now it’s one he shares with many others, largely Americans for Prosperity. So it’s no surprise Democrats have shifted their firepower from the man once referred to as “Bush’s brain” and onto the industrialist Koch brothers from Kansas.

“This is why there’s so much emphasis on the Koch brothers as opposed to liberal groups or even Karl Rove’s network,” said Robert Maguire, who researches political nonprofits at the nonpartisan Center for Responsive Politics. “Nobody comes even close.”

Hardly a week goes by that top Democrats don’t bring up the specter of the Koch brothers’ influence in the elections. But the condemnations only fuel conservatives to give more, said Americans for Prosperity President Tim Phillips

“When folks see the Senate majority leader on the floor of that chamber attacking, by name, individuals who simply disagree with him on the issues, it does motivate people to give — and that is one reason why our organization is growing,” Phillips said.

In the past, outside spending was used more as a tool to punctuate marquee races than to flood the zone in comparatively ho-hum midterm elections. But a decade of Supreme Court rulings has set off a raising-and-spending arms race.

The fall election is on track to have more undisclosed dark money spending than any other. The Center for Responsive Politics recently estimated such spending is running three times as high as it was at this point during the 2012 presidential race.

Democrats are doing their share to drive up spending thanks to the Patriot Majority group and wealthy investor Tom Steyer, who has promised to spend $100 million this cycle to bring attention to the problem of climate change.

One of the leading Democratic-aligned groups, the Senate Majority PAC, which was launched by allies of Majority Leader Harry Reid (D-NV) in response to Rove’s 2010 successes, has reported spending about a third as much as AFP’s $35 million.

“While we know that we will be outspent by billionaires like the Koch Brothers, we are going to continue to run efficient and effective campaigns to stop the Republican Party’s reckless and irresponsible agenda,” said Ty Matsdorf, spokesman for the Senate Majority PAC.

Despite Rove’s late start this election, few are ready to count him out.

“I’m sure they’re going to be back,” said Bill Allison, editorial director at the Sunlight Foundation. “They can pour it on.”

Democrats are bracing for his return. The Hagan campaign, which had been fundraising around the candidate’s birthday last month, sounded exasperated in a recent e-mail to donors.

“You aren’t the only ones who sent me a special birthday gift,” the senator wrote, as she thanked supporters for the $26,000 raised. “Karl Rove’s group Crossroads GPS sent me one, too — in the form of $3.6 million in new spending to defeat me. This is a gift I’d like to return.”

Photo: Carol H. Feeley via Flickr