Facing an avalanche of negative advertisements, labor’s most outspoken and visible champion in the Senate has, so far, been left to fend for himself.

With seven months left in the campaign, no candidate running for reelection -- save President Barack Obama -- has been targeted by outside interests more than Sen. Sherrod Brown (D-Ohio). Over $5.1 million has been spent on television ads opposing Brown in the 2012 cycle, according to data provided by a Senate Democratic campaign operative. The top spenders are the U.S. Chamber of Commerce, which spent $2.7 million, and 60 Plus Association, the conservative group that opposes health care reform, which spent another $1.4 million. Karl Rove's Crossroads GPS and Concerned Women for America Legislative Action Committee have also spent heavily in the race.

Brown isn't the only candidate these groups are targeting, but he has consistently been at the top of their list. The Missouri Senate race has attracted the second-highest amount of spending, with Republicans purchasing $3.45 million worth of ad buys against Sen. Claire McCaskill (D-Mo.).

The bullseye on Brown's back, however, has been largest and most inviting, and it's forcing officials to confront -- much sooner than envisioned -- one of the more critical questions they will face this cycle. How long can an out-gunned network of Democratic-allied interest groups wait to get involved?

To date, only one outside group has made a notable investment on Brown's behalf, and it wasn't even linked to one of his natural constituencies. The League of Conservation Voters (LCV), an environmental advocacy group, has spent in the "high six figures" in ad buys on Brown's behalf, but neither labor unions nor national progressive groups have dipped into their election season budgets to support him. The AFL-CIO has run a spot on behalf of another incumbent Democrat, Sen. Bob Casey (D-Pa.). AFSCME has run ads against Mitt Romney in both Florida and Ohio, in addition to being heavily involved in the recall effort of Gov. Scott Walker (R) in Wisconsin.

LCV recently launched its pro-Brown ads to thank the senator for voting extend the production tax credit and other manufacturing tax credits. Navin Nayak, LCV's senior vice president for campaigns, called Brown one of the "best allies in Washington" of progressives, saying, "Sherrod Brown really needs their help."

The imbalance hasn’t yet caused much discernible damage to Brown electoral standing. A recent Quinnipiac Poll showed the senator enjoying a 10-percentage point lead over his challenger, Ohio State Treasurer Josh Mandel (R). Over time, however, the Brown campaign anticipates that the battering will take a toll.

"I think it's tightening the race a little bit, and I think it will continue to tighten the race," said Brown campaign spokesman Justin Barasky. "That's why we believe that the race will be so much closer than it looks like right now."

Rather than wait for the race to tighten, however, strategists inside the Democratic tent are becoming more audible with their concerns, arguing that unions need to publicly show that being an outspoken labor champion has benefits if they want to recruit future candidates.

"Labor and progressives spent over $10 million to take out [former Democratic Sen. Blanche] Lincoln, but they have done nothing to defend Brown from the Chamber and Karl Rove onslaught," said a top Democratic operative with ties to labor, who hoped that groans of discontent might ignite some action from union groups. "It is just shameful. If they don't step up soon and emphatically ... it would send a horrible message to those members of Congress actually willing to stand up for labor and progressives."

Michael Podhorzer, the AFL-CIO's political director, said in an interview with The Huffington Post that the notion that labor had abandoned Brown was "preposterous." Not only had the union federation invested money in a major voter-to-voter contact effort in the Buckeye State, Podhorzer said, it had extensive plans to ramp up operations there -- both because of the critical role Ohio will play in the presidential race and the desire to see Brown win a second term.

"There is nobody that is up this year that we are going to work harder to make sure is re-elected. He has been as much a champion of working people as anyone. That's not lost or dismissed by labor leaders," he said. "We are going to put in whatever resources are necessary to win and it's our strategic judgement that what we are doing now is built around winning that campaign. We know we can't afford to lose Sherrod Brown in the Senate."

The challenge for the AFL-CIO, Podhorzer added, was to determine whether the Chamber actually thought Ohio was a race it could win, or whether the business lobby was merely trying to turn the state into a money-pit for labor.

Blair Latoff, a spokeswoman for the Chamber, declined to confirm the amount of money that the group was spending. But he did describe the goal as ensuring that "Ohio voters are educated on important issues affecting the economy" and telling them that "Sherrod Brown has been no friend of business."

Not everyone, however, is convinced that the money spent by conservative groups so far is anything more than a test run. After all, even if the ad campaigns don’t work to tighten the Senate race, they help set the stage for the presidential contest to come.

“In our view, it is really about Rove and others trying to put more Democratic Senate seats in play, forcing Democrats to spend money,” said Podhorzer. “And I don’t think it is a coincidence that it’s Ohio, because they want to lay the groundwork for the presidential race."

Daniel Mintz, MoveOn's director of national coordinated campaigns, had a similar take. "My guess is they're trying to spend a bunch of money now to see if they can make it competitive,” he said. "If come August or September, it's still a 10 or 15-point gap, you'll see the money dry up."

To date, MoveOn has focused its efforts on getting progressives elected in primary races. With many competitive Senate races around the country, the group is still deciding what to do with Ohio. But the notion of abandoning Brown if he does, indeed, need help was dismissed out of hand.

"He's stood up to the banks and he's not just been a reliable vote, but he's been a real leader on a lot of the issues that are critical to the 99 percent. We have no intention of putting him in the position where he doesn't have the resources to hold on to that seat," said Mintz.

None of that would seem to be a comfort for Brown. Barasky noted that hardly a week has gone by since this past November without an ad being run against Brown on television. The longer those ads go unanswered, the more likely voter impressions are to be molded.

As of now, however, there is no public fretting from the campaign.

"We have always known that the other side is going to have more money than us and our campaign is focused on building a grassroots operation and pushing out the good work Senator Brown has done as a senator," Barasky said. "We will leave all the spending decisions to those who can make them."

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