A flood of outside spending has quietly moved the Ohio Senate race from a second-tier contest to a single-digit competition.

“We know it’s going to get closer, and the numbers will tighten,” Ohio Democratic Party Chairman Chris Redfern said. “If you’re outspent 6-to-1, it will have an impact.”

Over the past several months, the Buckeye State has become one of this cycle’s battlegrounds in terms of early spending by outside groups. While allies of Sen. Sherrod Brown (D) have spent
$1.1 million to boost his re-election, conservative groups — led by the U.S. Chamber of Commerce — spent more than
$6.5 million to aid his opponent, state Treasurer Josh Mandel. The chamber’s spending accounts for more than half of the outside group spending on the right.

The political consequences have been brutal for Brown, who has slowly watched his poll numbers slide.

The latest Quinnipiac University poll, released last week, showed Brown ahead of Mandel 46 percent to 40 percent.

“It’s 6 points because they spent $5 million running ads against me since March of 2011. It’s amazing I’m not behind with that kind of money,” Brown said in a Monday phone interview. “No one in Ohio has had this kind of money spent against them this early — not even close.”

Democrats have said since the beginning of the cycle that they expected the Ohio race to be competitive — but they are surprised the contest has become this close at this point in the cycle.

Close Senate races are rare in Ohio. The state boasts a reputation as a battleground for presidential contests, but there hasn’t been a Senate race decided by a close margin since 1976. Since then, every Senate race victor has won by a margin of at least 8.5 points.

The stakes couldn’t be higher for Senate operatives, who increasingly view Ohio as key to control of the chamber.

Republicans need to pick up a net of four seats to win control of the Senate. Roll Call rates seven Senate races as Tossups. But out of the eight races in the next competitive tier — those deemed as Leans Republican, Leans Democratic or Leans Independent — the public independent polls are closing fastest in Ohio.

A February Quinnipiac poll showed Brown ahead by 13 points. Other public polls have shown a similar trend in the race tightening over the past several weeks.

It’s one of the reasons outside groups set their sights on this race early. It’s a state where cash-flush groups knew they could make a difference before the presidential race dominates the airwaves.

The chamber has spent about
$3.8 million so far on the race, according to a source who monitors ad buys. Crossroads GPS has poured in about $800,000 to target Brown, plus an additional $2 million for the presidential race in Ohio.

Comparatively, only two outside groups have aired ads on Brown’s behalf. The League of Conservation Voters spent $516,000 in late March, and Majority PAC, a Democratic super PAC, spent about $630,000 on TV ads through this week.

Mandel, 34, welcomes the spending discrepancy.

“While we do not coordinate with them at all, we’re obviously glad they’ve chosen to target Ohio as one of the key states to turn America in a better direction,” Mandel said in a phone interview. “It’s not lost on me that we’re one of the few races in the country where Harry Reid’s super PAC is spending money.”

But the discrepancy was enough to force Brown, 59, to respond with his own negative spot hitting Mandel last month. It’s somewhat rare for an incumbent to kick off his campaign with negative ads early in the cycle — especially if he is still considered the frontrunner.  

Both candidates are financially armed in their own right. At the end of March, Brown reported $6.2 million in cash on hand, while Mandel had just about $1 million less than that.

In the face of an increasingly competitive race, Democrats view their task to be defining Mandel as quickly as possible. Later in the year, there’s a strong chance the Senate race will be drowned out by a highly competitive national race.

“You need to define Mandel pretty quickly,” one seasoned Ohio Democratic operative said. “They’re basically running against not-Sherrod-Brown at this point. And in this case, he’s not well-defined. The calvary needs to come and help Sherrod.”

National Democrats allow that Ohio voters aren’t yet familiar with Mandel or his short political career. Local press has reported disparaging stories about Mandel’s hiring practices, his political ambitions and his unwillingness to answer their questions. And it was reported Monday that federal investigators have looked into one of Mandel’s large donors, but there is no allegation of wrongdoing on his part.

National Democrats argue that the average voter is not aware of Mandel’s record yet and that when they are educated, the poll numbers will move.

In the meantime, Brown’s allies remain nervous that the race could become an unexpected Tossup contest.

“If you go back 10 or 12 months, there were even folks on our side who said this wasn’t going to be a race,” said Tim Waters, the political director for the United Steelworks and a Brown supporter. “We never believed that — and when we saw the money getting dumped in there, we were justified.”