A consortium of Ohio newspapers released a new poll this morning finding Senator Sherrod Brown leading challenger Josh Mandel by seven points, 52-45 among likely voters. This comes after outside Republican-aligned groups have spent at least 18 million targeting Brown, according to Bloomberg’s calculations.

But Brown’s lead in the polls should not obscure the fact that in one key way, the contest remains the most important Senate race in the country.

That’s because the Ohio race is providing a clearer referendum than perhaps any other Senate race on the question of whether outside money can depose an incumbent and capture a Senate seat, largely irrespective of the candidates themselves. That isn’t just because this state is the target of more outside cash than any other. It’s also because there’s no other race combining this level of spending with such a clear disparity in the quality of the contenders. Brown has been a popular figure in Ohio (though Dems say his numbers have eroded a bit under the ad onslaught), while Mandel has committed a string of missteps. If all that money can engineer a win for Mandel despite that disparity, it will send a strong signal that outside cash can sway what have become otherwise unwinnable races.

In a recent interview with me, Brown, an old fashioned lunch bucket populist who is pushing several measures opposed by business groups, conceded that the outside cash means he could still lose. Asked if he were still at risk, Brown said: “Sure.” He added: “Anybody is at risk of losing if there’s enough money against him.”

The Brown campaign says a total of around $5.6 million in outside money has been spent by unions and Dem party committees for Brown; Dems say the Brown and Mandel campaigns are roughly at parity in their own spending.

Consider that Brown was leading Mandel by double digits in many polls before the crush of outside spending started. Even if Brown has a lead now, pollsters say this race may still be too close to call, given how much more cash is expected to flow.

“Ohio is where these outside groups have done the most of anywhere in the country to close the gap between a strong, popular incumbent and a relatively weak challenger," David Donnelly of Public Campaign Action Fund tells me. “If Brown loses, the level of outside money in Ohio is the wave of the future.”

Ohio, of course, is a true battleground state, so the race looked like a winnable one for Republicans, particularly after the 2010 Dem shellacking. But Brown developed an early double-digit lead in the contest, which held up in part because of the quality of Mandel’s campaign, and outside groups began pumping cash into the state, hoping to turn it into a real race. They succeeded in doing that. The outcome in Ohio will tell us whether that outside cash can turn a lopsided contest between a well-liked incumbent and a sub-par challenger into a win.