Liberals and Dems across the country are understandably paying very close attention to Elizabeth Warren and the Massachusetts Senate race. But there’s another Senate contest that deserves to be watched just as closely, and may be just as pivotal to Dem hopes of holding the Senate: The battle over Sherrod Brown’s seat in Ohio.

In what may come as a surprise to many Democrats, the Ohio Senate race appears to be the target of more spending by GOP-aligned outside groups than the Warren contest or any other Senate race in the country.

Outside conservative groups have spend nearly $5 million on ads attacking Senator Brown, according to data provided by the Brown campaign and its media tracker. National Dems who track ad spending say no other race in the country has been targeted by so much outside money.

Follow the money and you can see that conservative groups seem to view Senator Brown’s seat as key to the GOP’s efforts to take back the Senate. The 60 Plus Association, a self-proclaimed advocacy group for seniors, has spent over $1.4 million on ads targeting Brown. The U.S Chamber of Commerce has spent around $2.4 million on ads, the source says, over $1 million of that in the last couple months.

The Rove-founded Crossroads GPS spent $800,000 in 2011 on ads attacking Brown, a group spokesman confirms. Two other groups — the Concerned Women for America, and the Coalition to Protect Patients’ Rights — have spent another $270,000, the ad-tracking source says.

Total: Nearly $5 million in ads attacking Senator Brown alone.

Why the focus on Brown? There are several factors at play. One is that despite recent polls showing Brown with a sizable lead over his challenger, Josh Mandel, this very well could be a close contest. Ohio, unlike Massachusetts, is a true battleground state, and Obama and Mitt Romney are neck-and-neck there.

Second, Brown is an outspoken working class progressive who has crafted a populist message that resonates among the blue collar white swing voters the GOP is fighting for — making him a big prize for the right. Third, Ohio is pivotal to GOP hopes in the presidential race. And fourth, recent developments in Maine, and the fielding of a tough Dem challenger in Massachusetts, have increased pressure on the GOP to win in Ohio if Republicans are to take back the Senate.

At any rate, keep an eye on this race. It could prove decisive in the battle for control of the Senate, as you can see if you follow the money.