If there's one senator Republicans are itching to send packing more than any other in November, it's first-term Ohio Democrat Sherrod Brown.

At least that's the conclusion one draws when looking at how much money the GOP has spent to take him down.

Ohio Sen. Sherrod Brown (D). (Nick Daggy/AP)

But when it comes to the race for the majority, there are a handful of other contests that are closer, and could arguably use the cash more. So why Ohio? Part of it has to do with the nature of the state, and part of it involves Brown.

First, the basic data points: Republican groups have spent more than $20 million against Brown - more money than the organizations have collectively spent in any other Senate race. And Brown's Republican challenger Josh Mandel has been one of the cycle's most productive fundraisers. He impressively added another $4.5 million to his campaign account in the third quarter, his campaign announced Tuesday.

Polling shows that the race is competitive, but it's not in the uppermost tier of close contests. The last two live-caller surveys showed Brown leading Mandel by 10 and nine points, respectively. Automated Republican-leaning pollster Rasmussen Reports more recently showed the race tied, and Republican strategists contend it's a single-digit contest. But by and large, it hasn't looked as close as say, North Dakota, Montana or Wisconsin -- three prime GOP pickup opportunities. 

Ohio's population, though, is much larger. In big cities like Columbus, Cleveland and Cincinnati, it's more expensive to advertise compared to Fargo or Missoula (and advertising is where the lion's share of money goes in campaigns). Just because the GOP is spending big in Ohio doesn't necessarily mean it has bought more total gross ratings points (the unit of measurement in TV advertising) there than in other states.

Crossroads GPS, for example, has spent about $9 million in Ohio. But it didn't gear up its offensive on the airwaves there until the middle of this year. By comparison, it began its ongoing offensive in Montana late last year. When all is said and done on Election Day, even if Republican dollar-for-dollar spending in the Ohio Senate race is at the top of the heap, there may be other states where GOP groups bought more GRPs.

Aside from the nature of the state's population, Brown's record has also made him a ripe target for Republican groups. He's enjoyed a good relationship with organized labor during his tenure, making him an obvious choice for GOP opposition.

He's also to the left of his colleagues facing tough reelection bids this year. According to National Journal's 2011 Vote Ratings, Brown was the fifth most liberal senator, tied with four others. 

Democrats also have spent  quite a bit of money on the Ohio race, and their support is part of the reason Brown still has the upper hand in his reelection bid. Brown's allies have dropped about $8 million on the Ohio race -- a far cry from what the GOP side has been spending, but nothing to scoff at either.

Another reason for Brown's relative success is that Mandel's campaign has experienced its share of problems this year and Democrats have been there to pounce at every turn. The Republican nominee's missteps have jeopardized the utility of the immense financial support he is receiving. 

The GOP is accustomed to investing heavily in Ohio: No Republican has ever won the presidency without carrying the Buckeye State. At the top of the ticket this year, that state currently leans slightly toward President Obama. That's more good news for Brown. 

Notably, the National Republican Senatorial Committee has not yet gone up on the air in Ohio, even as the Democratic Senatorial Campaign Committee has. Part of the rationale for abstaining appears to be the high rate of spending Mandel and allied groups have committed to, freeing up the majority-focused NRSC to devote resources elsewhere.

If Brown wins in November, Ohio could go down as perhaps the costliest Senate loss ever for Republicans when it comes to sheer dollars spent. Such an outcome would no doubt prompt some post-game second-guessing about whether the money could have been used better elsewhere. But it will also be a reminder about the complex nature of outside spending, which isn't always dictated solely by the race for the majority.