In an increasingly fragmented television market, with hundreds of channels vying for viewer attention, political parties and candidates have to choose their targets wisely: Spend money on the wrong network and you risk missing the persuadable voters you need to win an election.

As cable networks take up a bigger percentage of viewer eyeballs, campaigns are spending more on those networks than ever before. A new study by Echelon Insights, a Republican firm that specializes in digital and advertising strategy, found that Republican campaigns tend to buy cable spots on channels aimed at men, while Democrats are buying more spots on channels that draw more women and young viewers.

It’s little surprise that the overwhelming number of political advertisements on Fox News come from Republicans: GOP candidates accounted for 84 percent of Fox’s ads. A similar number, 85 percent, of ads on MSNBC come from Democrats.

But the news networks are only the second-most partisan channels. Echelon found that 93 percent of political ads on the Golf Channel are paid for by Republican candidates and outside groups. And 94 percent of the political spots on the E! Network are sponsored by Democrats and their allies, making it the bluest network in America.

Republicans make up the bulk of advertising on The History Channel, ESPN 2 and the Discovery Channel. Democrats, who tend to advertise more on cable than their GOP counterparts, make up more than 80 percent of all political ads on Comedy Central, ABC Family, BET and Bravo.

The study doesn’t cover every cable viewer in America: While the Federal Communications Commission requires networks to file reports on political ad buys, it doesn’t require cable companies to do the same. But Time Warner Cable voluntarily posts its political ad buys online. That means data is available from big markets like New York and Los Angeles, and states with competitive Senate races like North Carolina and Kentucky.

Time Warner’s data allowed Echelon to analyze some 512,000 individual political advertisements that cost a total of $36 million — about 15 percent of all the money spent on television total, and a much higher percentage of the money spent on cable TV spots. That gave the company a broad look at where people are advertising.

“Though cable remains a relatively small percentage of overall TV buying, the variety of options for targeting mean that it can yield valuable insights as to the two parties’ targeting that broadcast can’t,” Patrick Ruffini, Echelon’s co-founder, wrote in an e-mail.

Fox News is by far the most popular channel with political ad buyers. Republicans alone bought more than 43,000 spots on the conservative news channel. Republicans bought more than 10,000 spots on ESPN, HGTV, the History Channel and A&E, the data show. Democrats bought almost 23,000 spots on CNN, more than twice the number of Republican ads. They also purchased more than 10,000 spots on ESPN, HGTV, USA, TNT, MSNBC and the Food Network.

The proliferation of digital video recorders, which allow viewers to fast-forward through commercials on shows they’ve previously recorded, means ad buyers are putting a premium on live sports, shows that viewers tend to watch as they happen. Republicans, especially, like advertising during Monday Night Football, which helps explain why ESPN is the most expensive cable news network on which to buy an ad.

The average spot on the all-sports channel cost a campaign $130.83 to run. An ad that runs on Fox News cost the second-most, at $128.69. No other channel commanded more than $100 on average, though USA and TBS ads cost more than $90 apiece.

Because of Fox News’ overwhelmingly Republican viewership, its relative value to political campaigns changes over time. Echelon’s analysis shows Republicans advertised most heavily on Fox News, almost to the exclusion of other channels, between February and late March, and again in late June — both times when expensive primary battles made access to conservative eyeballs essential to success.

After primaries, Republican cable dollars expanded across many other networks. Similarly, Democrats tended to gravitate towards MSNBC earlier in the year, when primary voters were about to cast their ballots.