The presidential primaries are only now just about to start, but conservatives investing money in politics have already bet their capital on the biggest return: defeating President Obama in the general election.

The shifting Republican nomination fight has been cheaper than a post-holiday sale compared with the 2008 race, but groups mobilizing against Obama are starting earlier than similar groups have in the past.

Spending on television advertising, traditionally the largest campaign expense by far, is dramatically lower than four years ago. In 2007, Republican candidates spent $33 million on television ads, according to estimates of ad spending created by Kantar Media. Democrats spent $40 million in 2007.

Kantar’s data show that all the Republicans in this year’s race, and the groups supporting them, had spent a combined $8 million through Monday — about a quarter of GOP spending in 2007.

While primary campaign spending is down, interest groups focused on defeating Obama are spending much earlier than the liberals who targeted President George W. Bush ahead of the 2004 election.

Republican groups with an eye toward capturing the White House have already spent more than $18 million on ads attacking Obama — more than twice as much as the entire GOP field. In the 2004 election cycle, the biggest liberal interest groups attacking Bush didn’t start spending on ads until March of the election year.

Karl Rove, a former Bush political adviser, emphasized the general election in a recent Web video for the conservative interest group American Crossroads, the biggest spender against Obama. Rove is helping run the group.

“We say it every election, but this time it’s true: 2012 is the most important election in 100 years, and I don’t say that lightly,” Rove says in the video. “We got a far more liberal, far more ideological president than we could have ever imagined. . . . We’ve got to fight to save our country.”

Even primary ads are heavily focused on Obama’s tenure. Data from Kantar show that almost half of the money spent was on ads pushing an anti-Obama message, making it the second-biggest issue tracked in the data after jobs.

There are a number of reasons for the low spending. An extremely fluid GOP race has meant that leading candidates have had little time to raise money before fading to the sidelines. And a big focus on the lively — and widely watched — debates means that campaigns haven’t needed to run a large number of ads to get their message out.

Mitt Romney is the only candidate who consistently has been at or near the top of the group. Given his prominence in the 2008 primary season, Romney hasn’t needed to reintroduce himself to voters. He also hasn’t started using his personal fortune as he did in his first bid. Romney spent $22 million on ads in 2007, according to Kantar’s data, compared with less than $1 million this year.

The Nelson bet

Senate Democrats lost a $1.8 million bet this week.

That’s how much the party has invested in ads boosting Sen. Ben Nelson (D-Neb.) this year. The party’s move was seen as an attempt to convince Nelson that he would be able to win reelection in the deep-red state.

The Democratic Senatorial Campaign Committee transferred $1.4 million to the Nebraska party, which used the money to produce and air ads featuring Nelson talking about his accomplishments. The Senate Democrats’ super PAC, called Majority PAC, spent an additional $400,000 backing him.

“Republicans will continue to have their hands full with a very divisive primary in the state,” said Sen. Patty Murray (D-Wash.), chairwoman of the DSCC. “We remain confident that we will hold the majority next year.”