His approval ratings are below 50 percent. Americans aren’t satisfied with the economy, gas prices or his handling of a number of situations abroad. A number of the states he won in 2008, such as Indiana, would be difficult for President Obama if the election were held today.

So why aren’t a line of Republicans ready to run against him? Haley Barbour, Mississippi’s governor, bowed out Monday, joining a number of prominent Republicans, such as New Jersey Gov. Chris Christie, who have opted against challenging the president in 2012.

Barbour said he didn’t have the “absolute fire in the belly,” while political strategists say it’s hard to imagine him winning the presidency. And Rep. Ron Paul (R-Tex.) is expected to form an exploratory committee today. But here are a few reasons the GOP field so far is smaller than expected.

1. Money: “$1 billion,” said Terry Holt, a former adviser to George W. Bush, when asked why more candidates haven’t joined the Republican field.

GOP strategists argue whatever Obama raises, be it the $1 billion figure that has been floated or simply matching the $750 million of 2008, is a major obstacle to many Republican candidates. Other than former Massachusetts governor Mitt Romney, most of the potential GOP contenders both lack an existing major fundraising base or personal wealth that would make it easy to fund a campaign.

2. Timing: A number of the GOP’s most impressive figures, such as Rep. Paul Ryan (Wis.), Sen. Marco Rubio (Fla.) and Christie, could opt against running in 2012 and instead run in 2016. Christie was elected in 2009, Rubio last year.

If Obama is reelected, 2016 would be a wide-open race with no obvious favorite from either party, as Vice President Biden is unlikely to be a candidate.

3. The Senate Republican Caucus: The U.S. Senate usually provides at least one presidential candidate. But this year, none of the 47 Senate Republicans is running.

In the case of one, Sen. John Thune (R-S.C.), who flirted with a candidacy, this could be related to Obama’s strengths. But this seems to be an unusual set of circumstances.

One group of Senate Republicans, such as Senate Minority Leader Mitch McConnell (Ky.), are legislative types who have never really tried for higher office.

Senators such as Richard Lugar (Ind.) and John McCain (Ariz.), have already run and lost. Other veteran Republicans are either too liberal (Susan Collins of Maine) or too conservative (Tom Coburn of Oklahoma.)

And one group, such as Sen. Bob Corker (R-Tenn.) and Rand Paul (R-Ky.), may be too new to the Senate to consider a promotion already.

This is marked contrast from 2008, when Biden, Obama, McCain, Hillary Clinton, and Chris Dodd (D-Conn.) all attempted to jump from the so-called World’s Great Deliberative Body to the Oval Office.

4. It’s just too soon: Some prominent Republicans, such as Ed Rollins, who ran Reagan’s 1984 campaign, say that with the House Republicans and the newly elected GOP governors dominating the news, there’s little incentive to jump into the race early. And they argue a fully-formed field is soon to emerge.

“The feeling out there is that there is still time for candidates to jump into the race,” said Ron Bonjean, who was an adviser to then-House Speaker Dennis Hastert (R-Ill.)

In 2004, facing an incumbent President Bush, Democrats had six candidates who seemed true contenders (Clark, Dean, Edwards, Gephardt, Kerry, Lieberman) and three more long-shots (Kucinich, Moseley-Braun, Sharpton).

By the time of the Iowa caucuses, the Republicans could have a similar-sized field. Already, businessman Herman Cain and former Louisiana governor Buddy Roemer are in the race, and neither has much of a chance of raising much money or winning. Romney and ex-Minnesota governor Tim Pawlenty seem like very serious candidates.

It’s possible five more Republicans will join the race, considering Rep. Michele Bachmann (Minn.), Indiana Gov. Mitch Daniels, former House speaker Newt Gingrich (Ga.), former Arkansas governor Mike Huckabee, former Utah governor Jon Huntsman, former Alaska governor Sarah Palin, Rep. Ron Paul (Tex.), ex-senator Rick Santorum (R-Pa.) and Donald Trump are all flirting with candidacies.

“Generally you only have two to three candidates with a real shot at winning the nomination and being a serious contender to challenge a sitting president, and Republicans have that,” said Tony Fratto, who was deputy press secretary for President George W. Bush. “Romney and Pawlenty, both former governors, are serious candidates, building serious campaign organizations. And we don’t know who else will enter, like Governor Daniels or Christie.”

He added, “What did the Democratic field look like at this time four years ago? Clinton, Edwards, a very untested Obama and a handful of guys who couldn’t punch themselves out of single digits. That’s usually the way it looks, and this race is no different.”

Other: You could argue that the Republican electorate is too conservative for a person like Barbour, full of people who falsely believe the president was not born in the United States. But it’s not clear Romney or Pawlenty or Huckabee is more conservative than Barbour, and all three have chances of winning the nomination.

Obama has more strengths than money, particularly his strong support among blacks, voters under 30 and self-identified liberals. But many swing states he won in 2008 elected a Republican senator two years later, and it’s not clear the first-time “Obama voters” will return to the polls in 2012.

Incumbents generally win in modern history, but Bill Clinton was aided in 1992 by a series of other prominent Democrats deciding to pass on the race, a fact well known among potential Republican candidates.