Being president of the United States is not the sweet gig it used to be. You still have Air Force One, but the country is running out of gas money. Unemployment is stuck around 9 percent, your approval ratings recently dipped into the 30s, and the Republican Congress keeps threatening to shut down the government to prove a point.

Now it looks like the Republican primaries will boil down to two challengers: former Massachusetts Gov. Mitt Romney and Texas Gov. Rick Perry. Which one do you want to run against?

At first blush, Romney looks like the stronger candidate. He kept his national network from his 2008 campaign and added talent from the other campaigns. His team stayed together and helped Scott Brown get a surprise win for Republicans in the special election for U.S. Senate in Massachusetts.

And what has Perry done? He beat a Democrat in Texas to win re-election to another four years as governor. Again. When it comes to the invaluable benefit of having an existing national network, the advantage goes to Romney.

Romney cannibalized the best fundraisers from the other 2008 campaigns and got a big head start on Perry, but he still hasn’t proven that he can compete with President Barack Obama at amassing campaign cash. Over the first half of this year, Romney lapped the Republican field with $18 million raised, but that was less than a quarter of Obama’s haul. Both Perry’s and Romney’s supporters have started super PACs to make up the difference, but Perry’s history of raising more political cash than the GDP of some emerging countries makes him potentially more formidable here. When it comes to fundraising, Perry has the advantage.

And though it’s impolite to make an issue of it, Romney is a Mormon. Many social conservatives in this country think that’s a cult, whereas liberals just think it’s something for the creators of “South Park” to mock on Broadway.

With a Mormon topping the Republican ticket, Republicans could face an enthusiasm gap akin to the lack of excitement that George W. Bush’s doctrine of “compassionate conservatism” engendered among social conservatives. No one throws red meat to those wolves like Perry does, giving him a huge advantage.

The real difference between Romney and Perry is how they perform on the campaign trail. A week and a half ago while standing atop some hay bales at the Iowa State Fair, Romney allowed a heckler to draw him into a peevish back-and-forth in which Romney claimed that “corporations are people” and then proceeded to explain his point without scoring any points.

Odds are pretty good that the Romney campaign didn’t send their candidate out to a state fair to engage with hecklers, much less to think out loud about whether corporations were people. Romney’s time would have been better spent eating deep-fried butter on a stick than trying to explain how money flows through corporations to real people. His intended message was washed under the wave of ridicule he took for his off-the-French-cuff remark.

If this were an isolated incident, Romney’s team could just blame it on the Iowa summer heat, but it was the second time that week that Romney unintentionally made the news.

It started when Romney blamed Obama for the Standard & Poor’s downgrade of the country’s credit rating. As a campaign tactic, this was a no-brainer. If you’re running against an incumbent and the economy is bad, you blame everything new that is bad in the economy on the incumbent. Right or wrong, blaming the credit downgrade on Obama is about as radical an opinion for a Republican as saying you probably won’t vote for Obama for re-election.

But before long, he appeared to back down from that position. In front of the Chamber of Commerce in New Hampshire, Romney explained: “I don’t think it’s simply the president’s fault. I’m sure there are many people to share responsibility for the excessive spending in Washington over the past couple of decades.”

Bad candidates explain. Good candidates declare. Winning candidates attack. These instincts are innate and can rarely be unlearned. Perry doesn’t have the problem of wanting to excessively explain his worldview to hostile people. He’s never betrayed the slightest worry about what the intelligentsia thinks of him. If an Obama official criticized him, Perry wouldn’t back down, he’d brag about it, and then he’d repeat the attack in what his advisers call “preacher mode.”

When Perry screws up, it’s for one of two reasons: Either he’s trying to break new ground governing (privatizing Medicaid delivery, building the Trans-Texas Corridor or mandating the HPV vaccine) or his blustery self-confidence causes a brief blip in an otherwise disciplined campaign. “Adiós, mofo,” anyone?

Perry wouldn’t lose to Obama because of a mistake. Perry could lose to Obama because of what he does on purpose, and that’s what makes the Texas governor the opponent Obama should be rooting to face.

The problem with running against Romney is that it’s like hitting Jell-O.

In his 2002 race for Massachusetts governor, he ran to the left of the Democratic nominee on abortion. Now he’s pro-life. He passed Romneycare, a state-based health care reform that the Obama administration used as the model for Obamacare, which Romney now opposes.

Romney has changed his position on so many issues — including stem cell research, amnesty for illegal aliens, the stimulus bill, tax cuts on capital gains, and many others — that is has become impossible to hit him on any one position. You can really only hit him convincingly on having no convictions, something a majority of voters believe about all politicians anyway.

It’s difficult to imagine Romney trying to defend the positions Perry took in his book “Fed Up!” If Romney had written a book that called Social Security a Ponzi scheme that should be dismantled, he’d disavow it before it hit the shelves.

If Romney’s book also blamed the 2008 economic meltdown on federal regulations, called the right to privacy “total and complete nonsense,” advocated for a national sales tax, or claimed that Obamacare funds abortions, he would have disavowed the controversial opinions and blamed his ghost writer.

And until recently, I would have predicted that Perry would respond to any challenges by squaring his shoulders, squinting his eyes, and asking, “Yeah, what of it?” Watch him speak, and Perry reminds people of George W. Bush, but read his book and he’ll remind you of Barry Goldwater. Perry has never lost an election, but his book — steeped in the states’ rights rhetoric of the tea party — seems like it is written by someone who would rather be right than win.

When Perry packed his bags to go on the campaign trail, he seems at first glance to have left his steadfastness back in Austin. His initial support of New York’s right to legalize same-sex marriage preceded a total reversal a couple of days later, and last weekend he disavowed his 2007 mandate that 12-year-old girls get vaccinated for the HPV virus.

Don’t expect Perry to become another Romney in this regard. Those flip-flops minimized the distance between Perry and the “teavangelicals” he needs to win the Republican nomination. Last week he stuck by his comment that Fed Chairman Ben Bernanke would be a traitor if he printed money for political reasons, and he is unlikely to flip-flop on other controversial positions that endear him to Republican primary voters, such as calling Social Security a failure and unconstitutional as a federal program.

The Perry in “Fed Up!” is the one who is getting the rock-star treatment on the presidential campaign trail, but it’s also the Perry whom Obama would want to run against. Somewhere in the White House, Obama is happier about running for re-election now that Perry’s in the race.