Challenging a sitting president

Rhodes Cook says that Obama's in relatively good shape for reelection because he's not facing a challenge from within the Democratic Party:

For some time now there has been a political rule of thumb: Presidents with little or no opposition in their party’s presidential primaries go on to win reelection, while those who must weather a significant primary challenge are defeated in the fall election.

At this point, there are no signs of a Democratic primary challenge to Obama.

If there was, he would have reelection problems of the first magnitude. For if a president has trouble uniting his own party, how can he successfully reach out to independents and voters from the other party in the fall? The answer over the last century has been that he can’t.

I must respectfully disagree with the direction of Cook's causal arrow. Obama is not facing a challenge from within the party because he's in decent shape for reelection.

Yes, Carter faced a primary challenge in 1980 and lost in the general election, and the same thing happened to Ford in 1976. But the only reason those presidents faced primary challenges was because high quality candidates (Teddy Kennedy and Ronald Reagan, respectively) and their funders and endorsers calculated (correctly!) that their incumbent president was weak, would likely lose, and would likely drag down others in their party should they be at the top of the ticket.

Obama's path to reelection is far from certain, but it will largely depend on what happens with economic growth over the next year. Democrats with presidential ambitions realize that the economy will probably not slip into a recession in the next year and that presidents rarely lose their reelection bids unless the economy is in a recession. Just that much information is enough to keep the high quality challengers at bay until 2016.