By Nate Silver, BaseballProspectus.com

The Contenders

Alex Rodriguez has to stay close to his current pace for years to have a shot at the crown. Bryan Yablonsky/WireImage.com

3. Prince Fielder
Projected Career Total: 489
Chances of Hitting 500: 45.2%
... 600 HR: 20.4%
... 700 HR: 7.9%
... 755 HR: 6.7%
... 782 HR: 5.6%
... 800 HR: 5.6%
... 900 HR: 5.2%

Top Comparables: Boog Powell, Mayberry, Canseco, Kent Hrbek, Eric Chavez.

He might be the best pure power hitter to come along since McGwire. Prince's comparables aren't terrifically flattering, and there's certainly the chance that by the time he turns 32, he's hung up his spikes and opened up Prince's Peach Pit out in the Miller Park netherworld. Nevertheless, the math is compelling: Fielder projects to become the youngest player ever to hit 50 home runs in a season.

2. Albert Pujols
Projected Career Total: 565
Chances of Hitting 500: 62.0%
... 600 HR: 40.9%
... 700 HR: 14.6%
... 755 HR: 6.9%
... 782 HR: 5.0%
... 800 HR: 5.0%
... 900 HR: 0.3%

Top Comparables: Murray, Orlando Cepeda, Robinson, Greg Luzinski, Jack Clark.

A year ago, A-Rod and Pujols would have been about tied in their likelihood of passing Aaron. But our model is very sensitive to any change in performance levels. It remains unclear whether there's anything wrong with Pujols, who is having a down year by his standards, but projecting him as a 35 HR/year guy instead of a 40 HR/year guy or a 45 HR/year guy makes a huge impact since that difference gets multiplied out over many seasons.

The Front-Runner

1. Alex Rodriguez
Projected Career Total: 731
Chances of Hitting 500: 100%
... 600 HR: 85.6%
... 700 HR: 60.7%
... 755 HR: 46.1%
... 782 HR: 29.6%
... 800 HR: 27.5%
... 900 HR: 9.8%

Top Comparables: Robinson, George Brett, Willie Mays, Bonds, Aaron.

You couldn't do much better in terms of comparables if your goal is to break the all-time record. Nevertheless, we have A-Rod as even-money at best to pass Aaron's mark, and a decided underdog to surpass Bonds' eventual total. What gives? Think about all the things that have to go right for a player to hit 755 (or 782) lifetime home runs. He has to stay healthy. He has to resist the temptation of early retirement, even if he already has several lifetime's worth of money in the bank. He has to avoid any sudden declines in performance. He has to not only stay at the top of his game, but keep the particular skill of power hitting intact.

Rodriguez could "devolve" into being Brett, and that still wouldn't be enough momentum to get him past Bonds and Aaron. It all sounds so easy -- if A-Rod heads into 2008 with about 520 lifetime home runs, then all he has to do is average around 30 home runs per season through age 40 to claim the record from Bonds. But remember when everyone assumed that McGwire -- or Sosa, or Griffey -- would challenge the record?

Remember 10 months ago, when many assumed Rodriguez's best days were behind him? A-Rod's right on pace, but he's too far from the finish line to be conceding any records to him; it's inherently dangerous to be predicting that someone will do something that nobody else in history has done. We might have to deal with this Barry guy for longer than you'd think.

More on PECOTA

Author's note: We've listed the top five comparables for each player, but as many as 100 comparables are used in formulating the forecast. Note that this is a somewhat different version of PECOTA than the one we use for our preseason projections. For one thing, we've adjusted our estimates of each player's ability level based on his performance through the All-Star break. For another, we've tweaked the model slightly to place more emphasis on home run power and longevity when selecting the comparables (at the expense of other factors like batting average and walk rate).

Our model figures that Bonds will end this season with 765 home runs. It also figures that he'll add 22 more home runs for a total of 787 if he continues to play after this year. Bonds latest public statements imply that he wants to continue playing.

On the other hand, this is a man who has been known to change his mind, and roughly half of Bonds' PECOTA comparables heading into 2007 played just one more season. We'll split the difference by assuming that there's a three-in-four chance that Bonds will play into 2008, and a one-in-four chance that he retires. Take a weighted average of the two cases, and that makes the magic number 782 home runs.

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