As to who has a good shot if they did run, I'd eliminate Condi Rice and Jeb Bush. Rice has no experience running for electoral office, and jumping to the most challenging electoral race of all is not the way to learn. Jeb Bush will discover that neither the party nor the country are inclined to elect a Bush for the third time in four presidencies (yes, I'm assuming a W win in '04). Spooks people about aristocracy and dynasty.
Likely "moderate" challengers (I think of them more as liberal Republicans) would have to include Colin Powell, John McCain, and Rudy Giuliani. These have to be the powerhouses, should they choose to run (and each one of them has serious reasons not to).
From the conservatives, I'd look to a number of governors and Senators. It's far too early to tell whom among them would even consider it, let alone be able to mount a serious campaign.
I still think the powerhouses to watch are the first three. And of course, should there be a Constitutional Amendment passed to allow foreign born citizens to become president, Arnold Schwarzeneger would move to the front of the list.
Jeb! Jeb! Jeb! Jeb! Jeb! Jeb! Jeb! Jeb! Jeb! Jeb! Jeb! Jeb! Jeb!! Jeb! Jeb! Jeb! Jeb! Jeb! Jeb! Jeb! Jeb! Jeb! Jeb! Jeb!
If we could forgive him for Toomey, what about Rick Santorum? He could do penance by endorsing Toomey for his vacant Senate seat (that is if he resigned it).
OTOH, pro-choice Arnold Schwarzenegger, is a liberal.
God help us if the Constitution is amended. When I listened to AG Ashcroft and Dir. Mueller say that one of the 7 wanted terrorists lived in the US for 15 years, I thought of Orrin Hatch's DREAM ACT. The Dream Act would allow a foreign born person who lived in the US for 20 yrs and had a HS diploma to run for President. So, if this terrorist lived here for 5 additional yrs and had the support of George Soros, this terrorist could buy the Presidency. The Dream Act could be a nightmare for America. I've called Hatch, Frist and many other Senators to forget about this "Dream".
I hope you don't mind the correction, but Senators, as a breed, are very ill-suited to the Presidency due to their generally deliberative and consensus-seekign attitudes. Not much leadership in that gene pool.