Of course, media focus turned out to be the undoing for other briefly-popular candidates Herman Cain, Newt Gingrich and Rick Perry. National Journal goes back to look at their rankings for how conservative a Senator's votes are, and Santorum was inside the top ten only once.
As you can see, Santorum was in the 20s and 30s an alarming number of times, portending what one might call that horrible "moderate" word.
Furthermore, Santorum's tax plan received the worst ranking of the major candidates from the Tax Foundation on their "Candidate Tax Plan Report Card," getting a D+. While listing his willingness to cut the corporate tax rate and eliminate the estate tax as major pros, they also say,
It fails to address the special-interest provisions and the preferential rates of the current code... Santorum has talked about taxing manufacturers at a zero percent rate, blatantly adding a targeted tax preference to the code... Though it drastically reduces the corporate income tax rate, the plan would still tax different forms of business income unequally... It lacks specificity, particularly regarding the corporate rate structure.
Santorum might be an example of a candidate peaking at the right time, but the media is ready with an onslaught.