This is an old story and Bush's homeland security advisor Frances Townsend denies Ridge was ever pressured. She gives a credible rebuttal to the charge of political motivation pointing out that raising the alert is the president's decision based on the recommendations of several security department heads.
In this case the alert was not raised, maybe for the political reason that it would look political. But Ridge's charge does serve to distance him from the administration he served. Something that could be useful to him should he decide to run for president himself in 2012.
Efforts to get him to run for Arlen Specter's senate failed, as I understand it, because he was making too much money on the lecture circuit. Maybe too, he has grander political ambitions. Getting a book out a couple of years before a presidential run seems to be the thing to do these days.
In the meantime, the Obama Administration, flailing as it over healthcare and other issues, seems to be doing everything it can to encourage ambitious Republicans to consider their prospects for the Commander in Chief.
Ridge has every reason to believe he's as plausible a candidate as anyone right now.