I don't think that Jim DeMint necessarily has Barack Obama's political fortunes in mind when he advises him to fire Tim Geithner, but it's good advice anyway.
In Washington D.C., it's not hard to predict what usually happens to a politician in Geithner's place. Several Republicans have called for him to resign; more will. The press will begin to ask many Democrats what they think. Several of them are clearly not far from calling for a resignation as well; one will before long. That will be the signal that Geithner is near the end of the road.
Obama still has a very ambitious agenda that depends on his Treasury Secretary. There's health care reform, the budget, cap-and-trade, a resolution for GM and Chrysler, and more efforts to stabilize banks. Administration officials are still saying that Congress is likely to need to approve more money for that bailout. That ship has probably already sailed, but at the very least it's impossible to imagine Geithner getting it approved. The sooner Obama cuts his losses, the better.