As you can see from this thread on Yglesias, some Democrats are pissed that progressives are refusing to donate to Obama due to his tack to the center, and instead are giving their money to more consistently liberal Democrats. That seems like a perfectly good compromise position to me: vote for Obama as the best presidential candidate (well, the best one with an actual chance of winning), but put your money behind the people who will bring about the change you believe in. That's not "enforcing ideological purity"; it's using your money wisely.
The "anti-purists" may be right that a full-on progressive couldn't win the presidency. But unless we rally behind truly progressive candidates, that will never change. "Get Obama elected" and "shift America leftward" are not mutually exclusive goals. Some folks will be more attracted to the former mission than the latter, and vice versa. Citizens should engage in politics in the way that best engages their passions. Demanding obligatory donations isn't the best way to set the electorate on fire.
The larger, more disgraceful, problem is that we're no longer voting with votes, but with dollars. But mentioning such things has fallen out of fashion.