Help Abu Muqawama Understand U.S. Politics (Updated)

Since I'm new to this town and don't really understand (American) politics, could someone please explain to me why the White House fears growing opposition from the Left on Afghanistan? As I see it -- and again, I am a political ingénue -- the Democrats are going to lose seats in the next midterm elections based on

a) health care reform and

b) other domestic issues, including the deficit.

But Afghanistan?

And I have a tough time believing, too, that Obama is going to face a tough primary challenge from an ati-war candidate in the 2012 elections or that he will lose the independent vote to a Republican in the general election because he is too hawkish on Afghanistan. So, why? Is this not like being scared of one's own shadow? Or is this just more evidence of an administration that seems to fear a fight with its own party more than a fight with the Taliban?

[NOTE: Although this blog normally steers clear of politics, I really am asking you political types out there for your opinion here. I'm not asking whether or not the war in Afghanistan is a good idea or a bad idea -- for those of you who want to debate that question, skip to the comments section two posts down. I really don't understand why President Obama should care what Russ Feingold thinks on this issue.]

UPDATE: A reader responds with what seems to be a reasonable explanation...

In 1994, Democrats lost control of the House not because Republicans turned out in higher numbers – but because Democrats didn’t. If liberals fear that Obama is leading the country down the wrong path in Afghanistan – and they become frustrated about the conduct of the war – they are more likely to say home on Election Day. Mid-term elections traditionally have lower turnout than presidential elections and depend on strong base turnout. Obama and congressional Democrats just can’t take the votes of progressives for granted.