What makes some people so sure that the anti-war people are undermining the troops?

I have heard some argue that the anti-war protesters are undermining the troops, because they are bad for their morale. Why does anyone honestly think that the soldiers give a toss about what those of us at home have to say? They're on the front lines, risking life and limb, so I seriously doubt that the opinions of the anti-war protesters are what's on their minds.