For the Democrats, it is. The Left will throw an all-out screaming fit if the rhetoric changes one bit toward acknowledging US success on the battlefield, and it could cost them just enough defections to the Green Party to tip the election to the Republicans. Clearly, the top Democrat candidates aren't suffering any MSM backlash (ha!) for making these patently false statements, so there is no downside for them in continuing to do so.
That's the whole story in a nut shell, you're exactly right!