No it’s stupid because stretching something generally means by using less like stretching resources by using less, or stretching a band by pulling it making it thinner.
That’s why it’s stupid because it contains none of the original meaning.
I can figure out in sports it probably comes from stretching out (muscles), like giving it a bit of extra effort. But the headline doesn’t work in that context either.

So in short, it’s a stupid use that is erroneously used out of the original context and meaning where it had a common denominator that made sense.
There is no stretching going on in Harris increasing her lead.