Ellipsis

[NewHope]

Context:
But in the past month, Kerry's personal image has deteriorated, with almost as many voters viewing him unfavorably as favorably.

I think "with almost as many voters viewing him unfavorably as favorably" means "with almost as many voters viewing him unfavorably as the voters viewing him favorably

In syntactical construction, "the voters viewing him" is necessary, but not necessary in understanding, so it is omitted.

Right?

[Casiopea]

Context:
But in the past month, Kerry's personal image has deteriorated, with almost as many voters viewing him unfavorably as favorably.

I think "with almost as many voters viewing him unfavorably as favorably" means "with almost as many voters viewing him unfavorably as the voters viewing him favorably

In syntactical construction, "the voters viewing him" is necessary, but not necessary in understanding, so it is omitted.

Right?

That's correct. It's redundant.