Using the gerund two times in a row
Here is the sentence:
Just as on smoking, voices now come from many quarters (insisting or insist) that the science about global warming is incomplete, that it's Ok to keep pouring fumes into the air until we know for sure.
Which is correct in the parentheses above, insisting or insist? Which role does it play here, attribute or some thing else? And could you help me analyze the grammar?