Marco Rubio wants the U.S. to insist on demanding something from Iran that it will never accept:

And so the bottom line in any negotiations should be clear: the only way sanctions on Iran will be lifted or suspended is if they agree to completely abandon any capability for enrichment or reprocessing.

This is the sort of demand that one makes if one wants to wreck any chance of reaching an agreement. Later on, Rubio says that everyone hopes that the talks in Geneva “work out,” but the only way that these talks could “work out” in a way that satisfies him is if Iran fully capitulated. Since that isn’t going to happen, we can conclude that Rubio doesn’t want negotiations with Iran to “work out,” because any deal that is remotely possible is one that he has already rejected. Like Kirk, he keeps claiming that he wants diplomacy to succeed while opposing the conditions that could make it successful. This position makes war with Iran more likely, since it effectively rules out the possibility of a negotiated compromise, and an attack on Iran makes Iran’s acquisition of nuclear weapons more likely and more certain.

In case issuing one deal-breaking demand wasn’t enough for him, Rubio also wants to engage in some explicit saber-rattling:

And at some point, Congress should consider making it very clear that if it becomes necessary, the United States reserves the right to take military action to prevent Iran from continuing to advance its nuclear weapons program.

The only reason to do this is if one wanted to start agitating for an attack on Iran. A vote to authorize such an attack would threaten to derail negotiations, and could kill off any chance of a peaceful resolution of the nuclear issue. Congress would be recklessly putting the country on a path to war with Iran, and all for the sake of “preventing” a “nuclear weapons program” that does not yet exist.

P.S. Colin Kahl and Alireza Nader explain at greater length why “zero enrichment” is a non-starter with Iran.