Transcript for Top Senate Dem: Trump in 'high wire act without a safety net' on North Korea

option is on the table. The best way to convince them is a bipartisan effort to do so. You gave me a segue, senator graham. Thank you for your time. Thank you. Want to bring in senator Bob Menendez. The top Democrat on the senate foreign relations committee. Let's start off where senator graham left off. I love my friend Lindsey graham. But I think first, we have to give a chance at peace. That's why we outlined clearly what a successful agreement would be. A complete, irreversible, and effort at the denuclearization of the Korean peninsula. Pretty much what the secretary of state, Mike Pompeo said, before the senate relations committee when I asked him questions. As well as the national security adviser, ambassador Bolton. Our standard is trying to define clearly what success is. Getting a deal with North Korea is not the difficult part. In fact, three administrations, president Clinton, president bush, president Obama -- have acquired some form of a deal only to ultimately have them fail. I think it's important where North Korea has advanced the program so dramatically in terms of ability and nuclear power and ballistic missile program to define the deal as the complete, irreversible and vary -- verifiable denuclearization of the peninsula. Let's see what that brings us, first and foremost. Do you have any belief that Kim Jong-un can accept something like that? That kind of a deal that is verified by inspectors inside his territory? Well, this is exactly the effort that was made with Iran. It -- and they did not have nuclear weapons. Our concern there was about their advancement of their nuclear enrichment and a pathway towards a nuclear weapon. In this case, when you already have nuclear weapons, the threat is clear. When you have nuclear weapons and you have developed intercontinental ballistic missile capabilities, the threat is real to the U.S. And our allies. Without a potential disbanding of the program. We went won't know if the threat is real or not. We need to know that that is taking place. And so that's why defining success here is so important. Especially when the administration has suggested that if they could reach a deal they might very well submit it as a treaty to the United States senate in which the senate would vote. So I think it's important to define it before. You would need those assurances before signing a peace treaty. What about the alternative. If this doesn't go as well as we all hope, and it leads to the prospect of military force, can you sign on to that use of force authorization that senator graham is talking about? Well, first of all, this is our concern here. The president has gone into a high-wire act without a safety net. And the preparation for this type of summit while we applaud robust diplomacy, the preparation for this type of summit to test the proposition of what Kim Jong-un is really willing to do or not has not taken place. So this is a bit of a risk. I want to see first what is produced here. I don't think you'll have an agreement on Tuesday. At the end of the day, is there a process where real agreement of irreversible denuclearization, verifiably so, is possible on the north Korean -- Korean peninsula? I'm not ready to give an authorization for use of force to this president or any. Until I understand that the path for peace is not attainable and the threat continues to be a threat to the security of the United States. We need to have the intelligence, a robust debate in the senate on such an aumf. I have prepared aumfs before. As the former chairman of the senate foreign relations committee. And voted for some. Voted against it. It depends on the totality of the circumstances. I can't just jump on to give the president the authority to have a switch in which he can engage in an attack, nuclear or otherwise, against North Korea. Does president trump deserve credit for getting us to this point? Well, look. As I say, we Democrats believe in robust diplomacy. I get concerned that the president thinks that this is a mano-a-mano engagement that he can achieve the success we want. Which is a completely denuclearized Korean peninsula. To the extent that Kim Jong-un has already gone from international pariah to being normalized internationally, you have to say that he's had some success here. A meeting as I said and a deal is not the hardest part. It's getting the right deal at the end of the day. We have had president Clinton, president bush, president Obama all got deals. And they did not meet with Kim Jong-un or his predecessor. In order to get a deal. And so -- this deal has given international recognition to North Korea's leader. I hope the president can succeed. We want him to succeed. I think success is defined not as a grand moment where you say we have peace in our time when, in fact, we don't have the verifiable elements of a denuclearization. Dismantling of its nuclear program. The ballistic missile program. And the chemical and biological weapons, as well, should be on the table in my opinion. If you avoid a nuclear challenge but you can deliver an icbm with chemical or biological weapons, that is equally a threat. We'll be right back with reports from Bob woodruff and

This transcript has been automatically generated and may not be 100% accurate.