Sen. Lindsey Graham, R-South Carolina, can't put a finger on why Republican frontrunner Donald Trump is doing so well. But he says it could spell disaster for the party.

"It's pretty hard to understand where this thing is going with the Donald," Graham said on "Face the Nation" Sunday. "But here is the state of play: Our leading Republican is embracing self-deportation, that all of the 11 million have to walk back where they came from. And maybe we'll let some of them come back. I just hope we don't go down that road as a party."

"Our leading contender, Mr. Trump, is going backward on immigration. And I think he's going to take all of us with him if we don't watch it," he added.

Trump earned the biggest criticism in Sunday's interview but Graham was also not impressed with what his rivals have had to say about the fight against the Islamic State of Iraq and Syria (ISIS).

"Nobody on our side seems to be willing to put a plan forward that truly would destroy ISIL," he said, using an alternate acronym from the group. Graham has said the U.S. will need to send in about 10,000 combat troops to defeat the militants who have taken control of a large portion of Iraq and Syria.

He argues that his experience - 35 trips to Iraq and Afghanistan, 140 days in those countries as an Air Force reservist and more than a decade in the Senate - sets him apart and has given him time to study the issues.

"I've learned from my own mistakes. I've learned from President Bush's mistakes, President Obama's mistakes. Senator [John] McCain and myself have been more right than wrong," he said.

Graham specifically mentioned two of his rivals: Former Florida Gov. Jeb Bush is "a fine man," he said, but lacks a ground component to destroy ISIS.

"It is really not a whole lot different than that of President Obama. If you're not willing to commit more American boots in the ground in Iraq, from 3,500 to about 10,000, if you don't understand we'll never destroy ISIL in Syria without a regional army of which we will have to be part, you're not ready to be commander-in-chief," Graham said.

The other Republican presidential candidate from Florida, Marco Rubio, is "good on foreign policy," Graham said. But again, he added, "you've got to realize you're not going to destroy ISIL without a ground force."

Graham also weighed in on the lingering questions over Democratic candidate Hillary Clinton's role in the 2012 attack on a U.S. diplomatic facility in Benghazi, Libya. Even though Clinton has handed over her email server to the Justice Department, Graham said "Congress has a role here."

He said that Clinton's assertions that she turned over all her email to the State Department and that she never sent classified information on her private server "are not holding water."

"There is a criminal investigation potentially afoot. Let the professionals do that. But she was in charge of Benghazi where four Americans died. She was in charge of their security. She failed them miserably before, during, and after," Graham said. "I want to see the Benghazi emails."