U.S. President Barack Obama has "a great likelihood of success" in his showdown with congressional Republicans on the Iran nuclear deal, Senate Majority Leader Mitch McConnell has acknowledged.

The Republican-led Senate and House are expected to reject the deal in a joint resolution next month amid near universal condemnation of the accord by Republicans.

But Obama has pledged to veto their disapproval resolution, and the question then becomes whether Republican opponents can muster the two-thirds vote of Congress needed to override his veto.

McConnell, at a speech before business leaders in his home state of Kentucky on August 17, talked about the difficulty in achieving an override. Obama needs 34 Senate Democrats to sustain a veto, and 21 have announced they are backing the deal. In the House, 146 Democrats are needed to sustain a veto and 48 have expressed their support for the accord.

"He can win by getting one-third plus one of either house," McConnell said. "So he's still got a great likelihood of success."

McConnell's public acknowledgement that Obama has the advantage reflects what Democrats and Republicans have been saying privately. His comments come as groups on both sides of the issue are spending millions of dollars on ads they hope will sway lawmakers during their August recess.

McConnell said he would be surprised if any Senate Republican supports the deal. He added that even if Obama prevails, the deal can still be reviewed by the next president, who might be a Republican.

"The campaign of the president to get it approved will be entirely among Democrats, probably Democrats in very safe Democratic seats," he said.

Three more Democrats came out in favor of the deal August 18: Senators Jack Reed and Sheldon Whitehouse, both of Rhode Island, and Mazie Hirono of Hawaii. Reed sits on the Senate Intelligence Committee and could help sway other Democrats' views on the matter.

But the deal has some notable Democratic defectors as well, including Senator Chuck Schumer of New York, the No. 3 Senate Democrat and the party's leader-in-waiting.

Schumer was joined August 18 by New Jersey Democrat Bob Menendez, who said he will also vote against the nuclear deal and would vote to override a veto by Obama.

Menendez, the senior Democrat on the Senate Foreign Relations Committee, said he opposes the agreement because Iran has violated several UN Security Council resolutions while advancing its nuclear program.

He also called on Obama to renegotiate some aspects of the deal.

McConnell said such opposition was helpful.

"I hope we can defeat it, but the procedure is obviously stacked in the president's favor," he said. "We'll see."

Arizona Senator Jeff Flake, the lone Republican senator who was considering support for the nuclear deal, recently announced plans to vote no.

"I was pleased that he finally reached the same conclusion the rest of us did, that it was not in America's best interests to support it," McConnell said.

The Senate leader said he hoped to have a "respectful, uplifting debate" when the deal reaches the Senate floor.

"What I've asked my colleagues to do is to sit at their desks, listen to each other, and treat the consideration of this agreement with the respect that it deserves," he said. "This is a major, major foreign policy decision."

Still, McConnell denounced the accord as a "very flawed deal" on several fronts.

"It leaves the Iranians with threshold nuclear capability," he said. "It has no impact on all of their other activities. They are the biggest sponsor of terrorism in the world today. The inspections are flawed."

Even though he opposes the deal, McConnell said he doesn't think the agreement will derail the U.S.'s long-standing alliance with Israel.

"The relationship between the U.S. and Israel is very strong, and it's not going to change as a result of this," he said. "But I think the prime minister of Israel has got it right -- this is a very, very bad deal."

With reporting by AP and Lexington Herald-Leader