A powerful senator has announced his opposition to�the Iran nuclear accord,dealing what's seen as a big blow to the administration as it fights to win congressional approval of the agreement.

In an op-ed in The Washington Post, Senate Foreign Relations Committee Chairman Bob Corker says, "Congress should reject this deal and send it back to the president."

Corker, a Tennessee Republican, "has been signaling his deep misgivings about the Iran deal, but the fact that he has now formally come out against it is significant," notes CBS News congressional correspondent Nancy Cordes.

"He's the closest the administration comes to a Republican ally on foreign affairs, and he repeatedly called for patience when the deadline for the Iran deal kept slipping," Cordes says.

Corker writes, "It is Congress's responsibility to determine whether this agreement will be in our national interest, will make the United States safer and will prevent Iran from developing a nuclear weapons program. I do not believe that it will."

He calls the inspections process in the deal "deeply flawed," and says the agreement strengthens Iran's hand in the Middle East.

"Absent a clearly articulated policy for the region, this deal will become the linchpin of the United States' Middle East strategy. We will be relying on Iran to help achieve our goals in Iraq, Syria and perhaps elsewhere. This abrupt rebalancing could have the effect of driving others in the region to take greater risks, leading to greater instability. Iran was fully aware of this, which helped the regime continually erode the deal to its benefit, and it will become an impediment when we try to push back against potential violations of the agreement."

Corker says he opposes the deal because it would enable Iran to get "hundreds of billions of dollars over the next decade, removes the conventional weapons and ballistic missile technology embargoes on Iran and allows for a U.S.-approved, industrial-scale [nuclear] enrichment program for which Iran has zero practical need."

"I came to these negotiations with an open mind," Corker writes. "Prioritizing engagement over coercion in an attempt to end three decades of animosity with Iran appeals to the American idealism in us all. And while we should strongly support diplomacy, the other side must believe there are real consequences in its failure. In this case, Iran never felt that, resulting in a very disappointing outcome for our country."

Separately, The Associated Press reports Senate Majority Leader Mitch McConnell "grudgingly acknowledged" Monday that President Obama has "a great likelihood of success" in rounding up the votes to sustain the veto Mr. Obama pledges in the event that, as expected, Republicans in Congress reject the accord. Two-thirds of the members of both houses of Congress would have to vote to override and, McConnell conceded, that's unlikely to happen.

By CBS News