Washington (CNN) Hours before top Obama administration officials began briefing Congress on the classified details of the nuclear accord with Iran, House Speaker John Boehner vowed Republicans would "do everything possible to stop" the agreement.

Senate Majority Leader Mitch McConnell, R-Kentucky, put the onus on the Obama administration to convince members on Capitol Hill the agreement deserved their support.

"It's always the administration, not Congress, that carries the burden of proof in a debate of this nature," McConnell said on the Senate floor. "And it seems the administration today has a long way to go with Democrats and Republicans alike."

Republicans are seizing on what they are calling a "side deal" Iran negotiated with the International Atomic Energy Agency regarding inspections as a reason to oppose the overall agreement.

GOP Rep. Mike Pompeo of Kansas questioned Kerry about "side deals" he said Iran negotiated with the International Atomic Energy Agency (IAEA) regarding inspections.

Kerry admitted he didn't have all the details on those agreements, but expected to be briefed on them.

At a State Department briefing on Wednesday, spokesman John Kirby said, "There's no side deals. There are no secret deals between Iran and the IAEA that the P5+1 has not been briefed on in detail."

"These kinds of technical arrangements with the IAEA are a matter of standard practice ... which we would be happy to discuss with Congress in a classified setting," Kirby said.

Pompeo told reporters after the briefing, "It's an enormous problem to be asked to vote on an agreement you have not seen in its totality."

He insisted the "secret side deals" were important because they deal with "important verification processes that are going to take place with respect to Parchin, where there were suspected explosive device testing take place for Iran that were nuclear related."

Pompeo and Sen. Tom Cotton, R-Arkansas -- another top GOP opponent of the deal -- sent a letter to Obama, along with Boehner and McConnell, demanding details of the supposed side deal with the International Atomic Energy Agency.

This week the White House has all hands on deck trying to prevent opponents from scuttling the deal. After Wednesday's closed-door sessions with members, the first public hearing on the Iran agreement is slated for Thursday.

Secretary of State John Kerry, Treasury Secretary Jack Lew and Energy Secretary Ernest Moniz are expected to face tough questioning from members of both parties on Senate Foreign Relations Committee.

The campaign against the deal is also in full swing -- roughly 40 House conservatives huddled for breakfast with Israeli Ambassador to the U.S. Ron Dermer, who outlined a long list of objections to the nuclear agreement.

Dermer's message, according to Iowa GOP Rep. Steve King, who hosted the meeting, was pretty straightforward: "Congress is the last stop to avoid this."

King said much of discussion focused on what happens 10-12 years after the agreement is implemented. The Iowa Republican warned if opponents on Capitol Hill don't shut it down now "then it paves the way not just for a nuclear Iran, but a very highly powered nuclear Iran that changes the dynamics in the region and changes the destiny of the world."

Virginia Rep. Dave Brat, who also attended the meeting with Dermer, said the overall thrust of the conversation was "to pay less attention to all the details -- the debate on centrifuges and years and committees and UN and all that -- and pay more attention to who's on the other side of the debate and that is Iran."

The White House got good news Tuesday night when Sen. Dick Durbin of Illinois, the No. 2 Democrat, announced he would support the bill.

He called the agreement "an historic opportunity to once and for all prevent Iran from obtaining a nuclear weapon, something no administration or Congress has yet to accomplish."

"Given a choice between the invasion or Iran or working in a diplomatic fashion toward a negotiation so that we can lessen this threat to the world, I think President Obama made the right choice," Durbin, a close ally of the President, said in a Senate floor speech.

But Republicans rejected the notion that the only choice before the Senate was to accept the agreement or go to war.

"There is a third choice," said Sen. John Cornyn of Texas, the No. 2 GOP senator. "There are tougher sanctions that will bring Iran to the table for a better deal and a good deal. It's simply unacceptable for the President to be misrepresenting what the options are to Congress and the American people."

Sen. Dan Coats, a respected Senate veteran who sits on the intelligence committee, echoed that view.

"We must ignore the coming public relations campaign that will trumpet this deal as a victory for diplomacy and the false premise that the deal is a choice between peace and war," Coats said on the floor.

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Of the deal itself, he said, "The more I read, the more my concern grows."

Durbin is in charge of counting votes for the Democrats. In recent days he's said is uncertain if there will be enough Democrats to either successfully block the a resolution of disapproval from coming to the floor or to sustain the President's expected veto of it if it passes. He said his fellow Democratic senators must first read the agreement and hear the administration's briefings before deciding.

Currently there are about 15 Senate Democrats who could vote against the deal. If they joined Republicans against it they could override a veto. Already those 15 are the subject of heavy lobbying by the forces for and against the agreement and will likely face intense pressure in the roughly 60 days before Congress must vote on the deal.

In the House where GOP opposition appears virtually unanimous, the President would need 145 Democrats to help him sustain a veto. The top House Democrat, Rep. Nancy Pelosi, endorsed the deal last week, which was a big win for the administration, and she expressed confidence members of her party would back the President.

Democratic Rep. Dan Lipinski of Illinois, didn't like Kerry's tone in the Wednesday meeting.

"I would appreciate if the secretary showed a little more respect for members of Congress," he said.

He declined to say specifically what the secretary of state said that was so off putting.

Another Democrat, Rep Jim Himes of Connecticut, said he was still studying the Iran agreement, but said Kerry and other Cabinet officials gave "a very, very strong defense of the deal" and "they are making a lot of headway."

Himes said administration officials urged members to view the deal in context of where Iran was recently -- on the threshold of obtaining nuclear capabilities - and where they are now.

"From my standpoint the burden of proof, given what I've learned so far, is for the opponents to explain why this is a bad deal relative to where we were and why this a bad deal relative to where we will be if the United States unilaterally walks away from it."