Republicans in Congress have for months been openly skeptical of the Obama administration’s nuclear negotiations with Iran, and when a deal was announced Tuesday morning, they denounced it and pledged to try to stop it.

Given the parameters of lawmakers’ ability to review the agreement, however, drumming up the necessary support to actually block it will prove difficult.

President Obama announced the pact in a statement from the White House early Tuesday, and said it “offers an opportunity to move in a new direction. We should seize it.”

Obama said the deal would cut off Iran’s pathways to nuclear weapons, be subject to strict verification standards, force Iran to get rid of 98 percent of its stockpile of enriched uranium and permanently prohibit Iran from pursuing a nuclear weapon. Needless to say, most Republicans in Congress disagreed with the president’s assessment. House Speaker John Boehner said Obama “abandoned his own goals.”

“His 'deal' will hand Iran billions in sanctions relief while giving it time and space to reach a break-out threshold to produce a nuclear bomb – all without cheating,” Boehner said in a statement. “Instead of making the world less dangerous, this ‘deal’ will only embolden Iran – the world’s largest sponsor of terror – by helping stabilize and legitimize its regime as it spreads even more violence and instability in the region.”

Once the full text of the agreement reaches Congress, it starts the clock on a process laid out in legislation that passed earlier this year after a tough fight on Capitol Hill that gives Congress a say on the agreement. Lawmakers will have 60 days to review the pact – an extended period of time thanks to several missed deadlines during the negotiations – and can vote on resolutions of approval or disapproval. Obama vowed to veto any congressional action that halted the deal, however, which means Republicans would need to sway a significant number of Democrats to join them in order to override a veto.

The process will start in the Senate Foreign Relations Committee, the panel that unanimously passed the legislation granting Congress the right to review the pact. Chairman Bob Corker, R-Tenn., used a series of briefings and hearings in June to prepare lawmakers on the panel for the review. Corker said he has had serious concerns for weeks about the direction of the negotiations.

“I begin from a place of deep skepticism that the deal actually meets the goal of preventing Iran from obtaining a nuclear weapon,” he said.

Once the panel approves or disapproves of the agreement, it will move to the full Senate. It’s unclear whether the House will wait for the Senate to act, or move forward on disapproval or approval of its own.

Many Democrats were hesitant to outwardly support the agreement Tuesday morning, saying they needed an opportunity to fully read and understand the details. But most were complimentary of the Obama administration, and some, like Senate Majority Leader Harry Reid, cheered the agreement while vowing to review the particulars carefully.

“Today’s historic accord is the result of years of hard work by President Barack Obama and his administration,” Reid said. “The world community agrees that a nuclear-armed Iran is unacceptable and a threat to our national security, the safety of Israel and the stability of the Middle East. Now it is incumbent on Congress to review this agreement with the thoughtful, level-headed process an agreement of this magnitude deserves.”

Whether or not Republicans can convince enough Democrats to join them in halting the agreement, the review process still presents significant hurdles for lawmakers. They will be out of session for the entire month of August – which is why the 60-day review was set – and likely won’t vote on any resolution until September. But timing could be a problem. Members of Congress face a July 31 deadline to finance the highway trust fund, and when they return from the August recess, they will have just a few weeks to solve a spending debate before the close of the fiscal year at the end of September – or face a potential government shutdown in October. That spending debate will likely take up significant floor time in both chambers.

The 2016 presidential race will also be in full gear by the time Congress begins reviewing the agreement, and could present problems, especially with five senators – four Republicans and independent Bernie Sanders – running for the White House. The first Republican debate is in early August, well before Congress will vote on this Iran deal, and could play a significant role. There will be plenty of pressure on Sens. Marco Rubio, Rand Paul, Lindsey Graham and Ted Cruz to stand tough against Iran and the agreement, given that they are the presidential candidates who could actually have an opportunity to halt it.

Several of the candidates were quick to criticize Obama and his agreement. Graham, speaking on MSNBC Tuesday morning, called it “a death sentence over time for Israel” and said it is a “terrible deal. It’s going to make everything worse, and I really fear that we’ve set in motion a decade of chaos.”

Rubio, a member of the Foreign Relations Committee that gets the first crack at reviewing the agreement, in a statement reiterated his opposition to it and said it undermines national security.

“Failure by the president to obtain congressional support will tell the Iranians and the world that this is Barack Obama's deal, not an agreement with lasting support from the United States,” Rubio said. “It will then be left to the next president to return us to a position of American strength and re-impose sanctions on this despicable regime until it is truly willing to abandon its nuclear ambitions and is no longer a threat to international security.”