The announcement of a landmark nuclear deal by Iran, the U.S. and five world powers is only the first step. 

In Washington, Congress will have a chance to weigh in, and ultimately vote, on the agreement. 

This is the process: 

First, it's important to note that the Obama administration and Congress are not handling this as an official treaty. If that were the case, constitutionally, only the Senate would have the opportunity to ratify the treaty, with a two-thirds vote -- or 67 senators. 

In this case, Congress has set up a process which requires both the House and Senate to approve or disapprove of the deal, with a simple majority. 

The original framework called for a 30-day review period. But since negotiators busted through last week's deadline, the review period has been extended to 60 days. 

The vote takes place after that, likely in September. And with a simple majority all that is needed to approve or nix the deal, it's quite possible Congress could vote it down. 

But President Obama already threatened Tuesday to veto the measure in that case. 

"I believe it would be irresponsible to walk away," Obama said. 

If the president vetoes, it would be up to Congress to override, which requires a two-thirds vote by both chambers -- 67 in the Senate and roughly 290 in the House. Otherwise, the deal stands. 

It's a high bar, but already Sen. Lindsey Graham, R-S.C., predicted in an interview with Fox News that lawmakers might be able to meet it.