From the moment President Barack Obama took office, the Iranian government had its doubts about his administration. There were two schools of “doubt.” The first questioned his intentions. They believed his rhetoric and promises were just that – empty words. In deeds, the argument read, he was no different from his war-prone predecessor.

The other school doubted Obama's abilities, not his intentions. Could an inexperienced, outsider president really shift America's longstanding policy and attitude towards Iran? Was he even the real decision maker?

“I do not know who makes decisions for the United States, the president, the Congress, elements behind the scenes,” Iran's Supreme Leader Ayatollah Ali Khamenei said in 2009. By now, however, there should be less confusion in Tehran. Obama is on a roll.

Tehran's doubts were not unfounded. What used to be perhaps an academic question, is today a very practical and pertinent one: The sanctions relief Obama is promising Iran in the nuclear talks are not his to lift. Obama has waiver rights, but only Congress can lift sanctions. Congressional opposition to a nuclear bargain with Iran has consequentially weakened Obama's hand in the negotiations, rather than serve as a 'Good cop, bad cop' Jedi mind-trick that would compel Tehran to lower its demands.

It's simple economics: Since the risk of dealing with a president that does not control the sanctions relief process is higher, the price Iran must ask for giving concessions must increase accordingly.

But the doubters in Tehran should take note. Some extraordinary changes to the political landscape in the United States have occurred that should prompt Iran to reevaluate Obama's abilities.

A few weeks ago, new sanctions on Iran were on the fast track in the new Republican Senate. The measure would, at a minimum, undermine the nuclear talks, at most cause their collapse. On paper, Obama was heavily outgunned. Historically, no piece of legislation passes as easily in Congress as an Iran sanctions bill. The Republican-controlled Congress has no time or patience for either Obama or his chats with Iranian nuclear negotiators, so sabotaging the talks and depriving the president of a much needed foreign policy success was a no-brainer. And mindful of Israeli pressure in favor of sanctions, many Democratic lawmakers would likely abandon the president and side with Israeli Prime Minister Benjamin Netanyahu instead, it was predicted.

But Obama stood firm. Rather than seek a compromise with the Senate, he threatened a veto and warned them about the consequences of sabotaging the talks. “The American people expect us to only go to war as a last resort, and I intend to stay true to that wisdom,” he said.

The threat worked. As of today, only two Democratic senators have co-sponsored the new sanctions bill. Unless sanctions supporters manage to get at least 14 Democrats to commit to the measure, they cannot override Obama's veto and will only embarrass themselves trying.

Perhaps more importantly, senators who supported a similar measure last year and who have historically been very close to the American Israel Public Affairs Committee's position on Iran, such as Kirsten Gillibrand (D-NY) and Cory Booker (D-NJ), have refrained from sponsoring the bill. “The president strongly believes it would gravely harm negotiations, and therefore, I am willing to give him more time before supporting this bill,” Gillibrand told CNN.

Even more shocking, perhaps, was Hillary Clinton's full backing of Obama in this contest. In the midst of preparations for her presumed 2016 presidential run, Hillary came out against both AIPAC and Netanyahu and called the sanctions bill “a very serious strategic error.”

Undoubtedly, the issue took on an even greater partisan dimension when House Speaker John Boehner secretly invited Netanyahu to address congress on this matter, which in turn added pressure on Clinton to close ranks with Obama. But for Clinton to come out and so strongly back Obama – at a time when she has sought to distance herself from his foreign policy – cannot be explained solely by partisan solidarity.

Rather, Obama has succeeded in changing the underlying politics of the matter. The debate over Iran sanctions is no longer about Iran, but about war with Iran. Diplomacy with Iran is the best way of avoiding both a nuclear Iran, and bombing Iran. Any measure that undermines diplomacy, such as new sanctions, automatically enhances the risk of war.

Passing sanctions on Iran used to be the safest political move in Congress. But today, imposing sanctions means supporting war, which is a move that carries a tremendous political cost. So high that Hillary Clinton chose to come out against AIPAC and Netanyahu instead.

This is not to suggest that Obama has taken control over the process of lifting sanctions. That authority remains in the hands of Congress. But what the recent wrangling in Congress shows is that Obama can redefine what is politically feasible and unfeasible. Two years ago, anyone who suggested that Congress would fail to impose new sanctions on Iran would be lucky not to be committed to a mental institution. Those advocating diplomacy over sanctions were in the political margins. Today, diplomacy is the policy, while sanctions proponents are considered extremists.

Tehran should be careful not to base its negotiation calculations on yesteryear's political realities.

Trita Parsi is the author of “A Single Roll of the Dice – Obama's Diplomacy with Iran,” and president of the National Iranian American Council. The opinions expressed here are those of the author.