The main person pushing this idea is Israeli Prime Minister, Benjamin Netanyahu. Israeli political leaders have been warning for more than 20 years that Iran is at a "breakout capability" for developing nuclear weapons.
It has developed into a boy-who-cried-wolf type scenario.
In 2012 Netanyahu stood before the UN General Assembly and promised that Iran were "within months" of developing a nuclear weapon.
In the most recent speech to US congress he said that negotiations with Iran were "paving the way for an Iranian nuclear weapon in a matter of weeks".
Netanyahu mentioned in his last speech to congress to ignore intelligence agencies, and to "just Google it". The reason why he doesn't want to rely on intelligence estimates is because according to both the US intelligence estimates and Israeli intelligence estimates Iran is not on a path to enrich uranium for the development of nuclear weapons, and in fact they've concluded that Iran have not even pursued research in that regard since 2003.
So what we keep seeing from Netanyahu in his speeches to the US Congress and the UN General Assembly is the same old rhetoric and fear mongering.
The fact that Iran has allowed the UN Nuclear inspectors to visit their uranium mines shows the level of ongoing cooperation Iran is willing to conduct, and that in itself shows these theories to be extremely weak.
Netanyahu's insistence that Iran have no nuclear capability whatsoever including those that are illicit for it to have and to develop under the Nuclear Non-Proliferation Treaty points to the hypocrisy of his stance, since there are four nuclear rogue states who refuse to sign the treaty, one of those ironically is Israel.
The reason Israel refuse to be part of the Nuclear Non-Proliferation Treaty is because it has its own undeclared arsenal of an estimated 80 nuclear weapons. The fact that US Congress and Netanyahu refuses to to address this issue while trying to punish Iran who is part of the Nuclear Non-Proliferation Treaty and is developing legitimate civilian nuclear energy under this signed treaty is hugely hypocritical.
Whats even more hypocritical is the stance the US has taken when it was the only nation in the entire history of mankind to actually use nuclear bombs on an entire population of civilians, twice, and has not been held accountable for the atrocities it inflicted in arguably the worst terrorist attack humanity has ever seen.
[cited: Josh Ruebner, Jewish-American author, political analyst]