This gives Romney a leg up on Obama who has refused to say that he would back Israel's play on Iran if it comes to that.
"If Israel has to take action on its own, in order to stop Iran from developing the capability, the governor would respect that decision," foreign policy adviser Dan Senor told reporters ahead of the speech, planned for late Sunday near Jerusalem's Old City.
Senor said Romney is careful to note the governor believes preventing nuclear "capability" -- not just a nuclear weapon -- is critical.
He later clarified his comments in a written statement, saying: "Gov. Romney believes we should employ any and all measures to dissuade the Iranian regime from its nuclear course, and it is his fervent hope that diplomatic and economic measures will do so. In the final analysis, of course, no option should be excluded."
Democratic U.S. President Barack Obama has also affirmed the right of Israel to defend itself, but in contrast to Romney, Obama has warned of the consequences of an Israeli strike on Iran.
"Already, there is too much loose talk of war," Obama told the American Israel Public Affairs Committee in March. "Now is the time to let our increased pressure sink in and to sustain the broad international coalition we have built."
Pentagon officials have spoken publicly about the difficulty of such a strike and American officials have expressed concern about the destabilizing effect such military action could have in the region, even if carried out successfully.
Romney, like Obama, believes the option of a U.S. attack should also be "on the table." He has said he will do "the opposite" of what Obama would do in his approach to Israel.
"Make no mistake: the ayatollahs in Tehran are testing our moral defenses. They want to know who will object, and who will look the other way," Romney plans to say later Sunday in a speech in Jerusalem. "My message to the people of Israel and the leaders of Iran is one and the same: I will not look away; and neither will my country."
The key difference -- and the one that matters most to the Israelis -- is that Obama would act militarily only if Tehran actually built a weapon. Proof of that would only come with some sort of test by the Iranians -- something that they aren't stupid enough to do.
Romney, on the other hand, agrees with the Israeli position that the Iranians must be prevented from developing the capability to construct a weapon. There are several red lines involved in this policy and will be just as difficult to judge as the Obama position. But the tripwire is thinner and there are some in Israel who believe the Iranians have already crossed the threshold.
If Israel decides to act, they are going to need American cover at the UN and in the international arena. Romney just may have signalled a willingness to provide that cover under most circumstances. It is a significant development and will no doubt be included in Israeli calculations as a decision is being made about what to do with Iran.