Gov. Romney mentioned a letter, signed by 38 Democratic senators, pushing back on a tense relationship with Israel. It was sent in Secretary of State Hillary Clinton and organized by AIPAC back in April 2010. A short excerpt:
We recognize that our government and the Government of Israel will not always agree on particular issues in the peace process. But such differences are best resolved amicably and in a manner that befits longstanding strategic allies. We must never forget the depth and breadth of our alliance and always do our utmost to reinforce a relationship that has benefited both nations for more than six decades.
Ben Smith has more background here.
Source: Ezra Klein