A key Senate Democrat drew a stark contrast with the Obama administration over its diplomatic engagement with Iran on Tuesday, in a sign of lingering tensions in some corners of the party.

In remarks at the annual AIPAC policy conference, Sen. Bob Menendez (D-N.J.) forcefully denounced the international nuclear accord reached with Iran last year, which President Obama has described as a pillar of his legacy.

Menendez was one of four Senate Democrats who voted unsuccessfully to kill the agreement last autumn, and made clear on Tuesday that he will continue to be a thorn in the administration’s side on the issue. 

“So long as I have a voice and a vote, I will not yield to those who wish to break my resolve on stopping Iran’s illicit nuclear program and on preserving the unshakable bond between Israel and the United States,” Menendez, the former top Democrat on the Senate Foreign Relations, said during the final day of the pro-Israel lobbying group’s Washington conference.