President Obama and Vice President Joseph R. Biden Jr., in separate interviews this weekend, said that the accelerating crackdown on opposition leaders in Iran in recent days would not deter them from seeking to engage the country’s top leadership in direct negotiations.In an interview with The New York Times, a day before his scheduled departure for Moscow on Sunday, Mr. Obama acknowledged the arrests and intimidation of Iran’s opposition leaders, but insisted, as he has throughout the Iranian crisis, that the repression would not close the door on negotiations with the Iranian government.“We’ve got some fixed national security interests in Iran not developing nuclear weapons, in not exporting terrorism, and we have offered a pathway for Iran to rejoining the international community,” Mr. Obama said.Mr. Biden echoed the same themes in an interview conducted in Iraq and broadcast Sunday on the ABC News program “This Week.” But in a rare foray into one of the most sensitive issues in the Middle East, the vice president argued that the United States cannot order Israel not to strike the plants at the heart of Iran’s nuclear program if Israeli leaders determine “that they’re existentially threatened” by the prospect that Iran would gain nuclear weapons capability.