Mr. Mitchell last visited the region in December, even as divisions between the Israelis and Palestinians hardened and hostilities deepened. Secretary of State Hillary Rodham Clinton appointed his deputy, David Hale, to take over as acting envoy.  Mr. Mitchell’s resignation comes at a time when the Middle East has been transformed by popular uprisings that have toppled aging autocrats in Tunisia and Egypt, prompted a NATO-led air war against Libya and led to harsh crackdowns in Syria and Bahrain.  The White House has announced that Mr. Obama plans to address the region in the wake of the upheaval — and the death of Osama bin Laden — in a speech on Thursday at the State Department. Coming a day before Mr. Netanyahu’s visit, it could set the stage for further disagreements. Mr. Obama was also scheduled to meet Tuesday with King Abdullah II of Jordan.  “We remain committed to peace in the Middle East and to building on George’s hard work and progress toward achieving this goal,” Mr. Obama said, though officials have said it is unlikely that the president will outline new American proposals to break the impasse, either in his speech or in his meeting with Mr. Netanyahu.  For his part, the Israeli prime minister is to visit Washington in search of support for heading off Palestinian plans to seek a vote in September at the United Nations General Assembly. The Palestinians hope for recognition of an independent Palestine with territory that includes all of the West Bank, Gaza and East Jerusalem. Mr. Netanyahu has made it clear that the turmoil in the region has heightened his country’s security concerns, making a negotiated peace an unacceptably risky gamble for now.  Last week, the two rival Palestinian factions, Fatah on the West Bank and Hamas in Gaza, signed a reconciliation agreement in Cairo, prompting a fierce denunciation from Israel, which says it cannot negotiate with Hamas, a group designated by the United States and the European Union as a terrorist organization.  American and European diplomats have insisted that any Palestinian government that included Hamas had to accept basic principles laid out by negotiators over the years, including renouncing violence and recognizing Israel. Mrs. Clinton, in Rome last week, did not slam the door on negotiations with the Palestinians that would include Hamas, but there seemed to be no intention by either side to resume substantive talks.  In Israel, the reaction to Mr. Mitchell’s departure mirrored the current state of the conflict, with Palestinian and Israeli officials blaming each other for his failure to broker a deal, though they joined in praising his work.  Zalman Shoval, a special envoy for Mr. Netanyahu who focuses on relations with the United States, said Mr. Mitchell had “made a major effort to try to move peace between Israel and the Palestinians” and deserved gratitude. “But at the end of the day, his efforts were undermined by the Palestinians’ refusal to engage in meaningful negotiations.”  Nabil Shaath, who heads Fatah’s foreign affairs department in the West Bank, said that it was Israeli behavior that needed to change. “The man was not given any support and he failed,” Mr. Shaath said of Mr. Mitchell in a telephone interview. “I don’t really blame him. He found himself without any initiative or ability to move ahead. He found himself doing a futile job. I liked the man. He is honest and hard-working, with lots of experience.”  Jackie Calmes contributed reporting from Washington, and Ethan Bronner from Jerusalem.  A version of this article appears in print on May 14, 2011, on Page A10 of the New York edition with the headline: Amid Impasse in Peace Negotiations, America’s Chief Middle East Envoy Resigns. Order Reprints| Today's Paper|Subscribe
George J. Mitchell Jr. is resigning the same day President Obama is to meet with Prime Minister Benjamin Netanyahu of Israel.