Former senator George J. Mitchell Jr., the veteran dealmaker picked by President Obama to lead Palestinians and Israelis to the peace table, has resigned his post in a move seen as emblematic of the frustrations and disappointments of the administration’s two-year effort to revive the Middle East peace process.

Mitchell, who brought warring Irish factions together for the 1998 Belfast Peace Agreement, told the White House he would step down next week as special envoy on Middle East peace, a job he was given amid high expectations on Obama’s second day in office.

He gave no official reason, but a White House statement on Friday said the 77-year-old former Senate majority leader was returning to his family after 28 months of difficult, and ultimately unsuccessful, negotiating.

“He took on the toughest job imaginable and worked grueling hours to advance the interests of the United States and the cause of peace,” the statement quoted Obama as saying. It said Secretary of State Hillary Rodham Clinton had appointed Mitchell’s chief deputy, David Hale, as acting envoy.

Whether intentionally or not, the shift came just as the White House was preparing for what is likely to be a pivotal week in Middle East diplomacy. Obama is scheduled to deliver a major policy address on the region on Thursday, followed by talks with Israeli Prime Minister Binyamin Netanyahu at the White House. Netanyahu’s visit to Washington will include a speech to Congress that will lay out Israel’s views on a possible peace deal.

Administration officials praised Mitchell’s efforts and stressed that his departure would not lessen White House resolve to work toward a peace settlement. Deputy national security adivser Denis McDonough said the decision to quit — first communicated in a letter to the White House on April 6 — was Mitchell’s alone.

“He made clear when he and the president first talked about the job that he would take on the assignment for two years,” McDonough said. “And now being north of two years, he made an assessment that he wanted to make good on what he had talked with president about.”

McDonough said the administration would “remain as committed as we have been since day one” on securing peace in the region. And he deflected suggestions that the administration’s efforts have been fruitless with Mitchell at helm. In early 2009, when Obama took office, the Gaza war was just winding down and neither side had spoken to the other for months, he noted.

“We have made good progress here on critical security improvements for the Israelis,” McDonough said. “We have continued to work very closely with the Palestinian security and institution-building efforts.”

Still, administration officials have acknowledged deepening frustration over the lack of progress in restarting peace talks, which resumed for a couple of weeks but broke down after Israel’s partial moratorium on new West Bank settlements expired. In recent months, as many of Israel’s Arab neighbors were caught up in the Arab Spring movement, Mitchell’s once-frequent visits to the region ended.

Palestinian leaders, sensing that the U.S. peace efforts had reached a dead end, turned their attention to an effort to have their lands recognized by the U.N. General Assembly as an independent Palestinian state. The U.N. vote in September would be largely symbolic and is strongly opposed by Israel.

Many longtime Middle East observers see little chance that any of the initiatives will lead to negotiated settlement. Meanwhile, with “no active peace process and no chance of one,” the continuing presence of a high-profile U.S. envoy is viewed as “awkward and incongruous,” said Aaron David Miller, a former Middle East peace negotiator and public policy scholar with the Woodrow Wilson International Center for Scholars.

“The sad reality is that the administration threw a highly talented envoy at a problem for which it never developed an effective strategy,” Miller said.

As the Obama administration’s point man in contacts with Israel on peace negotiations, Mitchell received a mixed reception in Jerusalem. He carried warnings to Israel against further settlement building in the West Bank and at one point even suggested that Washington could withhold loan guarantees if Israel did not change its policy. When relations with the White House soured over continued Israeli construction, Mitchell conveyed American displeasure even as he worked with the Israelis to salvage the negotiations.

Among Palestinian officials, Mitchell was perceived as representing a more even-handed approach to the conflict with Israel by the Obama administration as compared with the policies of its predecessor. The administration’s initial demand for an Israeli settlement freeze was adopted by the Palestinians, who now condition the resumption of direct negotiations on a suspension of further Israeli building in the West Bank and East Jerusalem.

“What’s needed,” said Jeremy Ben-Ami, president of the pro-Israeli advocacy group J Street, “is not a new envoy, and not a new initiative, but a firm decision from the administration that the status quo doesn’t advance American interests and that it’s time to engage more actively to help end the conflict.”

Special correspondent Joel Greenberg in Jerusalem contributed to this report.