WASHINGTON - US President Barack Obama defended his endorsement of Israel's 1967 boundaries as the basis for a future Palestine, telling America's pro-Israel lobby that his views reflected long-standing US policy that needed to be stated clearly.

He also said Sunday that the Jewish state will face growing isolation without "a credible peace process."

Obama tried to alleviate concerns that his administration was veering in a pro-Palestinian direction, placing his Mideast policy speech Thursday in the context of Israel's security. He told the American Israel Public Affairs Committee that those border lines must be subject to negotiated land swaps and said these principles reflected US thinking dating to President Bill Clinton's mediation efforts.

"If there's a controversy, then it's not based in substance," Obama said in a well-received speech at a Washington convention center. "What I did on Thursday was to say publicly what has long been acknowledged privately. I have done so because we cannot afford to wait another decade, or another two decades, or another three decades, to achieve peace."

The event was eagerly anticipated after Obama outlined his vision for the changing Middle East at the State Department on Thursday and then clashed in a White House meeting with Israeli Prime Minister Benjamin Netanyahu a day later.

The speech came ahead of a weeklong trip for the president to Europe, where he will tend to old friends in the Western alliance and look to secure their help with the political upheaval across the Arab world and the decade-long conflict in Afghanistan.

Netanyahu said in a statement after Obama's remarks that he supported the president's desire to advance peace and resolved to work with him to find ways to renew the negotiations. "Peace is a vital need for us all," Netanyahu said.

The Israeli leader's tone was far more reserved than last week, when he issued an impassioned rejection of the 1967 borders as "indefensible" and even appeared to publicly admonish Obama after their White House meeting.

Netanyahu was to address the pro-Israel lobby Monday night and Congress on Tuesday.

Obama didn't retreat from his remarks on what it would take to reach a two-state solution between Israelis and Palestinians. Repeating a large section of his Thursday speech, he said the result must come through negotiation, and that Israeli border security and protections from acts of terrorism must be ensured. An Israeli withdrawal from territory should be followed by Palestinians' responsibility for security in a nonmilitarized state.

"By definition, it means that the parties themselves _ Israelis and Palestinians _ will negotiate a border that is different than the one that existed on June 4, 1967," Obama said. That was before Israel seized the West Bank, Gaza Strip and East Jerusalem, and a half-million Israelis settled on war-won lands.

"It is a well-known formula to all who have worked on this issue for a generation," the president said. "It allows the parties themselves to account for the changes that have taken place over the last 44 years, including the new demographic realities on the ground and the needs of both sides."

Obama's emphasis on what is meant by "mutually agreed land swaps" reflected a part of the equation Netanyahu largely disregarded when he vociferously rejected the 1967 borders as a basis for peace.

Palestinians have expressed willingness to let Israel annex some of the largest settlements closest to the demarcation, as long as they are compensated with Israeli land equal in size and quality. In the last serious negotiations in 2008, the sides split over how much West Bank land Israel would keep in the trade.

Leading Republicans seized on Obama's Mideast remarks, insisting that he was imperiling Israel's security.

"This is the very worst time to be pushing Israel into making a deal," Senate Minority Leader Mitch McConnell of Kentucky told "Fox News Sunday," citing the uncertainty in neighboring Egypt and Syria.

Republican presidential candidate Newt Gingrich said the US shouldn't apply any pressure on Israel in light of the recent reconciliation agreement between Palestinian President Mahmoud Abbas' US-backed government and the Islamic militant group Hamas, which controls Gaza.

"How do you have peace with a Hamas organization whose stated goal is the destruction of Israel and driving every Israeli out of the country?" Gingrich asked on CBS television's "Face the Nation."

Obama acknowledged that he had touched nerves by outlining his principles for peace and that "the easy thing to do, particularly for a president preparing for re-election, is to avoid any controversy." But he said peace efforts needed to gain ground quickly.

"The march to isolate Israel internationally _ and the impulse of the Palestinians to abandon negotiations _ will continue to gain momentum in the absence of a credible peace process," he said.

Obama flatly opposed a Palestinian drive to win U.N. recognition for an independent state, even without a peace deal with Israel. He did note increased international impatience with what he termed the "absence" of a peace process. Arab, Latin American, European and Asian countries may be inclined to back the Palestinian bid.

"For us to have leverage with the Palestinians, with the Arab states, and with the international community, the basis for negotiations has to hold out the prospect of success," Obama said.

Palestinian reaction to Obama's speech was mixed.

Chief Palestinian negotiator Saeb Erekat refused to address the government's reconciliation with Hamas or Obama's opposition to Palestinian efforts at the United Nations.

"I want to hear from Mr Netanyahu," he said, calling for the Israeli leader to hold peace talks according to Obama's principles. "Before he says yes, it's a waste of time to talk about a peace process."

Hamas said it wouldn't recognize the Israeli "occupation" and that it, too, rejected Obama's reference to the 1967 borders. "It is a mistake to consider the US as an honest sponsor for the so-called peace process," spokesman Sami Abu Zuhri said.

Obama was to depart later Sunday for Ireland. He also will visit England, France and Poland this week.

The trip comes amid the continued NATO-led bombing campaign in Libya and a seemingly intractable conflict between Moammar Gadhafi's forces and Libyan rebels. Talks will also encompass economic concerns, as European countries make stark cuts in public spending and Obama and congressional Republicans try to hash out how to cut spending to bring US debt under control.