Unqualified bipartisan support for Israel's bombing raid on Iraq's nuclear reactor began to crumble today as opposition Labor Party leader Shimon Peres accused Prime Minister Menachem Begin of timing the mission to enhance his reelection prospects.

While stressing that he supports the bombing raid as essential to eliminating a major threat to Israel's security, Peres accused Begin of showing a "lack of responsibility" in failing to explore the diplomatic potential inherent in the charge of governments in France.

The increasingly bitter debate today came even as Israeli sources said the Jewish state was taking extra security precautions against the chance of an Iraqi retaliatory strike.

Israeli defense officials refused to discuss the security preparations. But they are understood to include heightened air defenses as well as closer monitoring of the country's eastern borders.

Israeli installations abroad were also said to be more alert for terrorist activities by those Palestinian guerrilla organizations controlled by Iraq.

As the political rivalry deepened today, Peres said that before the election of French President Francois Mitterrand, he talked with Mitterrand about France's sale of nuclear reactors to Iraq, and that the Socialist leader said he was opposed to giving Iraq any nuclear military option.

For his part, Begin accused the Labor leader of misleading the public about Peres' position on the reactor bombing by claiming opposition only to the timing of the attack. Peres had said he wrote Begin a letter last month urging that the raid not be conducted on May 10 because it would coincide with the French elections.

At a meeting of the parliament's Foreign Affairs and Defense Committee today, Begin distributed copies of Peres' letter -- dated May 10 -- which indicated the Labor Party leader's opposition to a bombing raid on Iraq at anytime.

"I speak from experience, and there are others who think the same way as I do," Peres is quoted in the letter as saying. "Israel would become isolated and lay herself open to similar attacks."

Begin tonight said in a statement that "Peres made an attempt that could not possibly succeed in justifying his misleading the foreign affairs and defense committee when he said that he had not opposed bombing the reactor in principle and only expressed opposition in carrying out the operation on French election day.

"The prime minister has proved in a document written by Mr. Peres that he definitely expressed opposition in principle to bombing the reactor near Baghdad, and the French election day was not mentioned in his letter at all." b

"As for the rest of the slander and vulgarity of Mr. Peres, it needs no reply," the statement added.

Peres tonight bristled at Begin's release of the letter, saying it was "more proof" that the timing of the air strike was politically motivated.

In a news conference, Peres said he had been opposed in principle to conducting the bombing raid until "all diplomatic avenues" had been exhausted. He said that because of his own good personal relationship with Mitterrand, he proposed going to Paris in an effort to convince the French leader to supply Iraq only with nonmilitary-grade uranium.

The Labor Party candidate said that because of the complicated international situation following the raid, he did not express his opposition to the timing and that Begin took advantage of his discretion for political purposes.

On Tuesday, Peres told reporters he expressed his admiration to the Air Force for a "brilliantly" planned and executed mission, but he did not comment further.

Candidates of all political parties running in the June 30 national election have supported the principle of the bombing mission, apparently mindful that to do otherwise would be regarded as unpatriotic in the face of the mission's overwhelming popularity.

Former foreign minister Abba Eban, Labor Party candidate for parliament, also criticized the timing of the raid, and said it was irresponsible of Begin to have invited Egyptian President Anwar Sadat to a summit conference in Sharm al-Sheikh last Thursday, three days before the bombing raid. The proximity of events, Eban said, caused Sadat unnecessary embarrassment in the Arab world.

Labor Party spokesmen noted Begin cannot but benefit from the wave of international condemnation of Israel for bombing the reactor, since it is an aciom of Israeli politics that the more foreign criticism an Israeli leader is subjected to, the higher his ratings in public opinion polls at home.

Meanwhile, sources close to the prime minister said that Iraqi President Saddam Hussein had been informed through indirect channels that reprisal attacks would be answered swiftly and forcefully by Israel.

"here and there, we have found the way to let them know. I hope they got the message. They must realize that if we can change the Middle East in two minutes, they better keep quiet," said a close adviser to Begin. He was referring to the length of time the Israeli military command said it took Israeli fighter-bombers to destroy the 70-megawatt, French-built nuclear reactor near Baghdad Sunday.

The Israeli Army chief of staff, Lt. Gen. Rafael Eitan, in a television interview last night, said that he had taken into account the possibility of Iraqi retaliation. But he did not specify what defensive steps Israel has taken.

Equipped with advanced Soviet-made Mig25 fighters and Tu22 fighter-bombers, Iraq could be contemplating a rapid-penetration, low-level strike in Israeli territory in hopes of avoiding radar detection on the way. However, Israel's air superiority and sophisticated detection network would pose sepcial hazards to such a mission.

Iraq also has Soviet-made Scud ground-to-ground missiles that could be deployed on launchers near the Iraqi-Jordanian border, about 240 miles form the Israeli frontier. One Scud model used against Iran has a range of approximately 180 miles, but a more advanced version believed by the Israelis to be in the Iraqi arsenal is effective for about 240 miles.

However, because Iraq has never engaged Israel in a direct, large-scale military clash -- limiting its 1967 effort to the deployment of an Army brigade in Jordan -- some military officials believe the most likely form of Iraqi reprisal would be a terrorist strike against an Israeli installation abroad, possibly in Europe.

"Rejectionist front" Palestinian guerrilla groups backed by Iraq include the Arab Liberation Front, established by the Iraqi Baath Party in 1969; the Popular Palestinian Struggle Front, and the Palestinian Liberation Front.

Israeli sources said that embassies abroad, as well as airline offices and other Israeli businesses, had been warned to be alert for possible terrorist action as a result of the reactor bombing.