With the increasingly bitter campaign for parliamentary elections in Israel turning against him, opposition Labor Party leader Shimon Peres has begun attacking Prime Minister Menachem Begin frontally over the Syrian missiles in Lebanon and the bombing of Iraq's nuclear reactor.

Peres' strategy in the last two weeks of the campaign will be to try to frighten Israeli voters away from Begin's ruling Likud Party by baiting the prime minister into "going one step too far" in making bellicose statements about Syria and Iraq, a senior campaign aide said.

"Our backs are to the wall. "We've got to go back to the basic instinct of telling the truth about the missiles and the reactor, even if we go down in flames for it," said the Peres strategist, who asked that he not be identified. He conceded that Peres risks the stigma of attempting to sabotage the government's handling of national security matters at a time it is already under sharp attack from abroad.

In what appears to be shaping up as a watershed in the Peres-Begin contest, Peres has also decided to redirect his effort to a more direct campaign against what he calls "Begin's regime of fear."

Peres, who is trailing Begin in every credible public opinion poll, took up the new approach in earnest today in a news conference, saying the Syrian surface-to-air missiles deployed in Lebanon the day after Israeli jets shot down two Syrian helicopters in April are still in place, and that the helicopters were downed for nothing.

The bombing raid on the nuclear reactor near Baghdad, the Labor Party leader said, unncessarily isolated Israel in world opinion, before serious efforts were made to neutralize an Iraqi nuclear weapons capability through diplomatic efforts. The purpose of the attack, he said, was to exploit a national security issue for electoral gain.

Peres accused Begin of fostering a "personality cult," and said, "from now on, we don't consider the government a government, but a Lukud election headquarters."

Peres conducted his press conference in Hebrew only for a domestic audience, saying that to answer questions in English for foreign television, as he usually does, could be interpreted as attempting to undermine Israel's position abroad. Later, in an Israeli radio interview used in overseas broadcasts, Peres said Israel must insist upon the removal of the missiles, "preferably through diplomatic means," but he refrained from attacking Begin directly.

Meanwhile, the Labor Party's campaign against Begin's economic policies suffered a setback when the government announced that the cost of living had increased only 3.3 percent in the month of May, the lowest increase in more than two years. Labor Party economists charged that the figure was kept low artifically by pumping treasury reserves into subsidies and tax-cut "giveaways" to reduce prices for campaign purposes.

Labor Party sources said the shift in campaign strategy, which they described as a "fundamentally new approach," had been under consideration for several days, but that Peres had opposed it for fear of being branded "unpatriotic" in the face of overwhelming public support for Begin's tough stance on both issues. For the most part, Peres had confined his criticism of Begin on the missiles issue to peripheral questions of tactical handling, and on the reactor bombing to Begin's loquacity in discussing in public classified details of the raid.

A catalyst for the decision to focus the campaign more directly on the two issues, a Peres adviser said, was Begin's suggestion at an election rally last night that he was on the verge of ordering the armed forces to strike the missiles in Lebanon if U.S. special envoy Philip C. Habib cannot assure him on Wednesday that they are about to be removed.

There is a growing perception in Labor Party headquarters that Begin's stridency of the last few days has increased fears among many Israelis that he may be leading the country into another war.

If Peres can draw out of the prime minister in the weeks ahead even more bellicose statements, the Peres strategists said, "then maybe we can scare the undecided vote away from him and pull even." The thinking in Labor campaign headquarters is that if the strategy fails, Peres could at least hope that a new Begin-led parliamentary coalition will collapse a few months after being installed, and that the Labor Party could recoup its loss in another election.

Even for Israel's usually rough-and-tumble political campaigns, this year's contest has been especially bitter and occasionally violent.

In a Labor rally last night in Petah Tikva, men identified by police as Likud supporters rolled barrels of burning garbage into the crowds and brandished knives, shouting, "Begin, Begin, king of Israel!" Peres accused the Likud of using "quasifascist" techniques, and asked rally participants, "Do you want this Khomeinism to take over Israel with idol worship?"

For his part, Begin enlisted the Cabinet to draft an official communique accusing the Labor Party of encouraging the "enemies of Israel." In a speech to the Cabinet, read to reporters later by his Cabinet secretary, the prime minister said he had not yet accused Peres of treason, but that there has been "something of sabotage" in Labor's statements.

Israeli police have reported numerous complaints by Labor Party campaign workers that they have been beaten up by Likud backers and that campaign booths have been burned. The Kilud has accused the Labor Party of provocation and of participating in similar violent acts.

Interior Minister Yosef Burg, who is in charge of the national police force, today deplored the wave of campaign violence, and said more police will be detailed to campaign rallies.

He blamed local campaign organizers, and said, "There are certain speakers who are tuned in on the violent wavelengths. . . . There are certain audiences where more violent, aggressive, offensive talk is more effective, and this is where it happens."

In last night's Petah Tikva rally, according to police, 18 persons were injured and 26 were arrested. In another rally in Ashkelon, Labor candidate and former foreign minister Abba Eban was repeatedly interrupted by hecklers he identified as Likud backers.

Begin, along with other party officials, today appealed to the elections commission to take steps to stem the campaign rally violence.

The league has submitted a draft resolution to the U.N. calling for an embargo of arms, technical and economic aid that Israel could use against Arab states. The issue was debated Monday by the U.N. Security Council; a vote is expected later this week.

Habib's diplomatic efforts on behalf of President Reagan started last month. Syrian-Israeli tensions, bringing the two countries near the brink of war, started April 28 after Israel's warplanes shot down two Syrian helicopters over eastern Lebanon and Assad ordered the SA6 missiles in the next day.

The Syrians had been engaged in fierce battles with Lebanon's right-wing Christian militias in the Catholic city of Zahle and Beirut, the capital. Israel supports the Christians because they oppose the Palestinian presence in Lebanon.

Officials who worked out a cease-fire between Christian militias and the Syrian forces in Lebanon under Arab League sponsorship were expected to meet later this month in Riyadh, Saudi Arabia on a solution to the Lebanese problems stemming from the 1975-1976 civil war.

The Syrians were sent in to enforce a truce in the war, in which Christian militias fought against an alliance of leftist Moslems and Palestinians.

The foreign ministers of Kuwait, Saudi Arabia and Syria, an Arab League representative and Lebanon's President Elias Sarkis, a Christian are the negotiating parties.