In an extraordinary political comeback from virtual ruin six months ago, Prime Minister Menachem Begin's ruling Likud Party has pulled ahead of the opposition Labor Party with less than a month left before a national election, according to the most reliable public opinion polls.

The unexpected turnabout of Begin's fortunes has resulted in deep concern in the Labor Party's Tel Aviv election headquarters, where a sense of helplessness over party leader Shimon Peres' inability to seize the campaign initiative has taken hold. A senior adviser to Peres, who asked not to be identified, said, "If things continue the way they have, I have no doubt the Likud will win it. We are in an almost impossible situation."

A national poll conducted in the last week of May shows Begin's party likely to win 45 seats and the Labor alignment 43 seats in the 120-member parliament, or Knesset. The poll, by the Applied Research Center, was commissioned by The Jerusalem Post, which published it today.

In a poll by the Dahaf opinion research organization, in which voters were asked who would be the most suitable prime minister, 38 percent favored Begin and only 28 percent Peres.

In January, when Begin's fractious Cabinet averted certain collapse in a parliamentary no-confidence vote by calling for an early election, the same polling groups projected a lopsided 58- to 20-seat victory for Labor. Other polls forecast 65 seats for Labor and only 12 for Likud. Peres was favored 44 percent to 12 percent for Begin.

Even if Begin fails to maintain his momentum and wins only the 45 seats predicted now, he appears for the first time since the campaign began to be in a fair position to put together some sort of coalition for the 61 seats necessary to form a government, albeit a weak one. In 1977, Likud won only 43 seats, but by enlisting the religious parties and the former Democratic Movement for Change, it formed the present coalition government that has lasted four years, longer than any other in Israel's history.

The turnaround is due, in large part, to two factors: Begin's success in exploiting the crisis over the deployment of Syrian missiles in Lebanon, thereby overshadowing domestic issues on which he is vulnerable; and his resurgence among the once-disaffected "Oriental" voters whose origins are in Jewish communties in the Middle East, particularly North Africa.

Paralleling those factors are two distinct phenomena that have vastly helped Beginhs campaign and undercut Peres' efforts: Begin has put his once-contentious and seemingly self-destructive Cabinet under wraps, thereby neutralizing the government's image of ineptness. At the same time, fighting within the Labor Party, stemming from the years-old rivalry between Peres and former prime minister Yitzhak Rabin, continues to erode support for the opposition.

The Syrian missile crisis has been particularly damaging to Labor.

"It's simple. We can either run for Begin or run for Assad," said a Peres adviser, referring President Hafez Assad of Syria.

There is no Arab country feared and loathed in Israel as much as Syria, given the costly battles in the Golan Heights in 1973 and the episodes of torture and mutilation of Israeli soldiers that are part of the lore of those battles. Peres has been put in the position of having to support Begin's position on the threat of the Syrian missiles to Israel's security, while feeling free only to criticize Begin's management of the crisis. Even then, Peres has been cautious, apparently fearing that any criticism could be seen as lack of patriotism.

Begin has adroitly blended anti-Syrian rhetoric, which is always popular, with studied military restraint, leaving the impression of a responsible stateman standing firm against the enemy but not willing to commit Israeli soldiers to battle unless it is absolutely necessary.

Practically every night for the last month, the first 10 minutes of Israeli television news has been dominated by the prime minister:

Begin emerging from meetings with U.S. special envoy Philip C. Habib. Begin talking tough against militant Arab states in unusually frequent news conferences. Begin assuring Jewish settlers in the West Bank that he will never yield an inch of occupied territory, and subtly linking the proliferation of settlements to the Syrian menace and to Israel's security.

When the April cost-of-living index was announced last month, showing a 10.4 percent monthly inflation rate -- 230 percent on an annual basis -- the news was buried in an avalanche of Syrian crisis stories, and it caused barely a murmur of complaint.

Most noticeably, there was not even an outcry from the inflation-weary Sephardic Israelis, who make up a large portion of the country's lower-income groups. Of the approximately 2.5 million voters in Israel, about 53 percent are Sephardic, or "Oriental," as distinct from the Ashkenazi Jews of European origin.

There has been a growing polarization of Ashkenazi and Sephardic voters in Israel for a decade, with the Likud's portion of the Sephardic vote rising from 35 percent in 1969 to 45 percent in 1973 and 58 percent in 1977, when Begin was swept into office largely on the basis of those voters whose origins are in Arab countries. Having fled their ancestral homes at times of Arab-Israeli strife, the Sephardic Jews have gravitated toward the Likud and Begin's hardline stance toward Arab states.

But even their support began to wane under the effects of inflation last year and as government food subsidies fell. There were food riots in Tel Aviv and Jerusalem, followed by increasing talk that the Sephardic Jews would look elsewhere in the next election.

However, acting like the astute politician that he is, Begin named Housing Minister David Levy, a highly popular immigrant from Morocco, as his campaign manager and placed him on the second spot of the Likud ticket. Other well known Sephardic Jews also got "safe" seats on the list.

In contrast, the top of the Labor Party list is made up mostly of Ashkenazi Jews, many of whom were in the government rejected by the electorate in 1977 following a series of scandals. Peres placed a Sephardic candidate in the second spot, but polls still show the Likud capturing half the Sephardic vote, while Labor is given only 20 percent.

Complicating the ethnic issue was the defection from the National Religious Party last month by Religious Affairs Minister Aharon Abuhatzeira, following his acquittal on corruption charges. Abuhatzeira formed his own splinter party but he has indicated willingness to take his estimated four or five Knesset seats to the Likud it Begin is able to form another coalition.

Begin also introduced his trump card, Finance Minister Yoram Arridor, who slashed taxes on color television sets, appliances and other popular consumer goods, as well as restoring food subsidies and cutting taxes on incomes. The bold "supply side" program, branded by Peres as crass election giveaways that will bankrupt the economy, has been popular with most voters, particularly Sephardic Jews who have felt priced out of the market on many consumer goods.

Begin has also succeded in disciplining his fractious Cabinet -- or what is left of it after several heated resignations -- and therefore removing from the campaign government.

"Israelis have short memories. Who's going to remember the days when [former defense minister] Ezer Weizman stormed out of Cabinet meetings, tearing posters off the wall in a rage?," a Begin aide said.

Indeed, for several months, the Cabinet has been a model of unity and propriety, as Begin has restored his control over the ministers. Simultaneously, Begin seems to have come out of the shell into which he retreated in the dark days last winter when he contemplated resigning. As if aroused for a keen fight, he has become as combative and tireless in his campaigning as he was when he set out in 1977 to end three decades of Labor Party rule.

At the same time, Peres, surrounded by intraparty squabbling and recriminations about what went wrong, appears to be snake-bittern by Begin. His public appearances so far have been less than electric and his advertising campaign -- even according to his own staff -- has been a failure.

The fight over control of the parliament and government is far from over, and the 10 to 15 percent undecided vote that could tip the balance has yet to be heard from. Moreover, it is still too early to indulge seriously in that curious blend of idealism and arithmetic that is coalition politics in Israel, and which will ultimately determine whether the next government is Likud or Labor.

But with the campaign entering a critical stage, Begin has proved that those who just a few months ago laughed at his reelection dreams may not be laughing last.