The nearer we get to independence, the less chance there is for unity" among rival Eritrean separatist movements, said Tesfal Wolde-michael, 25, the No. 3 man in the executive committee of the Eritrean Liberation Front.

Sitting cross-legged on the mat-covered floor of a palm-thatched but in ELF headquarters inside Eritrea, he acknowledged with resignation: The main problem is unity, not Ethiopia."

Less than an hour's jet flight away, in the headquarters of the Eritrean People's Liberation Front (EPLF), which broke off from the ELF, members of the ruling politburo voiced the same fears, although in less outspoken terms.

Sixteen years after they began fighting what has become Africa's longest was - and perhaps its most often forgotten one - the Eritreans could be expected to be ecstatic about their growing prospects of rapidly winning independence from Ethiopia.

On paper, the situation over the next six months promises to favor their cause as never before, thanks to changing alliances in the Horn of Africa, great-power rivalries and, especially, the destructive centrifugal forces at work within Ethiopia itself.

Yet the Eritreans acknowledge privately that independence is probably a long way off because of their inability to sink their own differences despite increasing pressure from their Arab supporters to do so. Some Eritreans admit that their stubbornness may already have cost them their chance for rapid victory.

In the field, however, hardly a month goes by without their overrunning another of the dwindling number of isolated garrisons that symbolize Ethiopia's tenuous hold on the Pennsylvania-size province and its 600 miles of strategic Red Sea coastline athwart one of the world's principal oil-shipping routes.

Ethiopia's increasingly erratic Marxist military regime faces armed revolts in three other provinces, the open hostility of its Sudanese neighbor to the north and west, and the possibility of war with Somalia over Djibouti when France grants that Red Sea port colony independence in June.

Meanwhile, the United States under President Carter has cut back on military sales to Ethiopia after more than a quarter-century as its most important supplier.

The decision provoked Ethiopia to order the closing of several American offices and installations and the departure of more than 300 Americans. The move, ostensibly to protest Ethiopia's long record of violating human rights, is thought also to reflect Washington's considered judgment that the regime in Addis Ababa is no longer capable of controlling the situation.

The Soviet Union is giving increasing support to the Marxist leadership of Tehiopia, and the leaders of the two main Eritrean secessionist movements - themselves leftists - realize that within six months they can expect to face the firepower of the arms the Soviets are shipping to Addis Ababa.

The Eritrean rebels also acknowledge that they unlikely to get the massive Arab financial and military aid that could help them finish off the Ethiopian presence quickly until they unify.

Still, all major Eritrean leaders move into and out of Riyadh, Saudi Arabia's capital, trying to change the minds of the Saudis, who have emerged as the most powerful force in the Red Sea and the rest of the Middle East.

So far, the Saudis have remained adamant although neither they nor their Sudanese allies - who regularly talk up the virtues of unity to Eritreans in Khartoum - have managed to get across the message the similar divisiveness has cost the Palestinian cause dearly.

Not even Iraq - which alone is regularly providing any of the Eritrean groups with arms and money - is willing to supply its friends with heavy weapons for fear they might be used against their rivals.

The destructive but inconclusive civil war the parent Egypt (ELF and the breakway EPLF fought between 1972 and early 1975 left no clear-cut territorial borders between the two groups, but rather a potentially explosive leopard-spot pattern.

The bitterness of the civl war and the changes wrought in the independence movement over the years - from a basically Moslem revolt against Haile Selassie's Christian-dominated Ethiopia into an increasingly radical revolution led by young men determined to install a new socialist order - have more to do with the groups' refusal to unit than any ideological difference, nearly a month of conversations with Eritreans of all persuasions suggests.

Both the ELF and EPLF favor a "National Democratic Front," both subscribe to similar leftist platforms calling for the sweeping nationalizations than are common liberation-movement fare.

Both appear dominated by Marxists but officially tolerant of non-Marxists and aware that the country's situation - backward, feudal, largely rural and illiterate - rules out any overnight revolution.

Despite disclaimers by all concerned, the basic differences appear rooted in personalities and rivalry cloaked in claims and counterclaims as to relative size, significance and ideological purity.

While exact numbers are probably moot - all concerned agree that the 24,000 Ethiopian troops spread out in isolated pockets throughout the province are outnumbered by the combined Eritrean strength - the ELF is generally believed to have between 22,000 and 25,000 troops in the field and the EPLF between 12,000 and 15,000.

A third, far smaller group - baptized the Eritrean Liberation Front-Popular Liberation Forces by its leader, Osman Saleh Sabe - is thought to number no more than 2,500 to 3,000 soldiers; but it makes up for its size by its leader's unrivaled access to Arab funds.

Although Sabe's forces have not engaged in any known fighting, both the ELF and the EPLF say he has more money than either of them, apparently because he is better than either of the more important groups at tapping Arab oil-money sources.

He recently flew a dozen French journalists in from Paris, all expenses paid, and ELF officials admit that he has cut them into $150,000 in Iraqi monthly payments and introduced them to the Saudis and other potential big donors.

The EPLF, with Sabe used to beassociated, accuses him of having stashed away as much as $20 million that was rightfully earmarked for the organization. It also claims that it is only now piecing together where various monies came from over the years.

Now, although there is discernible rank-and-file grumbling at the continuing lack of unity, each of the Eritrean groups is gambling that it can outwait the others and establish paramount power.

Each group claims massive defections to its side, but none of these claims can be proved; and while the ELF and EPLF are to hold a so-called "democratic dialogue" some time this spring, no one seems optimistic that they will be able to iron out their differences.

Instead, there is an almost universal mystic belief that "the people" will force unity much as they were credited with forcing an end to the civil war in the face of massive Ethiopian repression in late 1974 and early 1975.

Meanwhile, the Eritrean remain isolated in black Africa, which historically has distrusted them as agents of the Arabs and, somewhat inaccurately, jumped their cause with such purely secessionist adventures as Katanga's and the Biafran attempt to gain independence from Nigeria.

Having started out as Arab nationalists fighting against Haile Selassie's formidable feudal empire, the Eritreans now conceive of themselves as radicals battling the crumbling military dictatorhip in Addis Ababa that proclaims itself Marxist.

The only major power interested in helping the Eritreans is Saudi Arabia; but the Eritreans are suspicious of the Saudis and their Sudanese surrogates, seeing them as acting gor Washington.

Despite President Carter's cutback of military aid to Ethiopia, Eritreans of all persuasions remain suspicious of Washington, which they blame for having denied them the right to the independent state they feel is justified historically.

After Turkish, Egyptian, Italian and British occupation, Eritrea was forcibly federated with Ethiopia in 1952 thanks to a U.S.-engineered U.N. resolution in 1950, whose chief justification was to satisfy "Ethiopia's need for adequate access to the sea."

A decade later, mounting Eritrean opposition led Haile Selassie to dissolve the federation and to abrogate clauses protecting Eritrean cultural and political autonomy.

Now cynical Eritreans say Carter's apparent change of heart is dictated by the hardnosed calculation that Ethiopian control of the Red Sea ports Assab and Massawa - reachable now only by occasional and often ambushed armed convoys - may soon vanish altogether.

Some Eritreans express the fear that even if they win independence, their new state may be swept away in the kind of political chaos that characterized Ethiopia in the 18th and 19th Centuries and that the current regime in Addis Ababa seems to be rushing toward now.

"We've fought against American F-86s an F-5s," said an Eritrean commander matter-of-factly, "and at the rate the war is going we may soon be shooting at Mig-21s."