The unleashing of intense Zionist emotions under the banner of Eretz Israel - a larger, Old Testament Israel - is becoming a threat to President Carter's Mideast plans, with Prime Minister Menahem Begin cast as a modern Joshua.

Israel's new and immensely popular leader confides to intimates that he now knows he understimated Carter's angry reaction to the establishment of new Jewish settlements inside the Israeli-occupied Palestinian West Bank. But whatever Begin says privately, his government's blueprint for comprehensive, systematic movement of Jewish population into the West Bank seems to continue with a life of its own.

The plan, as explained in interviews and speeches by Arik Sharon, the man in charge of a new settlements, is not modest. It assumes population resources far beyond Israel's present capacity: urban industrial centers in the heart of the Arab West Bank, surrounded by satellite agricultural settlements.

This would require the movement of tens of thousands from the thickly populated coastal cities on the Mediterranean to the highlands of Judea and Samaria and a new network of highways. The object: virtual isolation of the centers of Arab poplation (about 1 million people) like Nablus, Ramallah and Hebron.

Whether Israeli voters would sanctify these colonization plans in a referendum is highly questionable. Certainly, the plans would die aborning in any conceivable peace settlement that might emerge from Geneva. But despite Begin's clear reluctance to press ahead right now, with Carter and the whole world demanding a halt, the plan to colonize the West Bank in the name of security seems to be proceeding against the prospect of a political settlement, which many political leaders here feel may be beyond reach. Should a political settlement fail, Sharon's ringing words to the Jerusalem Post in an interview last month will point the way: Colonizing the West Bank will be done because it has to be done.

The effect of this profound Zionist response to centuries of the Diaspara is hard to measure, but at the least Begin and Sharon have touched a past chord in the Jewish phyche that has reinforced the almost mystical drive to settle Eretz Israel.

It is a vision a young woman named [WORDS ILLEGIBLE] told us in the brand-new hilltop settlement of Beit Homotaim near the Arab village of Sanur. Army trucks and a front-loader had already completed an encirclement road around the windy, bleakly beautiful hilltop; gravel and sand were ready for poring; 21 tents were staked along one side of an old Jordanian police post; three prefab shelters, each for three families, were in place; a makeshift kitchen was serving lunch to the 23 children of Beit Homotiam; the inevitable barbed-wire fence was up.

"It is a vision and it will stay with us forever," Yacoov said. Most of her group (but not she) are members of the Gush Emunim, ardent religious believers in Eretz Israel. Evicted by the army three weeks ago when they first tried to settle, they returned one night and planted the Israeli flag on top of the hill. After personally interviewing the settlers, Sharon approved the settlement. He himself came to dedicate it.

But what if there is peace and they are out? Could this be only a temporary settlement?

"We say in Israel whatever is temporary is the most permanent," Yacoov said. "And do not forget," she went on, "that Begin promised the liberated West Bank would be open to all Jews anywhere." Did she vote for Begin last May?

"I was brought up to hate Begin," she said. "Whenever he came on the radio, it was turned off in my house. But I myself voted for him. My parents did not understand. But I understand. History tells us wherever the Jewish people settle, that will be the border."

With rare exceptions, that conviction is universal in the settlements we visited: the beginnings of the first industrial park between Jerusalem and Jericho, the army camps on the new, unpaved Allon Road along the spine of hills through the center of the West Bank, and in the agricultural fields of the Jordan valley.

The politicians claim all settlements that must be terminated in an Arab-Israeli peace will be terminated - whatever the cost of the Yael Ben Yacoovs who believe what they have been told. If so, the government's approval of settlements will end in cruel disillusionment.

But a far graver question is raised: whether the only peace Israel will approve is the one that must treat these settlements as permanent. That would be the kind of peace the Arab would never accept.