WASHINGTON (AP) _ President Bush, saying that Middle East peace negotiations might be in jeopardy, declared today that he would use his veto authority if necessary to delay action on Israel's call for $10 billion in loan guarantees to help house a flood of Soviet immigrants.

Bush said the world was on ''the brink of a historic breakthrough'' that could launch direct negotiations between Israel and its Arab neighbors. He told a hastily called news conference he was taking his action in the interests of Middle East peace.

Arab nations fear the loans could be used to settle immigrant Jews in disputed territory. Bush said he wanted to avoid a ''contentious debate that would raise a host of sensitive issues'' that ''could well destroy our ability to bring one or more of the parties to the peace table.''

Talks are expected to begin in October, but no time or place has been established. Bush said he would not say which country he thought might balk at attending a peace conference if Congress entered a debate on loan guarantees.

House Speaker Thomas Foley, asked whether the United States should link the housing guarantees with Israeli concessions at the peace table, said, ''No.''

Bush said he has ''absolutely not'' made any commitment that he will support loan guarantees even if the 120-delay is agreed to. He said such a commitment in advance would undermine his efforts for peace, although he noted his support in general for helping Israel absorb the immigrants.

Bush's veto threat seemed more symbolic one than real if his goal was to prevent debate. Foreign aid legislation is pending in Congress, and Bush could veto that bill or any other that reaches his desk with the loan guarantees included. But he has no Constitutional power to prevent Congress from debating any issue it chooses.

Bush also:

- Sidestepped a question on whether he believes the release of a western hostage in Beirut may be imminent. But he said Israel's recent release of 51 Arab prisoners ''is bound to be viewed as very, very favorable.''

- Said Supreme Court nominee Clarence Thomas is handling himself ''very very well'' in his confirmation hearings, and he is ''more confident than ever that he will be confirmed.''

- Defended CIA nominee Bob Gates anew, and said he had given ''absolutely no consideration for withdrawal because there's no reason for withdrawal.'' He noted ''feathery charges'' against him stemming from the Iran Contra affair, but said they are nonsense. Gates confirmation proceedings are set to begin on Monday.

Bush opened his news conference with a strong statement on his request for a delay in the loan guarantees, which Israel wants to help absorb tens of thousands of Soviet immigrants.

''I've worn out the phone in there,'' Bush told reporters, a reference to the lobbying he has done. The president said he was battling ''powerful political forces,'' in the controversy, but he found no fault with the pro- Israel lobby arguing on behalf immediate action for loan credits.

He spoke as supporters of Israel were fanning across Capitol Hill in hopes of persuading Congress to reject the delay.

Secretary of State James A. Baker III is due to return to the Middle East

in the next few days, Bush noted, in his efforts to convene a peace conference among Israel and Arab nations.

''Too much is at stake to let domestic politics take precedence over peace,'' Bush said.

Israeli Prime Minister Yitzhak Shamir says the United States has a moral obligation to provide loan guarantees to help pay the cost of absorbing tens of thousands of Soviet immigrants.

Some key congressional officials initially signalled their approval for a delay when Baker first mentioned the subject last week. But since then, others have said they might not be so quick to go along.

Bush met privately with members of Congress on Tuesday to discuss the subject. He promised to make up to Israel any losses caused by putting off the issue until next year.

''I'm committed to seeing that they (the guarantees) get considered. And we generally have been quite supportive of the idea of absorption (of Soviet immigrants by Israel.)...And, in principle, this concept of helping, we want to do it,'' Bush said in one meeting with lawmakers this week.

''I'm talking about world peace,'' Bush said today in pushing for delay.

He stressed that his request for a delay did not signal a diminished interest in helping Jews emigrate to Israel, or in support for the Jewish state itself. He noted that American troops had defended Israel during the Persian Gulf war from attacks by Iraqi Scud missiles.

And he said he had worked hard as vice president and president to facilitate the emnigration of Ethiopian and Soviet Jews to Israel.

Bush denied that there was any ''1992 politics'' involved in his view.

''We don't want a contentious debate on settlements or anything else at this juncture. We want to get these parties to the table. I don't think it's asking too much to have a 120 day delay, he said.

Bush said the United States had been Israel's ''closest friend in the world'' for more than 40 years, ''and will be as long as I'm president.''