President Reagan yesterday rejected Democrats' demands that he delay the reflagging of Kuwaiti oil tankers in the Persian Gulf, and Democratic congressional leaders predicted that Congress would not block the increased U.S. military presence in the region.

The Senate Foreign Relations Committee approved legislation to delay the reflagging, and the House is expected to consider a similar proposal next week. But the congressional leaders indicated that the measures are mostly an attempt to go on record against the administration policy rather than stop it.

"We don't honestly think, given the time schedule, that it is possible to prevail . . . to effect a delay in this operation," Senate Majority Leader Robert C. Byrd (D-W.Va.) said. "As a practical matter, this policy has been announced," he added, and "it is in the process of being implemented."

Reagan listened to objections to the reflagging plan at a White House meeting with congressional leaders of both parties and, minutes later, presidential spokesman Marlin Fitzwater read a statement to reporters.

Written in advance, it said Reagan is "moving forward with preparations" to put 11 Kuwaiti tankers under the U.S. flag and provide them with U.S. naval protection.

Fitzwater said the preparations would be ready by mid-July, "at which time we plan to proceed." Navy officials said the arrangements will be complete by Tuesday. The president also announced that he plans to undertake a renewed diplomatic initiative aimed at reducing tension in the region and ending the Iran-Iraq war.

Fitzwater said that Reagan has urged the U.N. Security Council to meet before the middle of July to vote on a cease-fire resolution and that the United States will be "consulting" at the United Nations on a second resolution calling for sanctions against any nation refusing to comply with the cease-fire terms. The first resolution has U.N. support, but the Soviet Union and China have opposed the second.

Fitzwater announced that Reagan is sending U.N. Ambassador Vernon Walters to "several capitals" for talks on the resolutions and that Secretary of State George P. Shultz is being sent to the U.N. Security Council meeting.

Meanwhile, a Washington Post-ABC News Poll shows that Americans strongly support the principle of a military presence to protect U.S. interests in the region but remain divided about the reflagging plan.

The nationwide telephone survey of 1,506 people between June 25 and June 29 found that 79 percent of those questioned favor maintaining a military presence to protect U.S. interests in the region, with only 20 percent opposed. Of those supporting the military presence, 43 percent said it is important enough to risk war while 56 percent said it is not.

Asked whether the United States should protect the reflagged Kuwaiti tankers even at the risk of being attacked, 49 percent agreed and 49 percent disagreed. This is a small decline in support since a survey a month ago.

The survey also found that Republicans favor the reflagging plan by 60 to 39 percent, while Democrats oppose it by similar margins. Men favor the reflagging by 61 to 38 percent, while women are opposed by 59 to 39 percent.

Defense Secretary Caspar W. Weinberger told reporters that Kuwait has agreed to provide oil for U.S. naval forces in the region and provide bases for helicopters involved in mine sweeping. He also said that Oman has agreed to provide military access rights and that Saudi Arabia has agreed to fly an additional sophisticated radar plane.

The White House took the unusual step of bringing Weinberger, Shultz, CIA director William H. Webster, national security adviser Frank C. Carlucci and Adm. William J. Crowe Jr., chairman of the Joint Chiefs of Staff, before reporters after Reagan rejected the Democratic appeals.

Webster said the U.S. reflagging could increase the threat of terrorist attacks sponsored by Iran, but Weinberger brushed this aside, saying Iran has sponsored terrorism worldwide and "they do that without regard for any shipping or any flagging or any other event."

After meeting with Reagan, Democrats all but abandoned efforts to delay the reflagging proposal, in some cases because they said it would further erode Reagan's credibility, already damaged by the Iran-contra affair.

House Speaker Jim Wright (D-Tex.) said Senate and House leaders urged Reagan to delay reflagging "because of concern that we are on one side of the war and we slip and slide our way into the role of an active participant."

He said Reagan responded that delay would embolden the Iranians. And, he said, "there is not, on the part of any of us, any disposition to leave the president of the United States bereft in the face of the world."

Senate Armed Services Committee Chairman Sam Nunn (D-Ga.), a critic of reflagging, said he doubts that the Democratic-controlled Congress would approve a binding measure blocking the reflagging, although it may pass some measure expressing dissatisfaction.

He said it would be "not good for America and not good for the young people we're sending over there . . . to have a bitter battle back home" over the policy.

Meanwhile, Democrats on the Senate Foreign Relations Committee, led by Claiborne Pell (R.I.) and Alan Cranston (Calif.), defied their leadership and voted to bar for one year registration under U.S. flags of any vessel belonging to a Persian Gulf nation.

Pell and Cranston conceded that they probably do not have enough support to override a presidential veto but said they want to go ahead with the bill to show the country where Democrats stand.

Also yesterday, an Iranian gunboat missile struck a Kuwaiti cargo ship, starting an engine-room fire in the third such gulf attack in the last four days, according to United Press International.

Staff writer David B. Ottaway contributed to this report.