President Reagan yesterday postponed his California vacation and will remain in Washington to pressure Congress to enact his tax and budget legislation, which includes the embattled three-year, $98.9 billion tax increase package.

He made the decision as Senate Finance Committee Chairman Robert J. Dole (R-Kan.) expressed concern that a rebellion among conservative House Republicans could scuttle the tax increase, which is now in a House-Senate conference.

"If the House doesn't follow through on the revenue bill, we've in effect destroyed the president's credibility in economic policy," Dole said.

"Some of my House Republican colleagues are sort of leading the charge to undermine the president," he added. "It reminds me a little bit of the early attacks made on Lyndon Johnson in the Vietnam war."

Some Senate Republicans also are jumpy, including Sen. John P. East (N.C.), who voted for the tax increase but indicated that he would vote against it when it comes out of conference.

Reagan's decision to delay the 14-day vacation, which was scheduled to begin next Wednesday on his mountaintop ranch near Santa Barbara, reflected not only the difficul- ties he is facing on the tax bill, but broader concerns as well. Deputy White House press secretary Larry Speakes said the crisis in Lebanon "had a bearing" on the decision.

Reagan has made no secret of the pleasure he gets from retreating to his secluded ranch, but some presidential aides were concerned that he not be vacationing if crises in the Mideast and on Capitol Hill spin beyond his control.

The president is said to be aware that newspaper cartoons have made light of his many trips to the ranch. "Doonesbury" cartoonist Gary Trudeau correctly noted recently that Reagan had split this year's planned month-long sojourn into two trips to mute the image of a remote, vacationing chief executive.

Speakes said the second half of the vacation would be delayed a week "or longer if necessary" because Reagan wanted to remain "at the forefront" of the effort to push the tax increase and companion spending cuts through Congress. Reagan would continue playing "a vigorous role" in lobbying House members for its approval, Speakes said.

There was fresh evidence yesterday that the thin margin of victory that carried the tax increase through the Senate July 22 on a 50-to-47 vote was endangered.

East, one of three senators who switched their votes at the last minute to assure victory in the Senate, said he will now oppose the tax boost. Sen. Harrison H. Schmitt (R-N.M.), who also voted for the Senate bill, signaled this week that he likely will switch and oppose it.

"It's a bad bill . . . . It's a fundamental reversal of the whole trend of what we were trying to do last year," Schmitt said. These and other signs of discontent threaten the conference report when it comes back to the Senate floor.

The president has brought 75 House Republicans to the White House over the last three days to emphasize his support for the tax bill, but White House officials said they believe the conservative revolt against higher taxes led by Rep. Jack Kemp, (R-N.Y.) has not been checked.

"We have indications it will be a difficult vote. We are starting with a number of votes against" the tax bill, Speakes said.

Kemp's campaign this week to scuttle the tax legislation has proven particularly nettlesome to White House aides and Dole.

"There are some, and some on the right, who don't want the facts, they want the issue," Dole said yesterday in an obvious but indirect reference to Kemp.

In support of the tax increase, the president "is not mincing any words at all about his feelings," said Rep. Trent Lott, (R-Miss.) after yesterday's White House meeting. While Reagan has called the tax increase "distasteful" and has been reluctant to make a public pitch for it, Lott said the president would be "plenty public" in his forthcoming push for the legislation.

"There will be no doubt as to his position."

Rep. Thomas N. Kindness (R-Ohio) said the president offered some good reasons for the tax bill, but "there are many of us who will find it difficult to support."

According to Speakes, Reagan told the House members yesterday: "We are beginning to see some real relief on interest rates with a somewhat dramatic decline over the past several days. Interest rates are going in the right direction. They must continue if we are to have economic recovery.

"If we do not get spending cuts and reduce the deficit, this downward trend on interest rates could be reversed," the president insisted. "While I am reluctant to raise taxes, the price is not excessive to get the deficit down and to ensure the continuation of economic recovery."

Reagan still plans to meet commitments to attend the centenniel celebration in Billings, Mont., and several GOP fund-raisers in Los Angeles during a two-day trip next week. He is to return Thursday. Congressional leaders hope to get the tax legislation to both chambers the following week.

Senior presidential advisers said yesterday that Reagan also wants to remain in Washington next week because the House is scheduled to take up an urgent supplemental appropriations bill, which he is threatening to veto even though the legislation includes some funds for his proposed Caribbean Basin Initiative.