Vice President George Bush came here tonight to bestow the Reagan administration's endorsement on Republican senatorial candidate Paul S. Trible Jr. and stump for the President's tax package.

"The president and I are eager to have you come to Washington," Bush told Trible while 300 Republicans attending a $50-a-ticket reception at the Virginia Beach Convention Center roared their approval. "I'll be standing on the steps holding out my hands to welcome you."

Bush also echoed the tax bill sales message that Reagan has been carrying around the nation, telling the group that "three-quarters of the measure will have hardly any effect on the honest middle-class taxpayer." The intent of the bill, he said, is to close tax loopholes and "cut off undeserved benefits to interest groups."

After Bush spoke, as a clutch of well-wishers pressed the vice president for handshakes, Trible told reporters he has not yet decided whether to support Reagan's $98.9 million tax increase package. "I really have to wait and see what the final tax package looks like," the three-term Newport News congressman shouted above the din, adding that he and some colleagues had attended a 90-minute Reagan tax lobbying session at the White House earlier in the week. "I think the thoughtful approach is to wait and see."

The apparent disagreement between Trible and the administration over the tax package here tonight seemed to underline what might become a touchy public relations dilemma for the Trible campaign in the coming months. Although Trible needs the President's support to cement the votes of the state's rapidly-growing Republican suburbs, he and his aides are also wary of tying his campaign fortunes too closely to Reagan's coattails.

Trible, they say, is eager to avoid repeating the mistakes made last year by unsuccessful GOP gubernatorial candidate J. Marshall Coleman, whose close identification with Reagan was believed to be a key factor in his election loss to now-governor Charles S. Robb.

"He Coleman submerged his own identity to Reagan. There's no question about it," said Neil Cotiaux, Trible's press secretary. "We're going to make sure that we establish Paul Trible's identity."

Appearing in a meeting room hung with red, white and blue Trible banners, Bush said that Reagan regards the Virginia senate race as one of the most crucial in the country for the Republican Party. "We need to strengthen our control of the Senate," he said.

The 35-year-old Trible accompanied Bush on a flight to Virginia Beach from Washington aboard Air Force Two, but Bush said he had not tried to lobby Trible on the tax package during the flight. "I didn't discuss it with him," he said.

The Reagan factor, as it is called here, seems likely to be a key aspect of the contest this year between Trible and Lt. Gov. Richard J. Davis for the Senate seat left vacant by retiring Sen. Harry F. Byrd Jr.

Black voters, who make up 14 percent of the state's electorate, are believed pivotal to statewide election results, and Trible has relied heavily on their support to win three successive congressional terms from a district that is almost one-third black. He created a scholarship fund at a black college with part of his congressional salary increase and supported a measure aimed at making the birthday of the Rev. Dr. Martin Luther King Jr. a legal holiday.

Some black leaders say, however, that Trible's vigorous support of Reagan policies on the economy, civil rights and welfare are sure to erode his past support within the black community.

"I definitely think that within the general black community the appearance of Mr. Bush will not help Mr. Trible at all," said Jack Gravely, executive director of the Virginia NAACP. A recent NAACP rating of congressional performance gave Trible only 44 points out of a possible 100.