Rep. Paul S. Trible Jr., Virginia's Republican candidate for the U.S. Senate, broke his longstanding support for Reagan administration economic policies yesterday, voting against the president's $98.3 billion tax increase proposal.

Trible's vote, one of 13 "no" votes cast by the 18 representatives from Virginia and Maryland (three of the four senators from the two states also voted "no"), may undermine what has been a key campaign strategy of his Democratic opponent, Richard J. Davis, the state's lieutenant governor.

Davis has made criticism of Reagan's economic policies the centerpiece of his campaign for the seat being vacated by Harry F. Byrd Jr., and has sought to portray Trible as an unquestioning proponent of those policies.

Trible, who was courted by Reagan at the White House last week, said he opposed the tax increase measure because it would place an undue burden on Virginia taxpayers and because he believed its passage could jeopardize the nation's economic recovery.

"I will support the president whenever possible, but I will also do what is best for Virginia," Trible said in an interview before the vote. "I've always been my own man, and will continue to do what I think is appropriate."

Trible, a three-term congressman from Newport News, has been a champion of Reagan's economic policies on the House Budget Committee, and some of Trible's more moderate supporters had worried that close identification with the president could hurt him in the November elections.

By opposing the administration yesterday, Trible appeared to be trying to avoid what some said was the mistake made last year by GOP gubernatorial candidate J. Marshall Coleman. His close ties to Reagan were believed to be a key factor in his loss to Gov. Charles S. Robb, a Democrat.

The Davis camp discounted the significance of Trible's break with the president. Davis campaign spokesman Will Marshall categorized the vote as a "conspicuous exception" to Trible's pro-Reagan voting record.

"As a rule, Paul Trible has given reflexive support for whatever has come out of the Republican White House," said Marshall, whose candidate also opposes the tax increase measure. "This is one of how many votes that he's cast since the administration has been in there?"

Trible said his decision to oppose the president was "difficult," but that it was by no means the most difficult decision he has made in Congress.

Trible said he believed the tax package would unjustly burden Virginians because it would double the federal tax on tobacco, one of the state's major agricultural products, and because it would increase the payroll tax for unemployment compensation without sending a greater percentage of those funds to Virginians.

"I'm confident that the administration will continue to support my candidacy and I will continue to support the president's initiatives," Trible said.

Trible press aide Gus Edwards said the congressman's office has been besieged during the last few days by letters, mailgrams and telephone calls from constituents and lobbyists. Among the lobbyists were representatives of The Tobacco Institute (which opposed the measure), and the life insurance and homebuilding industries (which favored it.)

The office also received a six-inch stack of petitions from Virginia voters opposing the measure. "It was the largest outpouring I've seen in six years here," Edwards said.