Virginia Democrat Richard J. Davis unleashed a stinging attack on his Republican opponent Rep. Paul S. Trible today, accusing him of "blatant lies" and "vicious" campaign tactics as the two met in the first of two scheduled debates in their U.S. Senate campaign.

Davis' attack, the strongest volley yet in the increasingly bitter contest, came as the campaign's first independent poll found the candidates for the seat of retiring Sen. Harry F. Byrd Jr. (I-Va.) locked in a virtual dead heat.

Trible, brushing aside the accusations, tagged Davis as a liberal in the mold of Sen. Edward M. Kennedy (D-Mass.) who would oppose Reagan administration policies and promote "the bankrupt, tax-and-tax, spend-and-spend policies of the past."

The new poll, scheduled to appear in Sunday's editions of The Richmond Times-Dispatch, shows Trible with a narrow 38-to-34 percent edge, a statistically insignificant spread given the poll's 3.5 percent margin of error. A sizable 28 percent of those polled were still undecided.

Republicans said they were jubilant at the survey's findings, insisting they show that Davis' hard-hitting assaults on Trible's campaign tactics have missed the mark. Trible aides have said that because Davis is Virginia's lieutenant governor and because he ran for office only last year, he would be expected to have a larger share of the electorate for him.

"The debate shows that Dick Davis has been engaging in campaign gimmicks and the people of Virginia aren't buying it," said Trible spokesman Neil Cotiaux. "The poll results show it."

Davis campaign officials said the poll results were expected. "Whenever you dump a couple hundred thousand dollars into the media like Trible has, that has an impact," said Davis campaign spokeman Will Marshall. "We fully expected we'd be six to eight points down."

The Times-Dispatch poll also found Trible with surprising strength among Virginia's crucial black voters -- apparently because of his active courtship of blacks in his Tidewater congressional district.

Though many political experts questioned the finding, the poll showed Trible receiving 29 percent of the black vote -- compared to the 5 percent black vote received by unsuccessful Republican gubernatorial candidate J. Marshall Coleman last November.

Yet Trible appeared to undermine that apparent strength during the debate when he all but recanted his earlier commitment to the D.C. voting rights amendment, considered a litmus test racial issue because the District is 70 percent black. Responding to a question, Trible said he was "unalterably opposed" to any measure that would give the District two senators -- failing to mention that he had voted for the D.C. amendment in 1978.

"He's a flip-flop artist," said state Sen. L. Douglas Wilder (D-Richmond), the state's senior black elected official.

Trible's backtracking on D.C. voting rights also disappointed one of the state's highest ranking black Republicans. "Maybe he's writing off some of the black vote," said Calvin Smith, a member of the Republican State Central Committee. "I want to talk to him about it."

The debate before the Virginia Manufacturers Association underscored sharp differences in style between the affable and avuncular Davis, 61, and the studied air of the 35-year-old Trible, a three-term congressman from Newport News. Davis' frequent quips and gibes won him hoots of approval from his supporters in the audience, but failed to rattle the businesslike Trible.

When Davis scolded Trible for two fund-raising letters that he said employed "scare tactics" and "lies," Trible refused to respond.

"The question, my friends, is not campaign memos or fund-raising letters, but rather how do we have America at peace, where people have jobs and homes and a promising future," Trible said.

On the issues, Trible seemed to score more points with the conservative businessmen in the audience than did Davis, stressing his support for a national right-to-work law and for a federal crackdown on labor officials who cross state lines to engage in union violence. Davis, however, sought to establish himself as a conservative, emphasizing his business background as a mortgage banker and his opposition to deficit spending.

Davis also criticized Trible for missing 78 of 321 roll call votes in the House this year. "He claims to put Virginia first, but if that is so, why does he have the worst voting record of any Virginia congressman over the last decade?" Davis asked rhetorically.

Responded Trible: "You know my opponent would prefer that I stay in Washington all the time and not campaign."

Both sides later claimed victory. "We cluster-bombed them," shouted Davis campaign manager James Carville, leaping from his front-row seat as soon as the debate was over. Trible manager Judy Peachee claimed she was equally pleased, saying that Trible's issue-oriented approach won out over Davis' snappy delivery. "I think Paul did very well," she said.

Trible's efforts in the debate to protray himself as supporter of the President seemed to mesh with the findings of the Times-Dispatch poll. Reagan, who flies here on Wednesday to stump for Trible, still meets the approval of 49 percent of Virginians, compared to 40 percent who disapproved of him, according to the telephone survey of 834 scientifically selected respondents conducted in mid-September.

This support of Reagan stands in stark contrast to nationwide polls which have found the president's support dipping, largely because of his handling of the economy. Yet, the Times-Dispatch poll found Virginia voters expressing confidence in Reagan's economic program by a 47 to 40 percent margin.

Moreover, despite a series of campaign gaffes that had thrown him on the defensive in recent weeks, Trible appears to have gained ground since July, apparently because of a carefully targeted summer-long media campaign, campaign officials said today. A poll taken by national Democratic pollster Peter Hart and leaked by the Davis campaign in early July showed Davis leading Trible by a 38 to 33 percent margin.

Trible campaign manager Peachee said the poll's findings are virtually identical to private polls done for the campaign by GOP pollsters Lance Tarrance and Ed DeBolt. "This confirms the poll numbers we've been getting," said Peachee. "We feel like the trends are good."

If nothing else, the poll suggests that Davis' attacks on Trible's campaign fundraisng tactics have had little impact on the voters, despite the extensive press coverage the attacks have received. Those attacks began last month after Davis' campaign obtained a copy of a Peachee memo alleging that Davis staffers had improperly solicited labor union contributions at a New York meeting of the AFL-CIO's Committee on Political Education (COPE). Davis emphatically denies there is any truth to the memo, yet Trible continues to stand by it.

The poll suggests instead that the Senate contest is shaping up as a classic party line confrontation, leaving the state's independents as the most crucial voting block. Trible, for example, has all but won the state's Republican vote, beating Davis among that group by 87 to 13 percent, according to the poll. Davis' strength among Democrats is nearly as strong -- 83 percent to 16 percent.