A new GOP poll shows Virginia Republican Wyatt B. Durrette has fallen behind Democrat Gerald L. Baliles by nine points in their race for governor, Republican campaign sources said yesterday.

The Nov. 5 race until now has been rated a tossup. The poll, conducted by Durrette's campaign, was completed late last week and showed Baliles ahead 43 to 34 percent, with 23 percent undecided, according to GOP officials who blamed the slippage on Durrette's failure to respond until this week to more than three weeks of media advertising by Baliles.

The Republican National Committee also confirmed yesterday that it is picking up the $100,000 cost of Durrette's new advertising campaign that began Monday, but disputed suggestions that the payments were intended as a rescue attempt for Durrette. The national committee has spent an additional $100,000 on radio and other media for the campaign throughout the summer, an RNC official said.

GOP campaign sources said the new national committee funds were committed after President Reagan canceled a July fund-raiser in Virginia and left the Durrette campaign about $250,000 short in planned contributions.

"We're not going to comment on any polling information from unnamed sources," said Don Harrison, Durrette's press secretary. "We will comment on any polling data that we release. But we have no plans to release our polls."

Bruce C. Miller, Durrette's campaign manager, said, "If there is any slippage, it would have been expected, having Baliles on television for three weeks . . . . Every poll I have seen up to now, we've been just about neck and neck. Of course I'm aware of information I'm not prepared to talk about."

The poll was completed before Tuesday night's statewide debate carried on public television, which both campaigns conceded was a draw. But both GOP and Democratic officials suggested the new poll could provide the spark for a campaign that until now -- only seven weeks before the election -- has failed to capture the attention of most voters.

"It's not devastating. You'd rather be ahead," one Republican strategist said. "It may have the useful purpose of waking some people up [in the campaign].

Democrats reacted cautiously to the new poll, saying it is traditional in Virginia for Democrats to lead until the final days of the campaign. Since early summer, polls by both campaigns have shown them virtually dead even, with about one-third of the vote undecided.

"Assuming the numbers are accurate, and I have no reason to believe they are not, it appears we have accomplished our first strategic goal -- telling people about Jerry," said David Doak, Baliles' political consultant. "It's been a vote-gainer for us."

Republicans have recently expressed concerns that the Durrette campaign has been hampered by internal disputes over how to attack Baliles, a moderate-conservative who upset a more liberal opponent to win the Democratic nomination. Durrette said this week that a strategy has been worked out and campaign officials are hoping that a rescheduled visit by Reagan Oct. 9 will ease any lingering fund-raising problems.

Democratic officials suggested that Durrette now faces the dilemma of trying to catch up with positive ads for Baliles, and at the same time risking negative advertising to chip away at him.

"Generally, you build up your positives before going negative. He'll have to do both," a Democratic strategist said.

The Durrette poll, according to the Republican sources, showed that the races for lieutenant governor and attorney general remain a virtual tie, with large numbers of undecided voters, which is largely unchanged since midsummer.