Democrat Charles S. Robb leads Republican J. Marshall Coleman by seven points in the race for governor of Virginia, but has lost some ground since September, according to a new poll by The Washington Post.

The poll, taken Wednesday and Thursday nights, after President Reagan's appearance at a Coleman rally in Richmond, shows Coleman trailing 51 to 44, meaning he has cut four points from Robb's 11-point lead in a survey of the same voters last month.

In other races, Democratic lieutenant governor candidate Richard J. Davis appears to be pulling away from his controversy-plagued GOP rival, State Sen. Nathan H. Miller, leading 52-40; while Republican Wyatt B. Durrette retains a slight lead, 45-41, over Democrat Gerald L. Baliles for state attorney general.

Coleman's press secretary, David Blee, said yesterday that he is "very skeptical" of the Post's poll, finding it "inconsistent with our own poll, which shows us leading," and with a poll by The Norfolk Virginian-Pilot, conducted in a six-day period that ended Thursday.

"The undecideds are breaking heavily in our favor, and we have the momentum," Blee said.

The bad news for Miller is that while both of his GOP running-mates are gaining on their opponents, he is falling further behind, according to the Post's poll.

With a margin of sampling error of almost 4 percent, many people just beginning to take the campaign seriously, and many others saying that their support for either candidate is not strong, the Post poll is unable to conclude that Robb will win on Tuesday. The poll by The Norfolk Virginian-Pilot, published today, has Robb and Coleman in a dead heat at 44 percent to 44 percent, with 12 percent undecided.

In the Post poll, 5 percent are undecided, compared to 8 or 9 percent who said they were undecided in September. Among the small number who said they have made up their minds since the first Post poll, Coleman holds a five-to-four edge.

The 807 people included in the Post poll were among 1,180 potential voters who were first interviewed Sept. 15-20. The new Post poll shows that the election continues to divide voters almost entirely along partisan lines.

Robb has solidified his overwhelming support among Democrats, with 89 percent of them saying they are for him. Coleman, who was supported by 76 percent of Republicans in the earlier poll, now is backed by 82 percent of the Republicans interviewed.

Much of Coleman's gain is coming from among independents, where Robb's lead has shrunk from 12 points in September to four, according to the poll. As a group, independents account for about four in 10 Virginia voters.

Almost half the backers of each candidate say they do not support him strongly. A total of11 percent of those who had been for Robb last month now say they are for Coleman, and9 percent who had been for Coleman have jumped to Robb.

In interviews after the poll was conducted, many of the switchers on both sides cite disgust with the media campaigns being conducted by the candidates. Beverly Brumbaugh, 44, a sixth-grade teacher from Colonial Heights, said that she abandoned Robb when she heard a radio commercial last weekend "accusing Coleman of making deals in behalf of drug dealers."

Jacob Tucker, 30, of Richmond, said he moved into the Coleman column after he saw a TV commercial in which Robb "was standing behind a plastic screen while someone threw mud at him. Robb had to have approved of that, and it was ridiculous."

Andrew Robinson, 29, a Richmond researcher, said he "can't stand the campaign tactics of either one," and "would like to vote for 'neither of the above,' " but probably will vote for Robb.

Coleman is counting heavily on the popularity of Ronald Reagan to propel him to victory on Tuesday, but it is not clear from the results of the Post poll how much influence the president is having on voters.

Patty Hughes, a 34-year-old Newport News housewife who likes Reagan and has seen the president on television with Coleman, recently decided to vote for Coleman and said that the Reagan factor "probably had a lot to do with" her decision. Reagan also was the factor that caused Glenna Butcher to switch from Coleman to Robb.

"I was against the AWACS sale," said the 45-year-old manager of an electronics system from Colonial Beach, "I didn't like the tactics he used in lobbying for the sale."

Reagan continues to be highly popular personally statewide, but a strong minority of Virginians are opposed to particular Reagan policies. In the Post poll, for example, 42 percent said they felt that Reagan cared more about serving upper-income people, compared to 46 percent who said he cared equally about serving all people.

The Reagan element clearly is a plus factor for Coleman among segments of the electorate. Some independents who are strong Reagan supporters are switching to Coleman, although they indicate that they have no more than lukewarm feelings for the Republican nominee.

But other independents, who like Reagan personally but feel that he sides with the rich, are splitting strongly for Robb. In addition, Robb appears to be retaining support among many Democrats who voted for Reagan and still think highly of the Republican president.

At the same time, some 25 percent of the probable Virginia electorate gives negative ratings to Reagan, and these voters -- preponderantly Democrats, and many of them black --overwhelmingly side with Robb.

Coleman's sharpest gains have been in the Richmond area, where he and Robb were tied in the earlier poll, and in Southside, where Coleman has overtaken what earlier was a large Robb lead. The Democratic candidate's strongest areas are Tidewater and Northern Virginia.

Statewide, the massive support of Robb by blacks continues to give him the margin of his lead. Whereas Robb led by 46 percent to 44 percent among whites last month, Coleman now is the choice of whites, 49 to 46, while Robb's lopsided 83-to-7 lead among blacks has increased to 88 percent versus 7 percent for Coleman.

The outcome of the governor's contest could be decided by the size of the black turnout. Blacks, who account for 16 to 18 percent of Virginia's registered voters, make up 11 percent of the poll's probable electorate. But if blacks represent far fewer than 11 percent of those who actually cast votes on Tuesday, Coleman could win, regardless of other last-minute changes. Estimates from the 1977 gubernatorial race put black turnout at 9 to 12 percent.

There also is a sharp split by sex, with enough men moving to Coleman to give him a 49-46 lead among men, compared to Robb's 48-43 lead among men last month, while Robb retained his lead among women, going from 54-36 to 56-39. One factor that helps explain this difference is that a substantially higher proportion of women than men say that they are Democrats.

There may have been few real differences on issues between the two candidates, but Joanne Skinner, 26, of Poquoson, said that she sided with Robb because of his opposition to placing anuclear waste disposal site in the state.

And a 60-year-old Richmond minister said that he moved from undecided to Robb after finding Coleman "too dogmatic. He has all the answers in advance: no tax increase, no early release for prisoners. In a complex society, you can't have everything in black and white."

The clergyman added that Robb has been accused of switching on issues, but he finds this acceptable because "if circumstances require, flexibility may be necessary."

Party identification helped Helen Craig, 50, of Richmond, opt for Colemand last week. A housewife and former teacher, she said that she still likes Robb "from a personal standpoint," finding him straightforward," but believes that if elected he would be presured by more liberal Democrats into "big spending policies."

Susan Pruitt, a 23-year-old data preparation clerk from Parksley, on the Eastern Shore, said she and her husband decided a few days ago that "Coleman is the more conservative, although I didn't think so before," and so he will get their vote.