Republican gubernatorial candidate J. Marshall Coleman has sent a "last minute election alert" to 22,000 financial backers saying he may lose next Tuesday's election to Democrat Charles S. Robb unless he raises $185,000 for a final barrage of television and radio advertisements.

The direct-mail appeals were received throughout the state in the last few days, and said that while Coleman's surveys show he can win, he will need the additional money to do so.

"If we don't get ads in the last week," Coleman said yesterday while campaigning in Williamsburg, "we can't get our message across just when everybody is paying attention to the race."

The final Coleman ad blitz would include commercials featuring President Reagan and radio spots made during the president's speech at a Coleman rally in Richmond on Tuesday.

"It's not hype," said Coleman press secretary David Blee. "It's to connote a sense of urgency . . . it underscores the importance of money in the race."

The Robb campaign also will rely on heavy weekend advertising to hold the lead polls show he has. Robb already has bought Monday night half-hour TV spots in Washington, Richmond, Roanoke and Bristol and is negotiating in Tidewater to air a taped election-eve production.

Robb took a break from the campaign trail yesterday to discuss new ads with his media adviser, Robert Squier, and to prepare for the final debate of the campaign today in Norfolk. But a Robb spokesman took time to label as "disgusting" Republican campaign tactics being used in the racially sensitive southern part of Virginia.

The Robb office released copies of a letter distributed under the name of a GOP fund-raiser in Prince Edward County -- where public schools in 1959 were closed for five years to resist integration -- that criticizes Robb for supporting postcard voter registration, which would make it easier to vote, and a proposed state holiday honoring the Rev. Dr. Martin Luther King Jr.

The letter also criticized Robb's in-laws, the late President Lyndon B. Johnson and Lady Bird Johnson, saying they "made millions of dollars during the Vietnam War while young men from Virginia were being killed."

"I have a dear friend . . . that lost his beloved son in Vietnam in order for Robb to be adequately financed to run for the governor of Virginia," the letter added.

"This is disgusting," said Robb press aide George Stoddart. "This stuff is orchestrated."

GOP radio ads being aired south of Washington's Virginia suburbs feature former Republican Gov. Mills Godwin -- once a strong supporter of the state's "massive resistance" approach to early integration -- who also attacks Robb for his support of those views.

The letter is part of a GOP mailbox assault in Southside, and it accuses Robb of trying to "buy the black vote" by giving campaign funds to black political organizations. It praises Coleman for opposing the King birthday holiday and postcard registration.

Coleman staffers disavowed any knowledge of the letter. "We can't screen every letter people send out to their personal friends," said Blee.

Besides postcard voter registration and the King holiday, two other issues Democrats say Republicans are using to make racially tinged assaults on Robb are the D.C. Voting Rights Amendment and setting aside a percentage of state business for minority contractors.

Coleman campaigned yesterday in Northern Virginia and Tidewater, the state's two most populous areas, where the outcome of next week's election may be decided.

Although he complained that the press was "being used by Robb" in reporting the Democrat's suggestion that Coleman is manipulating historical racial animosities in his campaign.

Coleman said the issues of a proposed constitutional amendment that would give the District two senators and at least one congressman and postcard registration of voters are "truly litmus test questions" of conservatism, a title both Coleman and Robb have claimed.

Coleman said he is "not uncomfortable" with opposing the D.C. amendment, although its backers have argued that most opposition is based on the likelihood that any D.C. House or Senate members would be black, liberal and Democratic. "I'm not against it because they would be black or white," Coleman said. "We should be colorblind and one of the things my record shows is that I am."

Robb's stand in favor of the amendment, he said, "shows that he is a liberal before some audiences and a conservative before others. You can't have it both ways."

Speaking before the Virignia Hospital Association in Williamsburg yesterday, Coleman said the D.C. amendment would "cancel out our two Virgnia senators." He said postcard registration is "going in the wrong direction" and could lead to voting fraud like that which existed in Virginia's 9th Congressional District years ago until strict state laws eliminated such abuses.