The Reagan administration, despite indications that some of its efforts to aid Republican J. Marshall Coleman actually may have benefited Democrat Charles S. Robb, is preparing to redouble its efforts to help Coleman capture Virginia's governorship.

A key element in that effort is expected to be announced later this week when the White House discloses plans for a personal appearance by the president for Coleman in Tidewater on Oct. 27, one week before election day, according to Reagan and Coleman aides. The visit will climax a two-week blitz of Virginia by Republican celebrities including Vice President Bush, former president Gerald Ford and Senate Majority Leader Howard Baker.

Also, the Republican National Committee, which already has sent one top political operative here and earmarked $100,000 for an independent media effort for Coleman, is considering assigning additional aides to shore up support for Coleman, who trails Robb in recently published polls by 5-to-11 points.

One RNC official said the committee, having downgraded hopes of winning the New Jersey governorship, would intensify its efforts in Virginia to avoid an election day shutout that would be interpreted as a blow to the president and the GOP. The official said RNC staff members were treating the Virginia election as a crucial test and that a defeat here could cost party chairman Richard Richards his job.

Interviews with a dozen politicians and analysts, buttressed by recent polls, indicate Coleman's constant invocation of Reagan's name has done little to help his campaign while it has aided Robb by solidifying the traditional Democratic base of liberals, blacks and union members. Members of those groups say they have doubts about the conservative Robb but far stronger fears about Reagan, according to the polls.

"The stronger Coleman's identification with Ronald Reagan, the stronger the traditional Democratic constituency goes to Robb -- there's a direct correlation," said Ira Lechner, a former Robb opponent who has played a major role in bringing liberals into the Robb camp.

The impact of the Reagan connection on liberals was made clear again last weekend when the Virginia Crusade for Voters, the state's premier black political group, endorsed Robb. Richmond Mayor Henry Marsh, one of Robb's key supporters in the group, cited "the excesses of the Reagan policies -- high unemployment among blacks, cuts in the free-lunch program, food stamps and Social Security," as a major factor in the Crusade's decision.

At the same time, because Robb has avoided taking any strong anti-Reagan stands, Coleman has not been able to cash in on Reagan's popularity with conservative groups in Virginia despite repeated efforts to do so.

Voters surveyed in a Washington Post poll published two weeks ago gave Reagan a 69 percent approval rating. Coleman only captured 52 percent of those pro-Reagan voters, while Robb took 39 percent, with the rest undecided. By contrast, Robb won 78 percent of those disapproving of Reagan compared to 11 percent for Coleman.

In Northern Virginia, where Reagan outpolled Jimmy Carter and John Anderson combined by 56 percent to 44 percent last year, The Post poll showed Robb in front of Coleman by a 59-to-40 margin.

Analysts offer a variety of reasons for Coleman's failure to capitalize on the Reagan connection. Several, including some in the Coleman campaign and the White House, believe the Republican candidate has overemphasized his ties to Reagan and Virginia Gov. John N. Dalton while failing to clearly establish his personal qualifications for governor.

"Time and again we've seen candidates for state office go off and preoccupy themselves with supporting some official in Washington," said one White House aide who asked not to be identified. "We urged them (the Coleman campaign) in June and doubly urged them this month to go out and develop their own local issues."

Coleman's opponents contend voters are having difficulty buying the proposition that the GOP nominee is a born-again Reaganite. The opponents note that Coleman four years ago ran for state attorney general as a progressive Republican, then derided Reagan's endorsement of his GOP rival for that job, and was late in boarding the Reagan bandwagon in 1980.

"Marshall's effort to tie himself to President Reagan's coattails hasn't been very successful because people generally understand that his record simply can't support that," said W. Roy Smith, a Reagan supporter who heads the conservative Virginians for Robb.

Many believe Reagan still can help Coleman recoup but only with a personal appeal. "It won't do much good for Marshall Coleman to keep talking about his support for the president," said University of Virginia political scientist Larry Sabato, who recently completed a poll showing Coleman trailing Robb by five points. "Ronald Reagan himself has to give Coleman a strong personal endorsement . . . without it, I don't think he can win."

Reagan already has aided Coleman with a two-page fundraising letter praising the attorney general as "exceptionally well-qualified" and with a 90-second videotaped message that Coleman aides say they may use in television ads later in the campaign. But the president cancelled a personal appearance in Richmond last month at the last minute because of a national speech on the economy and sent his wife Nancy, instead.

Campaign manager Anson Franklin expects the late October Reagan appearance to help give Coleman the momentum to overtake Robb by election day. "We've got a very popular president, he's willing to help and we would be crazy not to make the most of it," said Franklin.

As for the opposition, Robb said last weekend he has tried to keep the campaign focused on Virginia, not Washington, but added he is well aware of the impact Reagan and national Republican money could have here.

Robb, who has applauded Reagan's budget-cutting policies though not his tax-cut plan, added, "this campaign is a referendum on the future of Virginia," not on the Reagan administration.