Andrew P. Miller, who has gone to great lengths in his campaign to become Virginia's junior U.S. senator, tried for heights last week by climbing aboard a hot-air balloon at a Suffolk outing.

It was just one event in a schedule of appearances Miller has been keeping around Virginia, but this one did not work well. After several seconds, the balloon's gondola - and Miller - sagged back to earth.

What that symbolizes to many Virginia Democrats is that, despite his heavy schedule of appearances, Miller's campaign is having difficulty getting off the ground.

Some examples:

Bumper stickers and campaign literature. They are almost nonexistent in Northern Virginia, complain Democratic party workers. Last week Republican John W. Warner, whose campagin is eight weeks old, showed up at a Richmond debate with armfuls of literature, Miller had none.

Enthusiasm. Warner has been drawing crowds in the hundreds to some of his events, many of them lured by the attraction of being close to Warner and his celebrity wife, actress Elizabeth Taylor. But Democrats say people are not interested in the campaign, "I'm just of the opinion that the public isn't interested yet," said Senate Sen. Hunter B. Andrews, a Hampton Democrat.

Style. "Andy's not a great public speaker," concedes one of his campaign associates, who asks not to be named. Miller's friends say he is improving but he still is not an oratorical spellbinder. His remarks have the sincere rasp of a Hubert H. Humphrey, but also some of the late senator's length.

Visibility. Despite his toil, Miller's campaign to date has maintained a surprisingly low profile. Press schedules of Miller appearances have been sparse and, when issued, have sometimes arrived at some newspapers days after the events mentioned have occurred.

"You may not have been hearing from us as much material," he said, characterizing the Miller campaign staffs periodic "Andygram" memos said last week. "Certainly not as much as we had planned."

Miller's press aide, John K. Durst Jr., blamed the lapse in publicity on illnesses among the staff, but admitted the campaign had not been geared to grab headlines. "Issue stands per se may not be page 1 as you would like lately," one of the Miller campaign as "issue oriented."

Miller campaign director Allen F. Clobridge says the campaign strategy calls for emphasis on Miller's strong points - his experience as Virginia attorney general and his familiarity with voters.

Miller has carried a heavy schedule for more than six weeks, and each place he goes he picks out local issues. In Fairfax County, he praised the Metrorail project. In Norfolk, he talked about helping keep Navy ship repair work in the Tidewater area. Richmond conservatives heard about lowering taxes and the League of Women Voters were reminded that Miller increased the hiring of females in the attorney general's office. "I think the people of this nation are concerned about inflation, about governemtn," Miller says.

But many Miller supports on all shades of the political spectrum ask where the Democratic campaign organization is going. "I wouldn't say it's so strong right now," said Sen. Andrews.

Others are harsher, but ask not to be named for the sake of unity in campaign that gives Virginians a chance to elect a Democratic senator for the first time in 12 years.

"You couldn't even put (the Miller staff) in the same room (with the Warner organization), says one. "It's a joke."

Party regulars, like Fairfax County Democratic Committee Chairwoman Emilie Miller, recalling that this is an off year election, worry about light voter turnout, a fact that has not escaped Andy Miller either.

"The challenge is to get out the vote," he told a small crowd at a York County gathering last week. It was the inability of Miller's workers last year to get their supports to the polls that many say allowed the Democrat Henry E. Howell to upset Miller in a gubernatorial primary.

In Northern Virginia, Democratic leaders say people like Miller and are willing to work for him, but have been hamstrung by a lack of campaign material, which may be traceable to the candidate's persistent problem of raising money.