There's an unseen but potent presence hovering over the five well-qualified Democrats who are running for Congress from the 8th District in Montgomery County, and underdog candidate Leon G. Billings is none too happy about it.

It was Billings, a native of Montana, who suggested last week that Montgomery Democrats might do well to ignore for the moment the considerable imprint that outgoing Rep. Michael D. Barnes left on the local party before setting off on a run for the U.S. Senate.

Democrats Carlton R. Sickles and Stewart Bainum Jr. were singing Barnes' praises at a candidates' forum in Rockville, and when it came time for Billings to speak, he noted that Barnes "would be very pleased that he had so many supporters here on the rostrum.

"I suspect we might do him a little bit better if we all went down to Baltimore and spent some time with him down there," Billings said with more than a trace of wryness. Barnes, a liberal politician, is spending most of his waking hours in the port city looking for votes.

Billings, knowing well the folly of criticizing one of Montgomery's most popular officeholders, was not denigrating Barnes. Rather, he was delivering a simple message to his rivals and the 60 Democratic Party officials in the audience: Mike Barnes may be a model U.S. representative, well suited to his affluent and liberal district, but he is not running for election to the House. Several other Democrats are.

Billings' message was calculated. Trailing Bainum and County Council member Esther P. Gelman in fund raising, he badly needs the attention of Democratic Party regulars. To do that, Billings has tried to separate himself from the pack, attacking the policies of the Reagan administration with more spirit than the other Democratic primary candidates.

Billings even went so far last week as to suggest that candidates voluntarily limit their campaign spending, an idea that won nothing but silence from Bainum, Gelman, Sickles and Wendell M. Holloway, a lobbyist who also is running in the 8th District.

Still, many Democrats who are watching the primary race believe Billings is on to something. The electorate, they say, may soon tire of candidates who try to wear Barnes' mantle without first checking the fit.

Sickles, for instance, announced at the forum last week that his friends had urged him to run for Barnes' seat because "I could continue to do the work that Mike was doing." Sickles, who held an at-large seat in Congress in the mid-1960s, added that Barnes "has continued to support the very positions that I espoused when I was the congressman for the 8th District."

In the same vein, Bainum hailed Barnes as "the very best congressman Montgomery County has ever had.

"It'll be a real challenge to fill Mike Barnes' shoes for our next congressman," Bainum added. "This is a challenge which I welcome."

At a time when budget cuts threaten federal funding for Metro subway construction, highways and other programs vital to Montgomery's prosperity, some Democrats are growing tired of these candidates promoting themselves as the sole heirs to Barnes' tradition of national leadership on foreign policy issues.

The four months remaining before the Democratic primary is not nearly enough time for the five pretenders to the throne to duplicate that impressive record. Nor should they even try.

"Mike Barnes is the only guy who could do what he did," said one Democratic Party activist and longtime Barnes admirer. "He made no enemies and was never a threat to anybody.

"But he also has no flash, no pizazz, no oomph. All these new guys are trying to be like Mike, but it doesn't play."

One reason it will not play is that unlike Barnes, a shiny-bright newcomer when he was first elected to Congress in 1978, several of the Montgomery Democrats racing to succeed him will carry their own peculiar baggage into the primary in September.

Bainum, for instance, is an articulate and intelligent candidate, but his transparent ambition has irritated some Democrats over the years. Gelman, too, has made a number of political enemies, and now is using party elders such as former state senator Victor L. Crawford as go-betweens to make peace with other Democratic leaders.

Sickles, whose experience on Capitol Hill and spirit of fair play have won him many admirers, has his deepest political roots in Prince George's County, not Montgomery.

Similarly, Billings, who loves to play the role of a blunt-spoken Westerner, can be his own worst enemy sometimes. But in his wife Patricia, a tireless worker for the Democratic Party in the county, Billings has an asset that likely will translate into votes this fall.

"Leon is trying to be the cowboy because he knows he's a damn fool if he doesn't," said one prominent Democrat. "And he's right. Where's the spark, the fire, the new ideas in this race?"

"There are no philosophical differences here," said Crawford, a longtime observer of Montgomery politics. "The candidates are like soldiers in a row. Who do you pick?"