Rep. Michael Barnes is leading challenger Newton Steers in the U.S. House race that political prophesiers have called Maryland's closest, according to a new poll conducted by the Democratic incumbent as well as Steers' own telephone canvassing.

Barnes' poll shows him with an 18-point lead in the contest for the 8th District seat he wrested from Steers in an upset victory two years ago. The poll, a telephone survey of 452 registered voters, was completed Monday.

The results of Steers' unscientific phone canvassing of voters in Silver Spring, Wheaton and other traditionally Democratic areas, only spot Barnes a six-point edge, and set the volatile quotient of undecided voters at 24 percent. And, says Steers, in the last two weeks, the phone banks have shown them neck and neck.

The Barnes poll also showed Reagan still leading over Carter, but by a much slimmer margin than in a sampling last June. Reagan got 37 percent of the vote, Carter 32 percent. The poll also shows that the air has gone out of the Anderson balloon in Montgomery, with the candidate now getting only 13 percent of the vote, down from the summer when he outstripped all presidential contenders with 34 percent.

Not surprisingly, Barnes pronounced himself uplifted by the results of his poll, which he admitted was not an independent measure of voter sentiment, but was nonetheless an accurate, scientific reading, conducted by specifically trained volunteers.

Since his first poll last spring, Barnes' margin over Steers has grown from 8 to 18 percent, a shift Barnes attributes to the fact that Montgomery's voters "are turned off by the kind of campaign Steers is running."

Barnes' campaign manager, Brian Barkley, said that their phone banks were registering a shift toward Barnes over the past week in which Steers has hammered at Barnes' record of constituent service, in particular the "run-around" he gave a whistle blower who was battling with the federal bureaucracy.

In the campaign dialectic, it's also unsurprising that the Steers camp sniffs at Barnes' forecast, and even wonders if it is not a sign of some fearful trend that a candidate allegedly 18 points ahead would not just keep the good news to himself.

Whatever, Steers' campaign chairman, Howard Denis, brushed aside the results saying, "We do not view polls as oracles. There's too much tendency in political campaigns to equate polls with thunderbolts that come down from the eye of Zeus. The only effect of releasing this poll is to try to create a reality. If the Barnes' campaign wants to live in a fool's paradise they're welcome to."

Steers' campaign manager, Jeanne Miller, also laughed at Barnes's finding that 71 percent of Montgomery's sometimes fractious Democratic Party are behind him.

"Doesn't he wish," Miller snorted. "The undecided vote among Democrats is very large. The erosion of support is due to the feeling that he's not doing a good job.

Steers himself said, "I had five polls that showed me winning in 1978 and they were all dead wrong. We're trending upward. I'm slambang ahead."

Barnes claims one of the trends running in his favor is the high percentage his poll shows of Republican voters lined up behind him, as opposed to the number of Democrats -- 17 percent -- prepared to vote for Steers.

Barnes said his poll showed that Democratic support for President Carter had grown from 29 percent last June to 41 percent n October, indicating that the party in Montgomery is beginning to rally around its nominee.

There has been some good news, however, for the Steers Campaign. In recent days he has won the endorsement of The Gaithersburg Gazete, and The Baltimore Sun, which praised the "estimable" Barnes, but urged the election of Steers because his resume was more "Maryland oriented" than his "Washington-focused" opponent.