A year ago, the conventional wisdom in Maryland political circles was that the Democratic U.S. Senate nomination was practically Gov. Harry Hughes' for the asking. By September, Hughes had faded and political pundits were sure that Baltimore County Executive Donald P. Hutchinson and Montgomery County Rep. Michael D. Barnes would squeeze Baltimore Rep. Barbara A. Mikulski out of the Senate race.

The four Democrats will meet tonight for their first debate, and polls now show that Mikulski has a hefty lead over the rest of the pack.

With four established Democratic politicians in the fray to win the seat being vacated by Republican Sen. Charles McC. Mathias Jr., who is retiring after 18 years, the Maryland race is one of the Democratic Party's bright hopes in its struggle to regain control of the Senate. Two Republicans -- businessman Richard P. Sullivan and Prince George's County Del. Thomas J. Mooney -- are in the race, and a third, former White House aide Linda Chavez, is expected to enter it tomorrow.

According to four polls released since last fall, Mikulski is leading her closest rival by as much as 20 percentage points. "Barbara is genuinely popular," said John Marttila, a Boston political consultant working for Barnes. "She is a very formidable adversary. We will have to run a better campaign, raise as much money as she does, if not more, and get more endorsements."

Mikulski's opponents insist that it is foolhardy to give too much credence to polls six months before the September primary. "Polls at this stage don't bother me," said John T. Willis, Barnes' campaign manager. He said that polls showed Sen. Paul S. Sarbanes trailing former senator Joseph D. Tydings by more than 20 points only 10 weeks before their 1976 primary election. Sarbanes won with a 20 percent margin. Similarly, Hughes defied the polls with an upset victory in his first gubernatorial campaign in 1978.

The first polls, conducted from September through December by the Baltimore Sun, the Baltimore News-American/WBAL-TV, and Mason-Dixon Opinion Research Inc., came up with roughly the same results. Mikulski was favored by 33 to 38 percent of those polled; Hughes by about 18 percent; Barnes about 16 percent, and Hutchinson by 8 to 14 percent.

A Mikulski poll conducted in January came up with even more favorable results, showing her supported by 43 percent of those polled. The Barnes camp has not released its polling results, but Marttila said its campaign data showed that Hughes had slipped behind Barnes, largely as a result of the savings and loan crisis that has plagued the Hughes administration for 10 months. According to Marttila, Mikulski picked up most of those who were switching from supporting Hughes.

Currently, the biggest wild card in the contest is the support that Hughes will be able to muster. He has not formally announced his candidacy but has been raising money and making campaign appearances in recent weeks. Three independent polls show Hughes running neck and neck with Barnes.

"I don't think it's my place to write off Harry Hughes, but . . . [the polls show] pretty serious voter resistance to Hughes' candidacy," said Marttila. Hughes yesterday dismissed the early polls and said he can win over voters by Election Day by stressing the accomplishments of his administration.

Marttila added that Hutchinson "seems to evoke somewhat reasonable levels of popularity" but that voters see him as a local, not federal, official.

Hutchinson responded, "He's dreaming. He's fanatasizing." Hutchinson acknowledged that he is behind in the polls, trailing Mikulski even in his home base of Baltimore County. His strategy is to paint Barnes and Mikulski as liberals who are ineffective in Congress.

Barnes, a high-profile player on the House Foreign Affairs Committee, is presenting himself as the candidate with the most experience in foreign policy and the budget. During a recent Democratic forum, where he appeared with Mikulski, Barnes sprinkled his talk with references to encounters with foreign leaders, such as a private lunch with the president of El Salvador, and his role on issues such as Nicaragua and the Philippines. He makes frequent, if subtle, reference to his military service -- in the Marine Corps -- a claim that Mikulski cannot make.

Several Democrats said that Barnes' approach is to appear more "senatorial," based on a belief that Maryland voters like to elect senators who have broad vision and backgrounds in foreign affairs and economic issues.

Mikulski political consultant Robert Shrum said the "senatorial" approach will fall flat. "That's a way of saying, 'I'm a man and I wear a two-piece suit,' " said Shrum, who said there is no evidence in polls that voters are biased against women. "That's like saying that senators should fit some kind of cookie cutter image of what a senator is."

Mikulski's approach has been to present herself as a fighter for the common person. At a Democratic forum in Millersville, Mikulski told the audience that she would be an "advocate" who cared about jobs, "everyday schools" such as the University of Maryland, and where people will live and work in the 21st century.

And, in contrast to Barnes, she tried to play down her meetings with foreign dignitaries, talking instead about her contact with soldiers: "When I went to Lebanon, I didn't just go out there and talk to the Gemayel government. I also went out to the bunkers and talked to our Marines, and I knew firsthand what they were facing."