For 90 minutes the candidates slug it out over the issues and posture for the cameras, with the struggling underdog trying to get the best of the confident, photogenic incumbent who holds a comfortable lead in the polls.

Reagan-Mondale Sunday night? No.

Ferraro-Bush tonight? No.

In a Virginia version that parallels those national match-ups, it's Friday night's scheduled debate between incumbent Republican Sen. John W. Warner and Democratic challenger Edythe C. Harrison, who is trailing badly in polls, name recognition, fund-raising and organization.

Harrison's campaign is hoping that the 90-minute debate on seven public education television stations across Virginia will at least begin to do what many Democrats say her campaign so far has not done -- establish her as a serious challenger to Warner.

(The debate, from Channel 23 in Richmond, will be broadcast live beginning at 7 p.m. on WNVT channels 53 and 56 in the Washington area. WETA-TV (Channel 26) will delay broadcast of the debate until 11:30 p.m. Friday night.)

"It's not going to be a major mystery," said Steve Morrison, Harrison's press secretary. "He's going to talk about his record -- apple pie -- just like Reagan does. If we are able to have a truthful, factual debate on the issues, we can't lose."

Harrison has criticized Warner for going along with the steep increase in the nation's budget deficit under President Reagan. She also is critical of his support for expensive weapons systems that Harrison says drain resources for the nation's conventional forces. Harris says Warner is not concerned enough about waste and fraud in the Pentagon's budget.

She also contends that Warner has supported unwise cuts in federal student loans and other programs.

Warner press secretary Peter Loomis takes a slightly different view.

"I don't think there will be any surprises. There are substantial differences, particularly in the arms control area where she has vacillated," Loomis said.

Loomis said Warner would stick to his record as a first-term senator who is in tune with the Republicans who control the Senate and with Reagan, who is expected to easily carry Virginia on Nov. 6.

Unlike the national debates, Warner and Harrison will spend about 25 minutes of the program directly asking each other questions.

"I think we can safely assure you that there are some questions that will be posed to Mr. Warner that he hasn't thought of before," said Ric Ridder, Harrison's campaign manager.

Warner's staff initially was cool to the idea of the direct question-and-answer format, according to Jim Babb, the lead political reporter for WWBT-12 in Richmond and the moderator of the program.

"When you are the front-runner," said Harrison's press aide, "you want to keep your head down."

Warner rarely mentions Harrison's name and as the front-runner has not been anxious to give his opponent any more exposure than necessary.

But Warner, who likes to show his toughness in campaign appearances by reminding voters that he went "eyeball-to-eyeball" with the Soviets over incidents at sea during his days as secretary of the Navy, faced potential criticism if he did not go "eyeball-to-eyeball" with Harrison.

"I think this will be a somewhat livelier debate because of the interplay," said Loomis, Warner's press aide, who said Warner has always agreed to abide by rules set up by the host organizations.

There are other differences from the national debate.

The candidates had no role in choosing the panel of reporters who will ask questions during one 40-minute segment of the program. In the national debates, both campaigns were criticized for vetoing the appearance of several reporters.

The candidates can speak on any issues they want, with no restrictions. They will be seated together at a table rather than standing at podiums.

Warner and Harrison follow up Friday night's meeting with another joint appearance on Saturday at Oakton High School in Fairfax County. The one-hour meeting will begin at 1:30 p.m. and is sponsored by the National Association of Retired Federal Employees. The meeting is open to the public.