In 1973, when Robert Ashley Smith was convicted in a Prince George's County Circuit Court of double murder and arson and sentenced to life in prison, his weeping father quietly vowed "to fight this all the way."

It has been almost five years now, and Robert E. Smith and his wife Jessie still are as fervently convinced of their son's innocence as they were the day he was convicted. Every spare moment has been spent in tracking down new avenues of appeal, and virtually every extra dime - altogether more than $14,0000 - has been consumed in the long legal battle.

Sometime today, in another Prince George's County courtroom, their protracted legal battle "to free Robbie" will be resolved for good or ill. After two days of arguments, Circuit Court Judge Vincent J. Femia will issue his ruling as to whether the 24-year-old Smith should be awarded a new trial.

"I've sent out thousands of letters so I could learn about the court system, as well as the results of recent legal decisions that might affect my son's case," said Mrs. Smith yesterday. As a result, she said she has built up a library of arson-related materials and legal information that she felt might be of some use in freeing her son.

The long legal battle for the Smiths began back in 1972 when George and Lucille Brown were killed in their Bowie home, at 12305 Starlight La., across the street from the Smith home.

A fire investigator who arrived on the scene said the fire, which burned the front door and destroyed the living room of the Brown home, was caused by someone who threw flammable liquid under the door and set it ablaze.

Prince George's County police, working with fire investigators, turned up a single suspect - Robert Ashley Smith - who was described then by some neighbors as arrogant and strange.

According to the court records, county police took Smith to headquarters the night after the fire and began an intensive questioning session that eventually led to Smith giving police a statement in which Smith allegedly implicated himself in the crime.

The parents and their attorneys have argued that Smith, then 19 years old, was tricked into making the statement after 18 hours of questioning. It was that statement, they maintained, that formed the basis for their son's conviction.

In addition, the parents charge that poor advice by his attorney at the trial led their son to choose to have a judge rule on his guilt or innocence rather than a jury.

As a result, they say, their son should be entitled to a new trial at which new discoveries in the field of fire investigations, which they believe could help prove their son's innocence, should be allowed into evidence.

The post-conviction hearing under way in Prince George County is very unusual, according to Judge Femia, who said his role in the trial is not to decide whether Smith is guilty or innocent, but to determine whether the case has been accurately prosecuted according to law.

As a result, Smith's attorneys in this post-conviction appeal can raise only legal issues in their effort to win for him a new trial. Attorney Jesse Shiftman, who is one of the attorneys for Smith, said the range of legal questions that have been raised in the current hearing include the legality of Smith's arrest, the competence of his attorney, and whether recent United States Supreme Court decisions might not result in the overturning of his arson conviction because it placed him in double jeopardy.

Smith's father, a paint salesman, said that he had lost "a great deal of money" because of his son's trial. "I can't count the loss, but I know it's had an effect on my business."

As Smith sat outside the courtroom during a brief recess this week, he explained to a reporter that the thousands of dollars he has spent for his son's defense were "worth every penny."

"I was a real law-and-order-man until this thing happened to my son . . . now I don't know what to think," said Smith.

As Smith sat next to his wife he lowered his voice when asked what he thought the outcome of the trial would probably be. "I just don't have any faith any more. I don't think my boy is going to get a new trial despite what my wife thinks."

During the hearing the public defenders representing Smith have put on the stand lie detector experts as well as an expert in the causes of fire in an effort to challenge evidence aired in Smith's trial.

Michael Maher, the public defender's expert on flammability, testified that recent discoveries concerning polyurethane fabrics have provided another explanation for the fatal fire that cost the lives of George and Lucille Brown.

In the trial, Smith's attorney had argued that the fire was caused by a cigarette left on a couch and that fire damage involving the front door was caused by nearby faulty electrical wiring.

Maher offered a new explanation, saying that the polyurethane fabric on the couch was ignited by a cigarette, releasing an explosive gas that accumulated near the ceiling, which then exploded, scorching the door.

Mrs. Smith said more than 30 residents from her community have come to the trial in support of her son. "They all believe he's innocent."

The current hearing - based on a petition for habeas corpus - follows an unsuccessful appeal of the original conviction all the way to the U.S. Supreme Court.

Should Femia rule against the petition, the Smiths will be entitled to challenge that ruling in U.S. courts as well. Should that prove fruitless, Robert Ashley Smith will have exhausted all avenues of appeal.