IN April 1993 Denverites will finally hear "Play ball!" for their own major league team. The Colorado Rockies will take the field in Mile-High Stadium as one of two new National League expansion teams (the second goes to Miami).The Baseball Commission's official announcement last week was long-desired good news for the Denver economy: "The Chamber of Commerce's study predicted that [major league] baseball will bring in $90 million a year to the area - $35 million in new [tourist] dollars," says outgoing Mayor Federico Pena. "It is critical to the city to attract economic development." Six cities vied for the expansion teams. Only the Baseball Commission can add teams and expansion in either league is rare; the last expansion, on the American League side, was in 1977. With the two new teams, both the National and American Leagues will each have 14 teams. The Rockies will cost its new owners $95 million in expansion fees and at least another $15 million. It would have been cheaper to buy a more seasoned ball team and move it to Denver - certainly a fallback position for the four cities that contended unsuccessfully for a new team. But baseball teams are rarely for sale. Getting the National League to expand, however, was a long and grueling process. Denver had been clamoring for a major league team since the late 1950s. In 1985 US Sen. Timothy Wirth (D) of Colorado began investigating the possibilities of expansion and in 1987 formed a Senate task force to study the issues. The group met with then-Baseball Commissioner Peter Ueberroth, who, says Senator Wirth, was not interested at that time in expansion. "But 26 senators felt baseball ought to expand and had an obligation to do so," Wirth says. The task force generated research and held a number of meetings with Mr. Ueberroth and other baseball officials. "I think there was a lot of pressure coming from the Senate," Wirth recalls. "We tried to knock down all the arguments they had." Ueberroth argued that too many teams were losing money, the cities seeking teams didn't have baseball-only stadiums, and there were problems with TV contracts and players' salaries. But Senator Wirth countered, saying that by 1988 most franchises were operating in the black, that most big-league stadiums are not baseball only, and, most important, the talent pool had greatly expanded. Did pressure from the Senate expansion committee perhaps nudge forward the expansion process? "Yes," concedes Wirth, "it probably helped." So in a competitive field of cities bidding for one of the two new teams, how did Denver, a city of only 1.8 million people, shut out the others? A new stadium, for starters. The city's offer to build a first-class ballpark dedicated to baseball swung the expansion committee in Denver's favor. Last August voters approved a municipal tax to pay for construction, and in March, Golden-based Coors Corporation received naming rights with its pledge for an additional $30 million. "New stadiums attract people," asserts Robert Howsam Jr., former general manager of the Triple-A Denver Zephers. "And you need the luxury boxes for revenues and all the initial signage rights. If Denver hadn't voted in the new stadium, we wouldn't have had a baseball team here." Mr. Howsam says he firmly believes that choosing Denver over several other cities makes good sense to the expansion committee. There is no major club in the Rocky Mountain time zone. One new team is going east of the Mississippi River and the other west of the Mississippi, an appropriate geographical balance, he says. Though Denver's population is considered to be too small to sustain a major league team in the long-term, Colorado attracts 14 million visitors each summer. Already, 24,000 season tickets have been sold. The 2.2 million fans needed to make payroll the first year seems assured. But a ball club can take 10 years to become competitive and Denver fans are not known for their patience with losing teams, skeptics say. Fan support - and revenues - may wane. "We don't think it will take 10 years," Jacobs says. "The difference is, the commissioner made a ruling, gave some of the expansion money to the American League, and said the American League had to contribute players to the expansion draft. That's never been done before, so the pool of talent that we'll have to pick from will be deeper. And it should allow us to be more competitive more quickly."