Denver voters gave their energetic mayor, Federico Pen a, and his billionaire ally, Marvin Davis, a sharp slap in the face this week, overwhelmingly rejecting the mayor's plan for a new convention center.

Despite an intensive two-month campaign powered by the charisma of Pen a and the checkbooks of Davis and his business partners, the city voted almost 2 to 1 against a referendum proposal calling for a $138 million facility behind the huge Gothic railroad station at the edge of downtown.

Some political pros here had considered the election a mid-term referendum on the mayor's performance. But once the votes were counted, Pen a moved to distinguish the ballot from his personal standing: "People have been saying, 'Mayor, you're doing a great job, but on this one we don't agree with you,' " Pen a said.

Pen a, a Democrat, emerged as one of the nation's leading Hispanic politicians two years ago when he won the mayoral election here. The city seemed to be electrified by the earnest, hard-working young lawyer -- Denver's city hall was unofficially renamed the "Hall o' Pen a" -- and his opinion-poll ratings have regularly been high.

There has been no shortage of political challengers, though. At least three leading candidates for the mayor's job in 1987, when Pen a's first term will expire, came out against his position on the convention center referendum.

In various political forums, Pen a has championed a concept of "leadership by consensus."

"Inclusiveness is the watchword," he said at seminar on political power. "You can't go off marching by yourself." But the mayor found himself marching away from his constituency on the question of building a new convention center near the rail yards north of downtown.

Pen a talks constantly about Denver's future -- about his dream that it will be a "great city" on a par with Chicago and Los Angeles at the eastern edge of the Rockies.

But when that dream means spending public money, many Denver residents tend to talk about the past -- particularly the last five years of decline for the oil, coal and uranium industries.

Denver, population 490,000, is the capital of the energy business in this region; and that industry's decline has ravaged the real estate market here, particularly downtown. As a result, many Denverites are scared to embark on big, expensive new projects like Pen a's proposed new convention center.

Pen a's ambitious plan faced another obstacle, too: Some of the vacant land he targeted for the convention center development is owned by a partnership that includes Davis. The oil/motion picture/real estate billionaire is a Pen a ally and Denver's equivalent of a Getty or a Rockefeller, tied with David Rockefeller for eighth place on Forbes magazine's latest list of the richest Americans.

As the mayor's opponents noted repeatedly, approval of the referendum would provide the tycoon a windfall at taxpayers' expense. "Should our taxes support millionaires' high-risk pet project . . . ?" demanded one of the posters urging a "No" vote.

The major opponents of Pen a's convention center plan were also wealthy businessmen who had hoped the city would choose an alternative site on land they owned. But the mayor was never able to make that point stick with voters.

Pen a's side had some strong assets for the referendum campaign, though. With heavy contributions from business, they outspent the opposition 2 1/2 to 1, hiring the city's top-ranked political consultants and pollsters.

When Davis' involvement emerged as a campaign issue, there was a sudden spate of news leaks -- resulting in banner coverage in all the local news media -- that the billionaire had bought the Oakland A's baseball team and planned to move it here to replace Denver's minor-league squad. The stories have not yet proved true.

Throughout the campaign, polls had shown the outcome close. But Tuesday's election turned into a blowout. About 100,000 voters came out, a record turnout of 35 percent for an election not involving a candidate; and 65 percent of them voted "No."

After the deluge, Pen a was asked if his big loss would have a negative impact on his political clout. His answer was brief: "What political clout?" he asked.