DENVER -- "Where's the beef?" rides again.

In a deliberate replay of the strategy that Walter F. Mondale used to stave off the onrushing candidacy of Gary Hart in the 1984 presidential primaries, Denver's incumbent mayor, Federico Penåa, may have found a way to save his job in Tuesday's municipal election runoff.

Penåa, a liberal Democrat, has moved to blunt the surprising momentum built up last month by Don Bain, the Republican challenger. Bain, a well-to-do lawyer, surprised everybody in this strongly Democratic city when he came hurtling out of obscurity to a first-place finish in the first round of mayoral voting a month ago.

When Penåa and Bain began campaigning for the runoff, the Republican was far ahead in all the opinion polls. The Bain campaign, based on the argument that Penåa has been weak and indecisive during his four years at city hall, seemed to have unstoppable force.

But Penåa has closed the gap with Mondale's 1984 tactic: stirring up suspicions about the challenger. Day after day, Penåa has posed a series of questions to the voters: "Who is Don Bain?"; "Have you taken a close look at Don Bain?"; and, on occasion, "Where's the beef?"

The constant suggestion that Bain and his supporters in the business community must be hiding something has served two purposes for Penåa. It shifted attention from Penåa's performance in office, and it evidently has made voters think twice about the friendly Republican newcomer to city politics.

It is not clear, though, whether Penåa's counterattack will be enough to win the election. The intense, serious incumbent has had trouble rebuilding the coalition of Hispanics, blacks and business leaders that helped him win the mayor's race four years ago.

Penåa's victory in 1983 made him one of the nation's more prominent Hispanic politicians, and he has been working hard over the past few weeks to build on that status among minority voters. A fellow Hispanic mayor, Henry G. Cisneros of San Antonio, scored well for Penåa on a campaign trip here.

Pollsters say a big reason for Penåa's evident gain in the past two weeks has been that minority group members and other traditional Democrats are "coming home" to Penåa. Although the election is legally nonpartisan, everyone knows that this is a Democrat-Republican contest.

In size and setting, Denver has some things in common with Washington, D.C. Like the District, the city of Denver has lacked the political clout to force suburban commuters to help pay for municipal services. Unlike the District, however, Denver is no longer enjoying an economic boom. The collapse of the energy economy here has hit the city hard.

This is evident in a comparison of this year's mayoral campaign with the one four years ago. In 1983, the big debates were about channeling the city's growth: where to put new office complexes and shopping centers.

Today, the candidates are arguing over how to fill the empty shell of a big downtown department store that went out of business. Reflecting the general turn in this region toward a tourist-oriented economy, Bain and Penåa agree that a major mission of the next mayor will be to build a new airport and convention center.

Bain's campaign is based on frustration with Penåa's record: "He's had four years to get started on an airport and a convention center and we don't even know yet where we're going to build them," Bain says.