For the first time since the 1980 New Hampshire presidential primary -- which followed immediately the Iowa Caucuses where the heavily favored front-runner had ducked the Des Moines candidates' debate and then finished a shocking second -- Ronald Reagan is under pressure to recoup politically and publicly in televised competition with a political opponent. That year, with two winning performances in the Manchester and Nashua debates, Reagan silenced the whispers and routed the opposition. Now four years later, after a bad opening night in Louisville, the president faces a rehabilitated and refortified Fritz Mondale in Kansas City.

Reagan fans would be thrilled with a performance by their candidate comparable to the one he delivered in Cleveland in 1980 against Jimmy Carter. That was the night Ronald Reagan, by establishing himself as a Respectable Alternative to an incumbent chief executive American voters did not want to reelect, won the presidency.

In Louisville, through his unexpectedly forceful presentation, Mondale became a Respectable Alternative to Ronald Reagan. Mondale's and the Democrats' principal problem is that this year seven out of 10 voters personally like the incumbent president and rate him as a strong leader, so popular demand for a replacement is not clamorous. The challenger's task is just the reverse of four years ago: first, establish yourself as the Respectable Alternative, which Mondale has accomplished; next, persuade the voters that the incumbent president ought not to have his contract extended for another four years, which remains unaccomplished. It's the "next" that is the enormous task confronting Mondale tonight in Kansas City.

For the Minnesotan often scorned for his caution, risk must be his co-pilot in this -- the last time he will share a platform and the electorate simultaneously with the president. After tonight, unless Mondale can change things dramatically, the rest of the campaign will be the political equivalent of parallel skiing with Reagan safely ahead and that much closer to the Nov. 6 finish line.

After establishing himself as a Respectable Alternative in their first debate, Mondale cannot and must not worry about appearing desperate. Since Oct. 7, he has been taken seriously by both the incumbent and the electorate.

As somebody (it was either Rousseau or Richard Simmons) once said, an afternoon can be a lifetime in politics and tomorrow can be forever. Just four years ago, only four months before he was to be involuntarily retired, the Democratic president was told by his pollster that, "The American people do not like Jimmy Carter. Indeed, a large segment could be said to loathe the president." That same year, the Reagan pollster told him that he, the candidate, was "our most valuable resource" and that the campaign's "most effective asset is providing the electorate with in- depth exposure to Ronald Reagan." If that same belief still exists today, it has not become a major element of the Reagan campaign strategy. In- depth exposure is exactly what voters will be looking for tonight.

For Mondale, this debate is everything, and a little more. To "win," Mondale must convince doubting voters that he believes as much in strength as in peace. The challenger must force voters to think about what most of us resist thinking about: the actual risk of nuclear war. Then he must convince voters that he, not the president, could far better handle that risk.

To bring all this off, the Democratic underdog cannot wait for the president to make a mistake. At the risk of being branded too confrontational, he must confront the president in a way that forces voters to question how comfortable they would really be with "four more years." But unless he can convince voters that he is in the mainstream of Democratic presidents who believed in and sought peace through strength, his mission is doomed to failure. Voters do not doubt the former vice president's commitment to peace; they are skeptical about his and his party's commitment to strength. For the 1984 campaign, Kansas City is the World Series and the Super Bowl all rolled into one.