ST. PAUL, Minn. — The Republican National Convention is over (let's hear a loud shout-out for St. Paul!). All the 2008 presidential primaries and caucuses are history. Barely eight weeks remain before the nation's voters — in the civic sacrament of democracy we call a national election — will choose a new American leader.

Republican nominee John McCain's running-mate pick, Alaska Gov. Sarah Palin — along with the procedures McCain used to choose her — have already gotten wall-to-wall press coverage. But let us be blunt: Barring some genuine bombshell disclosure, both 2008 vice-presidential nominees will have minimal influence upon whom Americans will decide to be commander in chief for the next four years.

In the past 40 years, there have been just two elections when the vice-presidential candidates were manifestly superior to their opponents. The first was 1968, when the gifted Democratic senator from Maine, Edmund S. Muskie, was overwhelmingly — and rightly — preferred to Republican Richard M. Nixon's running mate, the mediocre and corrupt Maryland Gov. Spiro T. Agnew. The second was 20 years later, when Democratic Sen. Lloyd Bentsen of Texas all but annihilated his overmatched Republican opponent, Indiana Sen. Dan Quayle.

That was 1988, when Bentsen delivered the most memorable debate line in U.S. history, after Quayle had compared his own relatively thin resume to that of an earlier Democrat, John F. Kennedy: "I knew Jack Kennedy; Jack Kennedy was a friend of mine. Senator, you're no Jack Kennedy." If the Bentsen-Quayle debate had been a professional prize fight, any responsible referee would have stepped in and stopped it right then and there.

The important thing to know is that despite the clear advantages to Muskie and Bentsen, the tickets on which they were running lost. In 1968, Democratic presidential candidate Hubert Humphrey lost to Richard Nixon. In 1988, Democratic presidential nominee Michael S. Dukakis lost 40 states to Republican George H.W. Bush.

Understand this about presidential elections and American voters: We base our voting decision on who will be president — Barack Obama or John McCain — and not whether Sarah Palin or Joe Biden will be vice president.

That presidential decision will rest in large part on the three presidential debates scheduled for Sept. 26, Oct. 7 and Oct. 15. John McCain's best — maybe his only — hope is to prevail clearly over Barack Obama in those debates. Obama, in the respected judgment of Democratic pollster Peter D. Hart, "cannot seal the deal with voters unless and until he proves in the debates that he is the guy they want in the White House."

These presidential debates are the closest thing American voters have to a joint job interview where both "applicants," the Democratic and Republican nominees, are simultaneously and publicly judged on qualities of intellect, character, likeability and grace under pressure.

We have one American politician to thank for the gift of these presidential debates — President Ronald Reagan. After the first presidential debate in 1960 between Democrat Jack Kennedy and Republican Richard Nixon, the Democratic president in 1964, Lyndon B. Johnson — with a big lead in the public opinion polls — refused to debate Republican Barry M. Goldwater. In 1968 and 1972, Republican Richard M. Nixon refused to risk his lead in the polls by debating.

That changed in 1976, when President Gerald R. Ford, trailing Democrat Jimmy Carter, did agree to debate. In 1980, then-President Jimmy Carter met his Republican challenger Ronald Reagan in a single debate. But it was Republican incumbent President Reagan in 1984, with a big lead over Democrat Walter Mondale, who would pressure all future front-running candidates to meet their trailing challengers in televised debates by going against Mondale. So when you sit down to look over Obama and McCain in open debate, thank the Gipper for making it all happen.

To find out more about Mark Shields and read his past columns, visit the Creators Syndicate web page at www.creators.com.

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COPYRIGHT 2008 MARK SHIELDS