Barely 18 months before Iowa’s influential first-in-the-nation 2016 presidential contest, potential presidential candidate and former Secretary of State Hillary Clinton today holds a more commanding position for the nomination than any nonincumbent White House candidate has ever enjoyed, according to every national poll. National surveys continue to name her the first choice of basically seven out of 10 Democratic primary voters to be the party’s 2016 nominee.

But more important, according to the respected Wall Street Journal-NBC News poll, 55 percent of Americans now rate Clinton “knowledgeable and experienced enough to handle the presidency.”

This is a rare political strength. From Republican pollster Bill McInturff, who, with Democratic pollster Peter Hart, conducts this Wall Street Journal-NBC survey, comes this: “Never underestimate how incredibly powerful it is to (be able to ) say you’re knowledgeable and experienced enough to handle the presidency. To start a race with that is an extraordinary compliment.”

Anyone harboring White House ambitions (a group that almost surely includes a majority of U.S. senators) would kill to have Clinton’s numbers and standing. But, believe me, inevitability is not a winning campaign strategy.

In 1971, I was the political director for the presidential campaign of an exceptional public servant, Sen. Edmund Muskie, D-Maine. He was the strongest challenger in all polls, the only Democrat consistently defeating President Richard Nixon. To discourage other Democratic opponents and to demonstrate Muskie’s breadth of appeal, we rolled out an impressive array of public endorsements from dozens of governors, senators, and leading party conservatives and liberals.

In the United States, we no longer deliver milk or bread, and nobody delivers votes. Democrats, admittedly, are not publicly religious, but most party members seem to cherish the Bible story of David and Goliath. Democratic primary voters, as Clinton painfully learned in 2008, regularly reject the powerful, better-known favorite, falling head over heels instead for some previously unknown long shot.

Only twice in more than a half-century has a Democratic presidential nominee, in the year preceding the election, been the front-runner for the nomination in the Gallup poll: former Vice President Walter Mondale in 1984 and sitting Vice President Al Gore in 2000. Consider the track record of the first Democratic president since Franklin D. Roosevelt to win two White House terms — Bill Clinton. Just 13 months before he would defeat President George H.W. Bush, Clinton was running fifth among primary candidates, the choice of a thumping 6 percent of Democrats. President Jimmy Carter did not even register in any of the Gallup polls conducted the year before he was nominated and elected. Before they were nominated, Michael Dukakis, George McGovern, John Kerry and Barack Obama were all political “Davids.”

There will be, you can bet, at least one 2016 long-shot underdog candidate who captures both headlines and emotions, and excites Democratic primary voters. It’s in our DNA: Americans root for the underdog.

For Clinton, the road ahead is tricky. With only 25 percent of voters seeing the country as “headed in the right direction,” there is no popular groundswell for a third Obama term. Clinton must — without being disloyal — credibly establish daylight between herself and the incumbent administration in which, voters will be reminded, she was an important actor.

The strong temptation, understandably when her husband’s eight years are widely regarded as “the good ol’ days,” could be to invoke a “Back to the Future” theme. But American presidential campaigns are ultimately about the future.

Yes, Clinton’s position is dominant. But inevitability is never a winning strategy, and it’s not fun being Goliath.

Mark Shields is one of the most widely recognized political commentators in the United States. The former Washington Post editorial columnist appears regularly on CNN, on public television and on radio. Click here to contact him, or click here to read previous columns. The opinions expressed are his own.