Hillary Clinton says she's running for president in a video...

Hillary Clinton says she's running for president in a video posted to YouTube on April 12, 2015.

With Hillary Clinton's announcement Sunday -- finally admitting she's been running for president more or less since the day after she conceded the 2008 nomination -- we need to accept an important truth about presidential candidates.

For all practical purposes, there's simply no such thing as a strong general-election candidate.

There are strong candidates for a presidential nomination, and Clinton is about the strongest in modern times. There can be weak general-election candidates, too. Those perceived as ideological outliers (Barry Goldwater in 1964, George McGovern in 1972 and possibly Ronald Reagan in 1980) can cost their party a few percentage points beyond what a generic Democrat or Republican would have received.

It's also possible to imagine a candidate so inept that he or she forfeits votes a generic candidate might have won -- McGovern, with a mismanaged convention and a botched running-mate selection, might qualify.

But what about candidates who are so wonderful, whose appeal to swing voters is so strong, that they override the basic conditions of the election -- the economy, war and peace, the popularity of the president, how long the incumbent party has held the White House? In the entire survey research era, the only presidential candidate who can plausibly make that claim is Dwight Eisenhower, and he just may have benefited from too many consecutive Democratic terms in office.

Everything done by campaigns serves to build a superhumanly wonderful portrait of the candidate. There are those who are inclined to vote for that candidate anyway -- partisans who always vote for their party, swing voters who react to the economy or other fundamentals.

Those not inclined to do so probably won't believe the hype, no matter how gushing. It may feel as if we're drawn to vote for Barack Obama or Mitt Romney because we like them. In fact, we're just very receptive to liking candidates who we are (more or less rationally) likely to support in the first place.

Consider Obama. In 2008, it certainly seemed that he was a once-in-a-generation political force, a candidate who could truly bring new voters to the polls and shake up the status quo. And yet his final results looked a lot like what would happen if the 2004 electoral map were just shifted to the Democrats to account for Iraq and a deep recession -- just as predicted by political science models that know nothing about the candidates.

Even the downside of nominating a dud candidate is much less than it seems. By the time a candidate advances to the general election, he or she has been thoroughly vetted by the media and by the party, so there's a limit to how bad he or she can really be.

Granted, there's always the possibility something unprecedented will occur; we only have a very limited number of presidential elections to test for effects. It's not impossible, for example, that the chance to elect the first woman president will matter. But it's not especially likely, either.

The harsh truth is that especially in a partisan age, the candidates themselves aren't that big a factor in presidential general elections. Democrats may be wasting a lot of time and energy worrying what they would do if something happened to Clinton, but the truth is that they would do about as well with most replacements.

And the odds are that the same will be true on the Republican side, too.