It is far easier to argue that your opponent is not fit to serve if you can provide confidence that you are fit to serve yourself.
That's part of how Sunday's medical episode involving Hillary Clinton, followed by contradictory and incomplete explanations from Clinton's campaign, makes for a major campaign moment that has the potential to upend the race to the White House.
Fitness now takes on literal meaning and outsized importance in a tumultuous campaign that already seemed to see everything. Questions around Clinton's health figure to compete for space with how Clinton's campaign handled the fallout for at least the next few days, if not right through Election Day.
One bad health spell -– assuming it's only that –- could have the impact of legitimizing concerns raised by an opponent that Clinton has been actively seeking to delegitimize. It puts Clinton's allies in an awkward spot in what was already set to be a difficult phase of her campaign.
In a measure of how difficult this may be for Clinton to muddle through, Donald Trump and his campaign have shown uncharacteristic initial restraint. They realize that this is a mess of Clinton's own creating, and that how she deals with it could have far-reaching ramifications for the balance of the race.
Within moments of the episode itself, the campaign went into a vaguely familiar lockdown mode. When word did emerge from the campaign, the official account was that Clinton felt overheated at a 9/11 remembrance event and was feeling "much better" -- a story that seemed incomplete, based on video evidence that quickly surfaced.
Hours later came word that as that she had been diagnosed with pneumonia on Friday and placed on antibiotics. This came after days when coughing fits bad enough to stop her from continuing at public events had been blamed on allergies.
All of this may have been hard enough to believe on its face had Clinton and her aides not had a history of misleading the press and public when it comes to her health. The full story of her fainting spell in 2012, when she was secretary of state, may still not be known, and only began to be told when glasses she wore in public testimony revealed a more complicated truth.
Earlier this year, in relation to the unrelated matter of her personal email, Clinton declared that she was the "most transparent official in modern times." That statement strains basic credulity –- now more than ever.
Clinton allies will argue that she has provided more health-related details, not to mention vastly more financial and professional information, than Trump. That is inarguable.
It is also, for now, beside the point. After just this one ill-timed health scare, and in the course of just a few hours, Clinton's campaign reinforced concerns about her honesty and accountability. The timing also happened to play into a Trump campaign line of attack about her stamina and overall health.
Just last week, when asked about her health, Clinton declared, "I'm not concerned about the conspiracy theories."
But when events and actions reinforce those same storylines, it's time to be concerned. Those theories will harden and go mainstream, until or unless the campaign addresses them not with accusations and partial accounts but with solid information.