On a single day -- exactly a year before the Iowa caucuses -- look what happened to two Republican presidential hopefuls.

Gov. Chris Christie of New Jersey, while visiting a vaccine laboratory in London, said that parents should have "some measure of choice" over whether their children get immunized. With health officials reporting more than 100 cases of measles in 14 states, Christie's casual comment touched off a barrage of criticism from fellow Republicans as well as Democrats.

"Christie's Vaccine Stumble" headlined a critical editorial in The Wall Street Journal. "The governor panders amid an outbreak of preventable disease."

The New York Times, which ran the vaccine story on Page One, featured a second Christie piece as well, this one a lengthy investigation into the governor's "Fondness for Luxe Benefits When Others Pay the Bills."

Meanwhile, Sen. Rand Paul of Kentucky faced such an intense grilling on CNBC about vaccinations and other issues that he actually held his finger to his lips, told the female interviewer to "Shhh" and advised, "Calm down a bit here, Kelly."

Long before any real voters cast ballots in a real election, candidates for president face three other primary challenges: for staff, for contributions and for press attention. This "Media Primary" is the only one conducted in full public view, so it carries the most risk for White House wannabees.

As Christie and Paul just found out, no matter how experienced a candidate is, or how deft at answering questions, none of them -- and we mean none -- realizes how much harder it is to run for president than for any other office. The spotlight is far hotter, the scrutiny far closer, the bar far higher. And that's the way it should be.

Being president is a very tough job. The pressure that Christie or Paul or any candidate gets from the press at this stage is far milder than the constant, crushing demands of everyday life in the Oval Office. Those who cannot survive the Media Primary are clearly not qualified to serve as president.

Look at Christie. He went to London to burnish his thin foreign policy credentials. If he were not a likely presidential candidate, would the Post have sent a reporter to cover him? Would the paper have quoted him extensively and then interviewed a professor at the Massachusetts Institute of Technology, who called the governor "incredibly, incredibly irresponsible"?

The answers are no and no.

Consider the Times story, which detailed at length the high cost of Christie's foreign travels, and his propensity for letting rich friends pick up the tab. The governor likes to portray himself as "a cheese-steak-on-the boardwalk Everyman," wrote the Times, but in reality, he has "a taste that runs more toward Champagne at the Four Seasons."

Would the paper have assigned that story -- and allotted it so much space -- if Christie were happy staying in Trenton? Again, the answer is no.

Paul faced a similar level of elevated scrutiny on CNBC. Anchor Kelly Evans was relentless, pushing him to explain his positions on vaccinations and corporate taxes.

The senator was so discombobulated that he accused her of conducting a "slanted" interview "full of distortions" and lectured: "I think this is what is bad about TV sometimes."

Really? You mean they hit harder in the big leagues?

Mitt Romney found that out when he dallied, briefly, with running for president again. The chorus of criticism was led by The Wall Street Journal -- certainly no fan of the Democrats -- which accused him of squandering a "winnable race" in 2012 through a "clueless" campaign.

"If Mitt Romney is the answer," snapped the Journal, "what is the question?"

Democrats are not immune to this sort of laceration, either. When Hillary Clinton was selling her book last year, critics pounced on a comment, elicited during an interview with ABC's Diane Sawyer, that she and Bill were "dead broke" when they left the White House. The Post reported that her fumble "served as a reminder that candidate Clinton has never been as sharp and polished as many of her boosters might hope."

Of course the Media Primary has its flaws. Journalists can cross lines and invade a candidate's privacy. And they risk trivializing a serious business by emphasizing conflict and controversy in a relentless search for the elusive clicks that produce revenue.

But when citizens choose a president, they need and deserve all the information they can get. The candidates themselves will provide only poll-tested, pre-masticated, mightily massaged messages. Only the media can produce the rest of the story.