Now that Mitt Romney is the de facto Republican nominee, the political press corps can indulge in a compulsion as addictive as OxyContin and as frivolous as Angry Birds. The rules of the Pundits Guild stipulate that as soon as the news broke Tuesday afternoon that Rick Santorum was suspending his campaign, the vice-presidential frenzy had to begin without even a moment’s pause for rest or (yikes) reflection.

In truth, the press pack jumped the gun in the Heartbeat-Away Derby by anointing Romney as soon as he won the Wisconsin primary. Already we have been treated to a full-scale boomlet for Ohio Senator Rob Portman. The insider outlook was succinctly expressed last week by a Politico headline over a Maggie Haberman story: “Rob Portman tops veepstakes.” And the National Journal’s Major Garrett on Thursday offered one of those glib parallels that always pop up when speculating about the bottom of the ticket: “Portman is to Romney what Al Gore was to Bill Clinton.”

But not so fast, Vice President Portman. If the journalistic enthusiasm for Portman, a former George W. Bush budget director, is based on armchair psychological analysis about what Romney might theoretically want in a running mate, other reporters have been equally energetic in scrutinizing body language. Morgan Little, in the Los Angeles Times, recently observed, “If Romney’s campaigning in Wisconsin is any indication, Rep. Paul Ryan (R-Wis.) may be the new favorite.” Steve Holland in a Reuters dispatch Friday similarly noted, “The name most associated with Romney in the last week is Ryan who looked to have a personal chemistry with the former Massachusetts governor when they campaigned in Wisconsin.” There is always an inherent danger in allowing reporters this early in the vice presidential search to play with chemistry sets.

Meanwhile, Marco Rubio’s fortunes have bobbed like a flamingo’s head as it searches for food. The opening sentence of the Miami Herald story about the fledgling Florida senator’s late March endorsement of Romney included the obligatory “fueled speculation he is seeking a spot on the ticket.” But then skeptics began to take seriously Rubio’s boilerplate disavowals of interest. Time’s Michael Crowley was typical in concluding Friday, “I suspect Rubio may understand that his political stock may be overvalued.”

Whew. It’s going to be an exhausting four months until Romney takes us out of our collective misery by actually picking his own version of Mini-Me. Probing analysis and deft biographical portraits of vice presidential possibilities are valuable at any stage since nothing in a campaign for the White House is more important in future governing terms than the selection of a running mate. The problem comes when the press corps gets too far ahead of reality with the frenzied speculation about the results of an election with only one voter (Romney) who is keeping his thoughts to himself. Recent history suggests that treating the veepstakes like another political horse race invariably produces lame conclusions.

At this point in 2008, only a few conservative publications (most notably the Weekly Standard) were taking Sarah Palin seriously as John McCain’s running mate. (On the Democratic side, attention was still fixated on Hillary Clinton’s last stand against Barack Obama). While the pre-scandal-scarred John Edwards was on everyone’s 2004 VP short list, it did not prevent the New York Post from knocking wood with a banner, “IT’S GEPHARDT,” on the morning that John Kerry was scheduled to unveil his choice.

The campaign press pack would have done better with a dart board in 2000 than it did by ballyhooing the vice-presidential rumors passed along by political insiders. That April, Joe Lieberman was buried in long lists of possible Al Gore running mates and Dick Cheney had just been named as the impartial head of George W. Bush’s search committee. New York Daily News Washington bureau chief Thomas M. DeFrank captured the prevailing mood when he wrote on April 20, 2000, “Bush and Gore have said they haven’t yet compiled short lists of running mates, but Gov. Tom Ridge (R-Penn.) and Sen. Bob Graham (D-Fla.) frequently are mentioned by political insiders as leading contenders for their respective tickets.”

Walter Shapiro just chronicled his ninth presidential campaign. He writes the “Character Sketch” political column for Yahoo News. Follow him on Twitter @WalterShapiroPD.