Gio Gonzalez and the Nationals are sitting pretty and could meet Beltway-area rival Baltimore in baseball’s postseason. But first both teams and their fans have to get through September. (Toni L. Sandys/The Washington Post)

How do you hold a lead that ought to be safe but isn’t?

How do you keep from choking in September when you have just enough of a division lead to relax? Or gag and be overtaken in a final week that feels like a slasher flick come to life?

How do you avoid the trap of “if we just play .500 we’ll be okay”? How do you ignore your magic number? How do you keep grinding in Games No. 140, 150 or 160?

And how do you prepare to play the team that’s chasing you six times in September when it has the underdog’s desperation edge?

The Washington Nationals and their fans, with a six-game lead in the NL East, are about to find out. The Orioles, with a nine-game lead in the AL East, may be out of range of demons. But in 2011, two teams missed the playoffs after holding nine- and 81 / 2-game leads for postseason spots in September.

With just over a month left in the regular season, the Nationals have a solid starting rotation with the best ERA in the National League. Assuming the playoffs started tomorrow, the Post Sports Live crew debates which Nationals pitcher should start Game 1. (Post Sports Live/The Washington Post)

All baseball fans want their team to be in first place on Labor Day. A century of fact and lore says if you reach the head of the stretch with a significant lead, especially of five games or more, like the Nats and Orioles, you’re supposedly almost impossible to catch. The nature of the game, the difficulty of playing way above .500, even for one month, makes it harder to play catch-up in MLB than any pro sport.

But that’s not how it’s probably going to feel. Go on, say, “I’ll just relax and enjoy it. Isn’t it great to have so much quality baseball to watch?” Tell that to your four-seamer-sized ulcer. The Nats just hit 10 homers in three games in Seattle, but their lead on Atlanta has shrunk by two games over the last week.

You can munch probabilities for breakfast, digest stats for lunch and hug that postseason probability graph at that says Washington has a 97.4 percent chance to win its division and Baltimore a 98.2 percent likelihood. But it’s not going to help your stomach when they play. Because you know.

In just the past four seasons, nine teams that were in first place Sept. 1 were caught and passed for their division crowns. True, the biggest leads that any of them blew were four games by the 2012 Rangers to the A’s and three games by the stunned 2010 Braves (who ended up six games behind the Phillies). The kind of leads the Nats and Orioles now have are usually bullet-proof. But not always.

The biggest leads ever blown in September to miss the postseason altogether are the 2011 Red Sox (nine games), 2011 Braves (81 / 2 games), 1995 Angels (71 / 2) and five teams that blew seven-game margins — the 2009 Tigers, 2007 Mets, 1951 Dodgers, 1938 Pirates and 1934 Giants.

The Nats certainly have the harder road. Literally — “road.”

After concluding their current nine-game road trip that finishes against the Dodgers, the Nationals also have an 11-game mid-September jaunt with a visit to Atlanta in the middle of it. Their final six-game homestand at Nationals Park looks reassuring. If they get there with their lead and sanity intact.

But the Orioles have a headache, too. Nothing prevents collapses better than outstanding starting pitching. Every day, no matter what happened in its previous game, Washington is going to send Jordan Zimmermann, Stephen Strasburg, Doug Fister, Gio Gonzalez or Tanner Roark to the mound. That’s comfort food. The Orioles’ no-name rotation has been gritty. Can they avoid stretch-drive indigestion?

Winning is fun. After you’ve won. Until then, it’s tension. Being ahead is better, much better. August was a joy for both the Nats, who ran off a 12-1 blitz to get ahead by eight games, and the Orioles, who went 12-4 to reach a nine-game lead. But for teams such as these, which look 90-wins-plus good but not 100-wins-plus magnificent, getting to the wire is almost never a waltz.

Two years ago, the Nats led the NL East by 61 / 2 games on Sept. 1 and pulled away to 81 / 2 games by Sept. 12. Easy, huh? Yet they didn’t back into a division clinch until their 160th game.

Get ready for the glorious suffering. Our reward? The whole month of October could be even better.

Three parts of the country have a rare baseball condition at the moment. In Los Angeles, the Dodgers and Angels are both in first place in their divisions. If the season ended now, the Giants and A’s, on opposite sides of the San Francisco Bay, would both be wild cards, too. But no region is sitting nearly as pretty — and nervously — as Washington and Baltimore with their semi-insurmountable leads.

Don’t even say “Beltway World Series” or “Parkway Series.” It’s possible. But that’s begging for disappointment and getting a hundred miles (or 52 days, not that anyone’s counting) ahead of yourself. If I had to handicap today, I don’t think either team will make the World Series. Not quite good enough.

But I’d also guess, out of a jillion possibilities, that the NLCS would start with the Nats at the Dodgers and the ALCS with the O’s at the Angels. That doesn’t even qualify as predicting. Right now, that’s just a chalk bet. Are our nerves really prepared for this? The last time that Washington and Baltimore, which both had major league franchises as long ago as 1901, were in first place on Labor Day was — never.

Many have been waiting a long time for Washington to experience a really long baseball run. Until you’ve experienced it, you don’t really know what the sport offers. The idea of a bragging rights rivalry with the polarizing Orioles — who aren’t loved or hated in D.C. but certainly qualify as widely liked (as a team) or disliked (because of their Nat-swatting owner) — only adds a rich gravy.

Former Orioles owner Edward Bennett Williams, who was once president of Washington’s NFL franchise, loved what he called “competition living.” Compared with the intensity of football, he thought baseball would be soft stuff. Then his Orioles got in a pennant race. He hated to go to bed until the last out, even when games were on the West Coast and he had to be up early the next morning. In football, he could be on egg shells once a week. “With baseball,” he said, “I can be miserable every day.”

Now, as September arrives, baseball’s level of delicious torment turns up a notch. How do teams combat the pressure? There actually is an answer, easy to say, hard to do: with aggression. Never defend a lead, whether in a game or in the standings. Always try to build it.

Most important, be willing to live with your season-long identity and accept the results. As soon as a team starts flip-flopping its rotation, skipping cold pitchers or changing closers, using an ace on short rest or changing its lineup, then panic can start to arrive.

And look at the big picture as little as humanly possible. That only heightens emotions, makes every win more exhilarating, every loss seem worse than it is. That will drive any team crazy.

Besides, losing your mind, screaming and booing, sacrificing sleep to watch West Coast games, second-guessing managers and consulting oracles — all the manifestations of late-season baseball insanity as the Sept. 1 bell-lap arrives — that’s not a player’s task.

That’s our job. So let’s get started.

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