The Washington Nationals deserve their sunshine and optimism-filled spring training, but one huge storm cloud hangs over the fun — the unfinished $100 million-plus contract extension for Ryan Zimmerman. The thunder may start to crash on Saturday with repercussions that, worst case, could lead to the third baseman being traded by this season’s July 31 deadline.

If you think everything is puppy dogs and ice cream now, see how the mood changes and the five-year window alters if the face of the franchise leaves.

Both Zimmerman and the Nats’ front office want to push to get the deal completed by Zimmerman’s self-imposed but reasonable Saturday deadline. Team executives even suspect that they’re getting a modest bargain because Zimmerman, who has two seasons and $26 million remaining on his old deal, is willing to negotiate from a weakened position after an injury in 2011.

The Lerners, stung in part by their seven-year, $126 million free agent contract to Jayson Werth last year, are balking. They aren’t crazy. And it’s their money, even though it flows to them through their fans. But, in this case, they’re wrong. This deal, despite all its risks, needs to get done now.

This isn’t a dead-easy call. Zimmerman missed 55 games in ’08 and 61 last season but, overall, he’s averaged 138 games through his six full years, slightly more than Troy Tulowitzki (134 in five years), who signed a seven-year, $134 million extension a year ago.

“Zimmerman will probably stay healthy, have a great year and be a lot more expensive next season,” said one puzzled veteran baseball executive.

If the Nats want him, they aren’t going to get him any cheaper. If they don’t want him now, then trading him by mid-year, even though they won’t get 100 cents on the dollar, would be the logical next step for many teams. The Nats’ front office, grumpy of late, has already mulled over that possibility.

Some teams, like the Brewers with Prince Fielder, just let the star play out his deal and wave goodbye. But that sends a loud message to every other young player on your team. If you act perfectly and play productively, if you grew up in Virginia Beach and gave the team its only day-after-day symbol of hope for years, this is what you get. The Nats are loaded with such players, all of them watching: Stephen Strasburg, Drew Storen, Jordan Zimmermann, Danny Espinosa, Wilson Ramos, Bryce Harper.

If any player has earned a commitment it is Zimmerman, a model citizen who’s been an all-star, Gold Glove and Silver Slugger winner. Vast contract extensions are never safe. But the Nats are at a crossroads right now — the intersection of Risk and Reward — with their on-field success, their fan base and the enthusiasm in their clubhouse all skyrocketing.

So the implications of the Nats-Zim affair may be far reaching.

“If we could get Ryan extended, it would finish off a great offseason perfectly. It would be the last signal that all systems are go around here,” said a high-placed Nats executive who isn’t involved in the negotiations.

Flip the coin, however, and just days from now that perspective could be less cheerful. Not demolished by any means, but dented.

“You’ve got to take care of a guy like that after what he’s done for this team” the last six years,” said Storen, a 43-save reliever. “I sure don’t want to [face] him in a different jersey.”

Storen and Zimmerman have the same agent, Brodie Van Wagenen, who met with General Manager Mike Rizzo on Wednesday to talk about Storen. When will they deal with Zimmerman? Or will they get serious at all? Put out an APB on Van Wagenen’s car. If an agent leaves town just before a well-known deadline, that always means exactly what you think it means: major breakdown, potential long-term trouble, call the marriage counselors.

None of that aggravation is necessary, if everybody’s reasonable.

“Coming off an injury and playing third, not shortstop, you’re worth about 85 percent of Tulo, right? You’re a $17-to-$18-million-a-year player now. But next year, maybe more,” I teased Zimmerman on Wednesday.

“Maybe a lot more,” he said, laughing.

“I really want to stay here,” he said. “It might even be good that I’m coming off an injury year. It makes the price lower and a deal easier, you’d think. Whatever happens, it’s more than I can ever spend. So I don’t want to be greedy. You want to be happy. I saw what happened to Adam Dunn last year. You never know the impact when you change teams.

“But there’s a market [value] in this game. You can’t ignore that.”

In light of Werth, ownership appreciates the dangers of long-term contracts intensely. And they also see what looks like an accidental windfall in the way third baseman Anthony Rendon fell to them at sixth overall in last June’s draft when every board had him rated No. 1 or 2. Suddenly, a possible .300-hitting Zim replacement might arrive by 2014. Save $100 million, plug in a prospect and (maybe) spend the cash elsewhere. It’s always tempting to see prospect and think star. It seldom works so easily.

There’s also a certain element of old-fashioned common-sense shame that should be at work here. estimates that Zimmerman has been worth $125.6 million to the Nats in his career, based on wins above replacement; thus far, he’s been paid $19.5 million. So, maybe, the Lerners are already $106.1 million ahead of Zim. And they’ve got him signed well under market value for two more years. Maybe that’s how you become a billionaire, but that math might embarrass most people.

For me, the resolution should be simple. When you have first-rate baseball experts, leave decisions to them — even $100 million ones. What do the Nats’ “baseball people” — not one person, but their collective wisdom — have to do to persuade ownership to let them have the lion’s share of responsibility when the journey gets especially scary?

It’s not just that the Nats’ front office has, in three years, rebuilt the farm system from a joke to No. 1-rated or swung trades like Michael Morse for Ryan Langerhans and Ramos for Matt Capps. It’s also the past deals that were all teed up and, according to former Nats, got nixed, like a trade of Dunn for the Rays’ Matt Moore, now the game’s best lefty prospect.

Ted Lerner trusts his gut. And it has helped make him billions. But he has a builder’s gut, proven over decades in his business career. It’s not a baseball lifer’s gut instinct. Stop seeing Werth in your nightmares. Look at all the other decisions in the last three years that have been correct. Then do what D.C. fans of other pro teams so often want to scream, “Let the people that you were smart enough to hire make the calls.”

That doesn’t mean they’ll be right. It means that the process of making the decision was correct. That, terrifying as it may be, is the best you can do.

For Thomas Boswell’s previous columns, go to washingtonpost.