This year’s National League has often seemed divisible into two classes of teams: the Cubs, and everyone else. Lou Piniella’s club has held at least a share of first place in the NL Central every day since May 11, and a share of the league’s best record since May 26. The Cubs clinched their second consecutive division title on September 20, and Monday night’s victory over the Mets guaranteed them home-field advantage throughout the NL playoffs. Their competition remains a mystery, however. With just six games to go, the fight for the league’s three other playoff spots remains a war of attrition among teams who seem intent on backing into berths not by winning, but simply by waiting for their competition to lose more often.

Take the National League West. Through the end of April, the Diamondbacks built their own case as the league’s elite team, thanks to a torrid 20-8 start fueled by twin aces Brandon Webb and Dan Haren. While a prolonged slump dragged them under .500 by early July, the defending NL West champions maintained the division lead because their closest competition, the injury-wracked Dodgers, played even more poorly. Eleven times that month, the sun set with the Diamondbacks in first place despite a losing record. Their play improved slightly while the Dodgers rode a roller coaster of inconsistency: On August 29, the 69-65 Snakes opened a 4 1/2-game lead over a Dodgers club that had lost eight straight.

Then the flaws in the Diamondbacks’ blueprint caught up to them: an offense short on on-base skills, a bullpen among the shakiest in the league, and a defense compromised by the season-ending injury of second base wizard Orlando Hudson. Over the next two weeks, Arizona went into a 3-10 tailspin, plummeting nine games in the standings as the Dodgers, riding the white-hot bat of Manny Ramirez (.399 AVG/.493 OBA/.751 SLG with 16 homers since being acquired from Boston via a July 31 blockbuster), reeled off a 12-1 run and captured first place on September 6. Nothing comes easily for this Dodgers club, though. With their first division title since 2004 in sight, they’ve split their last eight contests against the sub-.500 Pirates and Giants. Their lead is now back down to two games, thanks to a 7-1 sprint by Arizona.

If the West appears dominated by mediocrity, consider the circuit’s wild card race. On the morning of September 1, fresh off a three-game sweep of the Pirates, the Brewers held an 80-56 record, good for a 5 1/2-game lead over the Phillies, a record second only to the Cubs’ among all NL clubs. Absent from the playoffs since 1982, Milwaukee appeared poised to celebrate the success of an arduous rebuilding effort. Baseball Prospectus’s Postseason Odds Report estimated the Brewers’ shot at making the playoffs at 96%, accounting for both their wild card lead and the narrow 14.8% chance that they could yet wrest the Central Division title from the Cubs.

It’s been all downhill since then. Not only have the Brewers stumbled to a league-worst 5-15 record this month, on September 16, following a four-game sweep by the Phillies, they shocked the baseball world by firing manager Ned Yost. The desperation move was virtually unprecedented for a contender this late in the year. But with the team still smarting from last year’s similar late-season fade — they had shared first place as late as September 18 — owner Mark Attanasio and general manager Doug Melvin felt compelled to shake things up. Paramount in their decision was Yost’s mismanagement of his bullpen. Normally a slave to mid-inning matchup shenanigans, Yost’s penultimate game at the helm saw him turn a 3-3 tie into a 7-3 deficit by calling for lefty specialist Brian Shouse (a vastly inferior pitcher against right-handers) to intentionally walk lefty slugger Ryan Howard (a vastly inferior hitter against southpaws), then remain in the game to face righty Pat Burrell. Burrell stroked an RBI single, followed by righty-swinging Shane Victorino’s three-run homer. Since his firing, the Brewers have gone 2-4 under interim skipper Dale Sveum, who announced his presence by installing Mike Cameron in the leadoff spot despite a .326 OBA, third-worst in the lineup. Of greater consequence, the team may have lost co-ace Ben Sheets for the rest of the year due to an elbow injury.

Milwaukee’s postseason hopes would be dead and buried were it not for the cushion they’d built prior to September. As it is, through Monday they were one game back in the wild card race, clinging to a 34% chance of winning. Barely ahead of them are the Mets, who appear equally haunted by the ghosts of yesteryear. After an up-and-down season that saw general manager Omar Minaya fire manager Willie Randolph back on June 18, they’ve battled the Phillies for the NL East lead since the All-Star break. As of September 10, they led by 3 1/2 games, thanks to a 9-3 run.

Despite that lead, both ends of the Mets’ pitching staff were already in tatters. Starter John Maine, a 15-game winner last year, had been sidelined since August 23 by a sore shoulder, while the once-great Pedro Martinez was carrying a 5.44 ERA. Worse, closer Billy Wagner had been lost for the year due to a torn ulnar collateral ligament. Replacement closer Luis Ayala and company haven’t provided much relief in his absence: The Mets’ pen blew late-inning leads over the Braves twice in the next three days, kicking off a 4-7 slide that allowed them to be overtaken by the Phillies, who’ve gone 10-1 in that span while opening up a two-game lead in the East through Monday’s games.

The Postseason Odds show the Mets with a 65% chance of hanging on and slipping in. But with two games to go against the Cubs followed by three against the Marlins, the remainder of the Mets’ season is no cakewalk. The Brewers, with two more against the Pirates and then three against the Cubs, have an edge despite the one-game deficit.

The struggles of both teams remind one of the immortal words of sportswriter Walter Brown. In analyzing the war-depleted rosters of the Cubs and Tigers before the 1945 World Series, he famously quipped, “I don’t think either of them can win.” Observers of this year’s NL races can certainly relate.

Mr. Jaffe is a writer for Baseball Prospectus. For more state-of-the-art commentary, visit baseballprospectus.com.