As the season progresses, baseball’s pennant picture is supposed to gain clarity — not become even more confused. Perhaps it’s the addition of the wild card, or perhaps it’s the spread of an industrywide parity that would warm NFL commissioner Pete Rozelle’s heart. Whereas once the commissioner of baseball infamously — and weakly — cited cooked studies and postseason wins as a reflection of the lack of competitive balance, we have lively title races in all six divisions. If six — plus the equally dramatic pair of hunts for the wild-card slots — is a number that creates too much uncertainty for old-school fans, it does provide current fans with a playoff picture as exciting as any in baseball’s history.

The contrast between the leagues is interesting enough. In the American League, there are pairs contending in all three divisional races. If the existence of the wild card might lower the stakes overall because one second-place team will make it, two still will not. In the National League, we may only have five of the 16 teams that we can consider as truly out of it, although clubs like the St. Louis Cardinals and Houston Astros are still in the running because of the race between the Brewers and Cubs to get back to the top of the pack in the NL Central. That’s just a repeat of last year’s near-collapse of the Cardinals down the stretch, though, a meltdown that kept Houston and the Cincinnati Reds in meaningful September baseball. The Senior Circuit’s competitive crowd makes for a repeat of last year’s wild-card slam dance, where the Phillies achieved the rare feat of giving up and then getting back into the race.

The AL’s wild-card picture is similarly desperate, even if it has fewer entries. It doesn’t get much tighter than the Yankees, the Cleveland Indians, the Detroit Tigers, and the Seattle Mariners within a game of one another going into yesterday’s action. The specter of a Boston Red Sox meltdown as devastating as that of 1978 — the Bucky Dent year — colors everything to come in the AL East, while the Indians and Tigers are knotted up in the Central. The stakes are no less important for a team like the Mariners than it is for those history-haunted clubs. They’ve been shut out of the postseason for five years, and if catching the Anaheim Angels might prove difficult, they could still end up boxing out their more famous rivals for the wild card.

The competitive dynamism of so many races going on should earn some deserved comparisons to great playoff races from baseball’s past. Certainly, one of the key criteria for selecting the playoff races is that races with more than two teams in them are that much more interesting, so you can imagine what fans think of this year’s NL West, which has four legitimate contenders. The 2007 AL East race might not get the same sort of honored place in the game’s history as the 1934 NL pennant chase, when the stakes were allor-nothing, and the defending champion New York Giants blew a big lead — a situation the Cardinals could exploit, but that the well-regarded Chicago Cubs could not. But that’s not entirely fair, and as last year’s victory by the Cardinals in the World Series makes clear, getting there is the battle. If the Red Sox hold off the Yankees, who knows what’s supposed to come next?

Consider the NL Central race this season. You’ll find echoes of the stretch drive to come among the persistent quartet in the 1973 NL East’s playoff race, a season that ranks as one of the all-time best in Baseball Prospectus’s book “It Ain’t Over ‘Til It’s Over” (Basic). That’s not because there’s something great about playoff spots going to .500 ballclubs, or that there’s a sort of strange symmetry between Willie Mays’s last season and what will be Craig Biggio’s. Instead, what’s fascinating about both races is that there’s so much that could happen now, and could have just as plausibly happened then. That year, the Mets came back out of last place on August 30 to win the division with a final record of 82–79, and if the Mets’ Tug McGraw had faith in “You Gotta Believe,” that sentiment was just as applicable to the Pittsburgh Pirates’ trying to overcome Roberto Clemente’s tragic death and Steve Blass’s infamous loss of command of his pitches. The Cubs led that race early, and the Cardinals late, but both faded down the stretch, creating the opportunity that the Mets of McGraw and Mays exploited. Think that scenario shouldn’t inspire fans — and the teams — in Houston and St. Louis right now?

Similarly, the Arizona Diamondbacks’ currently improbable place atop the NL West might make them seem ripe for a fall, no less than the now-infamous 1964 Phillies. This year’s Snakes have moved out in front of more highly regarded contenders like the Los Angeles Dodgers and the San Diego Padres (two-time defending division champs), despite surrendering more runs than they’ve scored, the sort of thing sabermetricians consider a warning sign of a future fall. But the D’backs are a generally young team with an outstanding bullpen and helpful tools in the lineup, an allworld rookie in Justin Upton, and have propelled themselves forward by adding quality veterans to their rotation. Gene Mauch’s infamous Phillies had the benefit of an equally well-managed broad assortment of hitting help — rookie Richie Allen rated among the best players in baseball — and the addition of veteran ace Jim Bunning before the season gave them enough to mount a seemingly improbable bid. What’s remembered from 1964 is that the Phillies blew it, not that they were unexpected contenders. What will people say about Arizona if their in-season performance weighs heavily on them down the stretch? That they never should have gotten this far, that they blew it, or that they’ve had an outstanding season in any sense of the word?

In a nutshell, it’s these sorts of comparisons than keep so many of us in our seats, in the stadium, or on the sofa, and they fuel the sorts of arguments that baseball fans of any age enjoy having. As we gear up for a stretch run that has more than half of the teams still involved, and so many possibilities, it’s not hard to enjoy the present every bit as much as the past.

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