It’s well established that the Phillies boast a fearsome offense, but unlike last year, Philadelphia’s 2008 success is due in nearly equal measure to the pitching staff. While Philadelphia’s prolific attack ranks second in the National League with 457 runs scored (through Monday), its pitchers have allowed only 380, the fourth fewest in the senior circuit. Contrast that to the 2007 squad, which blasted its way to a memorable division-clinching comeback by scoring a league-leading 892 runs, overcoming the shortcomings of a staff that surrendered 821, the fifth most in the NL.

This year’s showing by Phillies pitchers is especially impressive given that Citizens Bank Park is one of the better hitting venues in the major leagues — last year, it played as the fifth-best NL hitter’s park. Since the park opened in 2004, Phillies pitching has ranked 13th, 8th, 12th, and 12th in the NL in runs allowed. Short fences are certainly not the only reason for Philadelphia’s poor pitching performances, though; one has to go back to 1994 to find a Phillies club with one of the top four staffs in the NL, and 1983 for the last team to do so in a non-strike season.

The improvement of this year’s staff might initially appear to be a mystery, given that all of the pitchers who have started games this year were also on last year’s club. The rotation has improved, specifically in the greater amount of innings it has gobbled up, but the real key has been a better bullpen. In 2007, the Phillies pen had a 4.41 ERA, blew 21 saves, and put up -1.2 Adjusted Runs Prevented (ARP, a Baseball Prospectus statistic measuring relief performance compared to average). So far this season, Phillies relievers sport an NL-best 2.78 ERA, have blown seven saves (fewest in the league), and lead the majors with an ARP total of 50.8, which has added up to almost eight wins above what a replacement-level bullpen would produce.

The major reason for this transformation is closer Brad Lidge. No other move has had a bigger impact on a current contender’s success. Lidge has been nearly untouchable, with a 1.00 ERA and perfect 19-for-19 conversion of saves, joining Mariano Rivera as the only closers with more than seven saves this year yet to blow a chance. Lidge leads all relievers with four wins added above replacement level, and has also fanned 49 in 36 innings of work.

General manager Pat Gillick was able to acquire Lidge at a discount for middle reliever Geoff Geary and outfielder Michael Bourn because of the closer’s struggles the past two seasons in Houston: Lidge blew 14 of his 33 save chances in 2006-07, a slump that followed his infamous blown save in the 2005 NLCS, when Albert Pujols hit a titantic three-run homer against him to win Game 5. The embattled fireman maintained his historic strikeout rate, however — Lidge is the all-time leader in K/9 IP (minimum 400 innings) at 12.6 — and it turns out that a change of scenery was the only thing needed to return the stopper to his former dominance.

Another Gillick transaction that was even more low-key than the Lidge trade was signing Chad Durbin to a one-year deal after Durbin was non-tendered by Detroit. The 30-year-old right-hander had been used primarily as a starter in his major league career, but the Phillies stuck him in the pen to serve as their long reliever, and he has given up just 10 earned runs in 50 innings of work, and less than 7.5 H/9, leading to an ARP total that ranks second in the league only to the Dodgers’ Hong-Chih Kuo.

Lidge, Durbin, J.C. Romero, and Ryan Madson have formed a formidable foursome coming out of the Philadelphia bullpen, and while the trio of Tom Gordon, Clay Condrey, and Rudy Seanez has been weaker, it has at least provided stability. Up until Monday, when Gordon went on the DL, the Phillies had used just seven relievers all season, the same seven with which the squad broke camp at the end of spring training. No other team this year has used fewer than 10 relief pitchers so far, and in 2007 Philadelphia employed a total of 21. The Phillies rotation deserves some of the credit. While Philadelphia’s starters have not been especially effective — a 4.48 ERA and 30-30 record — they’ve also been a highly stable unit, with an NL-low six pitchers used, and throwing more innings than any other NL rotation, over six per outing. That saves the pen from overwork, and it ranks near the bottom in innings and games pitched.

That durability may determine whether or not Philadelphia can hang on for its second straight division flag. The Phillies’ offense is an established force — it finished first or second in runs in the NL each of the past three seasons, and will most likely do the same this year, but the pitching staff is far less of a certainty. If the starters can continue logging innings and sparing the relievers, the Phillies will have a shot at maintaining their outstanding endgame. There is also a good chance the Phillies will be unable to match their first-half pen performance during the second — as they failed to do last weekend against the Mets — which could allow another one of the division’s teams to take their place as the division’s comeback heroes.

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