The quip has become a chorus.

In debating whether Barry Zito will earn his $126 million from the San Francisco Giants after posting a combined 3.85 earned run average with the Oakland Athletics the last two seasons, doubters say, “But he is going to the National League.”

Roger Clemens is considering a return to the Yankees or the Boston Red Sox after steamrolling opponents in Houston for three seasons, and warnings sound, “But he’ll be moving back to the American League.”

Randy Johnson gets himself traded from the Yankees back to the Arizona Diamondbacks, and everyone expects his E.R.A. (a vertiginous 5.00 last season) to plummet: “He’ll be much better back in the National League.”

For all its status as the senior circuit, the National League has become a shelter of sorts for pitchers, a safe haven from A.L. lineups that tend to pelt them with three-run homers and force early showers. Like swinging two bats in the on-deck circle, pitching in the A.L. — with its designated hitter and far stronger No. 9 batters — makes moving to the N.L. feel downright liberating.