NBC’s powerful latenight combo of “The Tonight Show With Jay Leno” and “Late Night With Conan O’Brien” opened bigger advantages over rivals during the May sweep, each posting its largest margin of victory in four years.

According to Nielsen nationals for the April 24-May 21 period, “Tonight” claimed its 29th straight sweep victory among adults 18-49 over CBS’ “Late Show With David Letterman” and its 31st straight in total viewers (includes July sweeps). And “Late Night” remained unbeaten in sweep months, making it 34 straight triumphs over CBS’ “Late Late Show With Craig Kilborn” (also including July).

Leno defeated Letterman by 60% in adults 18-49, up from a 41% advantage a year ago and his biggest winning margin in any sweep since May 1999.

The month’s averages for the 11:35-12:35 a.m. slot: “Tonight,” 2.4/10 (even vs. a year ago); “Late Show,” 1.5/7 (down 12%); “Nightline,” 1.4/6 (up 8%); “Jimmy Kimmel Live,” 0.7/4 (down 11% from 12:05-12:30 a.m. vs. year-ago “Politically Incorrect” results). In total viewers: “Tonight,” 6.3 million (up 5%); “Nightline,” 4.3m (up 2%); “Late Show,” 4.0m (flat); “Kimmel,” 1.7m.

A “Tonight” highlight was the May 12 telecast featuring a guest-host appearance by Katie Couric, which attracted 4.8 million viewers in the 18-49 demo — the largest Monday tune-in since April 1996.

ABC’s “Nightline” delivered its best May sweeps figures in three years among total viewers and adults 25-54 (1.7/7), pulling into a tie with CBS’ “Late Show” in the demo.

In the 12:35-1:35 a.m. slot, O’Brien’s “Late Night” on NBC (1.3/8 in adults 18-49, up 8% year-to-year) defeated Kilborn’s “Late Late Show” on CBS (0.7/4, down 12%) by 86% in adults 18-49 — up considerably from the 50% margin of May 2002 and the show’s largest advantage for a May sweep since 1999.

In overall total viewers, O’Brien hit his highest May figure ever (2.6m).

NBC’s “Saturday Night Live” also easily led its time period (3.7/15 in adults 18-49, 7.5 million viewers overall), and has now defeated Fox’s competing “Mad TV” (2.3/8 in adults 18-49, 4.9 million viewers) every sweep since Fox entered the latenight race in the first quarter of 1994.