Fox demonstrated last week how formidable it might be if it could become a Sunday force once again, as the launch of its night of animated laffers — anchored by “The Simpsons” and the return of “Family Guy” — helped the net open up some distance on its rivals in key young-adult demos.

While CBS prevailed once again among adults 25-54 and total viewers, Fox won in adults 18-49 by its widest margin since early March and has virtually locked up at least a share of the season’s 18-49 crown.

Fox rolled on Tuesday and Wednesday as usual with “American Idol,” then capped the week with a strong second-place finish to ABC on Sunday, a night the net let slip from its grasp in recent years. It also led Saturday with “Cops” and was competitive on both Monday and Friday.

For the week of April 25-May 1, which included the first four nights of the May sweep, Fox’s 4.3 rating in 18-49 was a healthy 19% ahead of ABC and CBS (3.6 apiece) and a big 34% better than NBC (3.2), according to Nielsen. The Peacock went from a dominant No. 1 in the same frame a year ago to a listless fourth.

CBS wins in viewers

CBS won by a narrow margin over Fox in adults 25-54 (4.7 to 4.5) and rolled in total viewers (13 million), nearly 3 million ahead of Fox for its ninth straight win by this measure.

In the season’s tight 18-49 battle, Fox (4.05 through Sunday) now appears locked in at a final 4.1 rating, good enough for its first victory ever. CBS (3.98) will need a boffo finish to round up to a 4.1 itself and also claim a share of its first seasonal demo title.

For Fox, “American Idol” was up week-to-week and year-to-year, with Tuesday’s edition (10.8 rating/29 share in adults 18-49, 25.52 million viewers overall) ranking No. 2 for the sesh — behind only ABC’s smash hit “Desperate Housewives” (11.5/25, 25.69m). And Wednesday’s “Idol” results show was nearly as strong (10.5/26, 24.43m), ranking third for the week in 18-49.

A repeat of “House” also won on Tuesday for Fox (6.1/15 in 18-49, 16.45m), the net’s new comedy ‘Stacked” led in the post-“Idol” slot on Wednesday (5.1/13 in 18-49), and the 600th episode of “Cops” highlighted another Saturday demo victory.

Biggest story, though, came on Sunday, where “Family Guy” (5.8/13 in 18-49, 11.87m) returned with burly scores, actually building on the second of two firstrun segs of “The Simpsons” (5.6/14, 11.92m). “American Dad” also looked good in its bow at 9:30 (4.7/10, 9.32m). Overall, Fox had its strongest Sunday with all-entertainment programming since December 2003.

Event hurts Eye

CBS was hurt the most by President Bush’s Thursday news conference, losing one of its highest-rated hours of the week (“Without a Trace”). Net was paced as usual by the two shows that did air on the night: “CSI” (8.3/21 in 18-49, 23.68m) and “Survivor: Palau” (21.21m).

Also contributing for the Eye were the series finale of “JAG” on Friday (2.9/9 in 18-49, 13.98m), which posted its best demos in 19 months; “CSI: NY” (5.3/14 in 18-49, 14.84m), which led its Wednesday hour; and Sunday movie “Riding the Bus With My Sister” (3.5/8 in 18-49, 14.98m), which performed well despite tough competish from ABC’s dramas.

ABC’s second-place tie with CBS in 18-49 marked the net’s most competitive weekly ranking since the Academy Awards frame in February.

ABC trio stays on top

For the sixth straight week, ABC claimed the top three programs of Sunday in “Desperate Housewives,” “Grey’s Anatomy” (8.2/20 in 18-49, 17.88m) and “Extreme Makeover: Home Edition” (6.5/17, 16.36m), with “Grey’s” hitting a series high in 18-49 for the fourth straight week.

Net had the week’s top-rated newsmag in adults 25-54 in “20/20” (4.2/12) and put up pretty good numbers for a “Lost” recap episode on Wednesday (5.0/15 in 18-49, 13.75m), but caught its biggest break when President Bush’s news conference reduced its low-rated Thursday to a single hour.

Over at the weblets, the WB topped UPN in adults 18-34 (1.6/5 vs. 1.4/5), with the former led by its early-week dramas but also getting a boost on Thursday with a special night of “Blue Collar TV” programming (Daily Variety, May 2).

UPN, meanwhile, continues to be in growth mode, rising vs. the same week a year ago on most nights of the week. Combo of “Eve” (1.1/3 in 18-49, 2.84m) and “Veronica Mars” (1.1/3, 3.04m) perked up on Tuesday, and Friday’s “Enterprise” (1.2/5, 2.92m) is building to its finale with momentum.

TNT easily led the ad-supported cable networks for the week in 18-49 (1.05 rating), total viewers (2.57m) and other categories, as its first 15 NBA Playoff games have outperformed last year’s averages.

On Monday of the current week, CBS rolled to victory with its highest 18-49 average on the night (excluding sports) since January (5.5/14).

Two weeks from its finale, “Everybody Loves Raymond” (6.3/15 in 18-49, 19.42m) drew its largest overall audience of the season, and “CSI: Miami” (7.3/18, 20.70m) was the night’s top draw on any net, easily besting rival NBC drama “Medium” (4.7/12, 12.74m).

Fox’s “24” (4.7/11 in 18-49, 10.88m) won the drama battle at 9 over NBC’s “Las Vegas” (3.9/9, 10.96m), while the Peacock’s “Fear Factor” prevailed in demos at 8 (4.0/11, 9.53m).

(Each 18-49 ratings point reps roughly 1.3 million viewers.)