NEW YORK – With Tom Brokaw’s exit from NBC’s “Nightly News” nearing, the ratings race between his program and ABC’s “World News Tonight” is heating up.

Last week, they finished in a virtual dead heat, according to Nielsen Media Research. In the previous four weeks, NBC’s show was more popular for two, and ABC’s for two.

“Nightly News” won the important May “sweeps” in total viewers, their 21st sweeps win out of the last 22. But ABC was crowing about a victory among viewers aged 25 to 54, for the first time since 1996.

“Peter (Jennings) and I have made some changes to the broadcast and it’s gratifying to see the audience has noticed them,” said Jon Banner, executive producer of “World News Tonight.” He said the show has beefed up investigative reports and brought back its popular “Person of the Week” segment.

NBC believes the close ratings can be summed up in two words: Oprah Winfrey.

Winfrey’s popular talk show, which pushed hard for viewers during sweeps, is on ABC stations in many markets. With Oprah’s help, “World News Tonight” begins each night with a 261,000 advantage in viewers over “Nightly News,” NBC News spokeswoman Barbara Levin said.

During May, that advantage swelled to 573,000 viewers, she said.

“In spite of this, we are gratified that we were able to grow the “Nightly News’ audience so significantly from its lead-in,” Levin said.

One other plus for NBC: during the full season that began in September, NBC increased its viewership lead over ABC compared to last year, she said.

ABC believes NBC’s lead-in numbers are deceptive. Winfrey is on ABC stations in only half the nation, and in no market does her show lead directly into “World News Tonight.” Local news is usually on in-between. Besides, the audiences for Winfrey and Jennings are significantly different statistically, said ABC News spokeswoman Cathie Levine.

“There isn’t a relationship,” Levine said. “They make this up because this is their thing to cling to.”

Think this isn’t competitive? Wait until Brokaw steps down and is replaced by Brian Williams in December.

Last week, the two broadcasts averaged 8.72 million viewers (6.1 rating, 13 share). The “CBS Evening News” had 6.8 million viewers (4.9, 11).

Fox rode the successful conclusion of “American Idol” to a victory in the prime-time ratings race. The talent show’s conclusion was seen by 28.8 million.

For the week, Fox averaged 11.5 million viewers (7.0 rating, 13 share) and easily won among viewers aged 18 to 49. CBS had 9.6 million viewers (6.3, 11), NBC had 7.3 million (5.0, 9), ABC had 6.4 million (4.4, 8), UPN had 2.8 million (2.0, 3) the WB had 2.3 million (1.6, 3) and Pax TV had 880,000 (0.6, 1).

A ratings point represents 1,084,000 households, or 1 percent of the nation’s estimated 108.4 million TV homes. The share is the percentage of in-use televisions tuned to a given show.

For the week of May 24-30, the top 10 shows, their networks and viewerships: “American Idol” (Wednesday), Fox, 28.8 million; “American Idol” (Tuesday), Fox, 25.1 million; “CSI: Miami,” CBS, 21.3 million; “Everybody Loves Raymond,” CBS, 18.3 million; “Two and a Half Men,” CBS, 18.2 million; “CSI: Crime Scene Investigation,” CBS, 14.9 million; “Without a Trace,” CBS, 12.5 million; “24,” Fox, 12.3 million; “Law & Order,” NBC, 12.2 million; “Still Standing,” CBS, 12.1 million.