It was bound to happen: Airing head-to-head against Fox’s “American Idol” is starting to hurt CBS’s new hit terrorist drama “The Unit.”
 
“Unit” averaged a 4.2 Nielsen overnight rating among viewers 18-49 last night, down 14 percent from the 4.9 it posted last week and off 18 percent from the 5.1 it earned for its premiere episode two weeks ago.
 
The show’s rating dropped despite the fact that “Idol” fell off slightly week-to-week in the 9 p.m. slot, from a 14.7 last week to a 14.4 last night.

Part of the problem for “Unit,” however, was that it had a weakened lead-in. The drama “NCIS” was a repeat last night, the first rerun during “Unit’s” three-week run, and it posted a 2.6 among 18-49s last night, off 41 percent from the 4.1 an original episode earned last week.
 
Up against the top show on TV, losing a solid lead-in hurts.
 
Still, “Unit” remains a strong performer, and it should benefit when the singing competition returns to one-hour episodes next week, meaning “House” will air at 9 p.m. The medical drama is strong in its own right, but certainly doesn’t perform as well as “Idol” does.
 
Elsewhere last night, ABC’s comedy “Hope & Faith” took over the 8:30 p.m. slot after “According to Jim,” moving from Fridays at 9 and pulling a 2.1 among 18-49s. That’s exactly what the network had averaged in that slot over the previous four weeks for two episodes of “Jim” and two episodes of “Rodney.”

Fox took the night easily among 18-49s, averaging a 13.6 rating and a 33 share. CBS was second at 3.6/9, NBC third at 3.1/8, ABC fourth at 2.2/5, WB fifth at 1.0/2 and UPN sixth at 0.7/2.
 
Fox bettered the combined broadcast competition in each of its two hours last night, beginning with a 12.8 rating at 8 p.m. for the first half of “Idol.” CBS was a distant second that hour with a 2.6 for its repeat of “NCIS” and ABC third with a 2.1 for “Jim” (2.2) and “Hope & Faith” (2.1). NBC was fourth that hour with a 1.9 for “Most Outrageous Moments,” WB fifth with a 1.1 for a repeat of “Gilmore Girls” and UPN sixth with a 0.9 for an “America’s Next Top Model” repeat.
 
At 9 p.m. Fox led again, improving to a 14.4 among 18-49s for the second half of “Idol.” CBS finished second that hour with its 4.2 for “Unit,” with NBC third with a 2.7 for an hour of “Scrubs,” ABC fourth with a 1.8 for an hour of “Sons & Daughters,” WB fifth with a 1.0 for another “Gilmore Girls” rerun and UPN sixth with a 0.5 for a repeat of “Veronica Mars.”
 
With Fox out at 10 p.m., NBC took the lead with a 4.8 for “Law & Order: SVU.” CBS was second that hour with a 3.9 for “The Amazing Race” and ABC third with a 2.8 for “Boston Legal.”
 
Fox also led the night easily among households with a 19.2 average rating and a 28 share. CBS was second at 7.7/12, NBC third at 5.5/8, ABC fourth at 4.7/7, WB fifth at 1.9/3 and UPN sixth at 1.2/2.