The guitar gods gather for “Eric Clapton Crossroads Guitar Festival Chicago” on “Great Performances” (8 p.m., PBS, check local listings).

Taped before a live audiences at Chicago’s Toyota Park last July, the benefit concert for Clapton’s Crossroads rehab facility offers many a thrill for fans of blues, country and classic rock, including an 81-year-old BB King breaking into “The Thrill is Gone.”

Actor and Chicago booster Bill Murray acts as host and court jester, changing into wigs and costumes reflecting various stages in Clapton’s career. He opens the proceedings with a fan’s rendition of Van Morrison’s “Gloria,” the only song he knows how to play on a guitar.

Highlights include several generations of musicians from John Mayer to Buddy Guy and Vince Gill, Johnny Winter, Willie Nelson, Robbie Robertson in between. Clapton’s fellow Yardbirds guitarist Jeff Beck wows the audience and is accompanied by a striking young bassist who doesn’t look old enough to have her driver’s license. Clapton also reunites with Blind Faith alumnus Steve Winwood to perform “Presence of the Lord” and “Can’t Find My Way Home.”

¢ Every year brings new Christmas specials, but few linger in our memories and become annual favorites like “Dr. Seuss’s How the Grinch Stole Christmas” (7:30 p.m., ABC), a holiday staple since 1966.

¢ Mike Myers, Eddie Murphy, Cameron Diaz and Antonio Banderas add their voices to “Shrek the Halls” (7 p.m., ABC), a new holiday offering spun off from the popular CGI film franchise.

¢ In other Yuletide news, Al Roker hosts “Christmas in Rockefeller Center” (7 p.m., NBC), the 75th annual tree-lighting celebration. Scheduled musical guests include Celine Dion, Josh Groban, Taylor Swift, Ashley Tisdale and Carrie Underwood.

¢ Anderson Cooper moderates a debate between Republican candidates for president (7 p.m., CNN), with questions provided by citizens who submitted them through YouTube, the online video-sharing service.

While dismissed by some as a gimmick, last summer’s YouTube debate between Democratic candidates presented a series of voices (and accents) rarely heard in the processed and formulaic realm of network and cable news. Yes, somebody made a cute video with a snow globe, but another questioner posed a sobering and reasoned question about Iraq that seemed to change the tenor of the debate on that subject.

The YouTube debates may be scary for candidates used to more scripted situations. But politicians who learn to master new forms of communication often end up on top, or at least in the mix. Franklin Roosevelt was said to have owed his success to his radio addresses that allowed him to enter the living rooms of his fellow Americans. John F. Kennedy made better use of network television than his opponent Richard Nixon in 1960.

Cable television sparked several campaigns. In 1988, “700 Club” host Pat Robertson made a surprisingly strong run in the Republican Iowa caucuses. In 1992, Ross Perot used a few guest spots on cable TV’s “Larry King Live” to launch a quixotic independent campaign and win nearly 20 percent of the total vote. Cable commentator Pat Buchanan also ran strong races in the Republican primaries in 1992 and 1996.

Will an Internet candidate emerge in 2008? Democrat Howard Dean used the Internet as a great organizational and fundraising tool in 2004 but failed to get far in the primaries. This year, Republican maverick Ron Paul also has used the Internet to raise funds and reach a young crowd. But who will become the first candidate to use the Internet to go all the way?

The YouTube debates are a good place to find out.