During the last US presidential campaign, a group of musicians, including Bruce Springsteen and REM, embarked on a "Vote for Change" tour of crucial swing states.
The message from the stage each night was that a vote for the Democratic candidate, John Kerry, would help remove George Bush from power. Springsteen himself went even further, writing an editorial in the New York Times calling for "regime change".
Vote for Change succeeded in attracting audiences and raising money, but none of the swing states swung Kerry's way - and Bush did even better in "Vote for Change's" heavily targeted state of Ohio than expected.
Some of Springsteen's fans erupted in fury at his political stance. "Boycott The Boss" postings appeared on the Internet, with comments such as "He thinks making millions with a song-and- dance routine allows him to tell you how to vote. Here's my vote: Boycott him. If you don't buy his politics, don't buy his music" and "Springsteen is a left-wing tool. I won't be buying any of his albums in the future, nor concert tickets".
Previously, The Dixie Chicks made an anti-Bush remark at one of their concerts and found themselves banned from many radio stations. There were even ritual burnings of their CDs. Many other American entertainers, such as Linda Rondstadt and Barbra Streisand, have been loudly booed when they voice anti-Bush sentiments at their shows.
It is precisely because of the fear of a backlash vote that both Hilary Clinton and Barack Obama have been studiously avoiding the rock star endorsement. To launch her campaign, Clinton used a Celine Dion song. Obama, while he has used music from U2 and others, has yet to actually appear with any significant member of the entertainment industry.
Though not officially Obama-endorsed, a video on YouTube, called Yes We Can, featuring a host of musicians and actors, has adopted subtler approach. Directed by Jesse Dylan (son of Bob) and featuring will.i.am from Black Eyed Peas, John Legend, Herbie Hancock, and Scarlett Johansson among others, the video sets one of Obama's "inspirational" speeches to light hip-hop beats and soft guitar sounds.
The fact that the video is absolutely atrocious - it sounds like it was written by Bob the Builder and features a stream of meaningless platitudes that would shame even the most hopeless self-help guru - hasn't deterred 13 million viewers from clicking on it. It is believed to be the most watched political speech ever.
Tactically, it is superb. The people behind it obviously learnt from the Vote for Change tour. There is no Bush-bashing or overt politicising. It's just a vacuous, feel-good video, portraying Obama as some sort of sweet and cuddly JFK/Martin Luther King hybrid. It sounds like an overlong Hallmark greeting card.
In the past fortnight, when Obama seemed close to securing the Democratic nomination, musicians including Arcade Fire began jumping on the bandwagon. Following his setback in this week's primaries, they may go quiet again.
Again, Obama himself is keeping his distance, but unilaterally The Grateful Dead are to play a one-off reunion show to raise money for his campaign, and Fall Out Boy and Pearl Jam (minus Eddie Vedder) are doing similar musical endorsements.
Still, the last three are hardly high-wattage and, in this still close election, and any endorsement that might alienate voters needs to be scrutinised.
You would expect rock stars to be falling over themselves to be associated with a liberal black candidate (and self-confessed cocaine user!). But it would appear that the Obama campaign is saying "just send the cheque please - you might scare off the Christian voters". With stakes this high, can you blame him?
