From Jonathan Adler at National Review:
"On the eve of the Iowa primary, several news outlets (including the apparently anti-Fred Politico) reported that Thompson would drop out of the race following a poor showing in Iowa and endorse John McCain. The sources were allegedly Republicans "close" to the Thompson campaign, but those of us who support Fred smelled a rat, and actual Thompson advisors vehemently denied the rumor.
Robert Novak reports what many suspected: The rumor came, in part, from a rival campaign."
"Published reports that Fred Thompson soon will withdraw from the Republican presidential contest and endorse Sen. John McCain have been traced in part to Mitt Romney's campaign, trying to stir up strife between McCain and Thompson." (source)