The letter, splashed across Thai newspapers' front pages this week, has been denounced as self-serving, sycophantic and "shameful."
Critics say it's Thaksin's attempt to spin the political crisis so he emerges as a victim, and accuse him of unsolicited kowtowing to Washington.
"Uncle Sam, I'm being bullied," read The Nation newspaper's banner headline Thursday, above an article calling Thaksin a "crybaby" for talking to outsiders about the crisis that has followed massive anti-Thaksin protests months ago, leaving the government in political limbo.
In a commentary headlined, "Dear George letters are shameful," the English-language Nation said "Thaksin's pleas to U.S. leader have ridiculed Thai democracy."
Thaksin's letter said "there has been a threat to democracy in Thailand since early this year."
The letter outraged Thaksin's opponents, who said they would deliver their own accounts of the situation to the embassies of the U.S., Britain, China, France, Japan and Russia on Friday.
Thaksin has penned missives to leaders around Europe and Asia, trying to explain Thailand's political crisis since it began in January.
He met with British Prime Minister Tony Blair, French President Jacques Chirac and Russian leader Vladimir Putin during unofficial trips in April.
But it was the June 23 letter to the U.S. president that hit a nerve, the AP reports.