Not to put too fine a point on this, but mainstream North American parties don't like the word "manifesto." It's a word associated with declarations by extremists, radicals, anarchists, communists, etc. (should I mention the archetypal "Communist manifesto"?) Googling, for instance "Republican Party election manifesto," returns a series of entries that don't relate to the mainstream Republican Party's self-description of its policies and proposals. If you google "Democratic Party election manifesto," you get a series of entries related to Japan, India, Britain, etc., and virtually none related to the US. If, instead, you googled "---platform," you'd get lots of entries for American parities. In North American English, there is a clear usage distinction between "platform" and "manifesto."