"There's nothing in the behaviour of the the US capitalist class which suggests it would peacefully defer to a radical left wing party genuinely bent on stripping it of it's wealth and power rather than engage it by mobilizing the reactionary sectors of the population for civil war. At present, there's every indication that it would find enough willing foot soldiers from a Republican base which would be breaking further to the right than the left in a deeper crisis. "
Which I find a bit speculative. There is no need for capital to mobilise reactionary sectors of the population for a civil war, because there is no working class movement to put down. (The Tea-baggers are more indicative of the break up of the Republicans, than of a proto-fascist militia for capitalism, surely?)
Marx and Engels might not have entertained the possibility of peaceful transformation with much enthusiasm, but they did not think that revolution was an abstract moral postulate, outside of historical time, either. In certain times they wound down their involvement in radical groups, because they saw them as being out of time: you can see their irritation with the German emigres in London after 1849; and more important, you can see it in Engels satisfaction that the International Working Men's Association had been effectively wound up (under the subterfuge of relocating it in New York). Engels, then, was happy to anticipate that a new international would be many years ahead, once Marxist theory had been better understood. (see Engels' letter to Bebel 20 June 1873, and to Sorge 12 September 1874.