Revolutions in America do not involve violent overthrow of the government - but massive turnout at the voting booth. I do not know of anyone on the left (or the right) who is taken seriously that considers armed (or unarmed) revolution an option. The majority of radical leftists and progressives (and reactionaries) advocate electing candidates that will adopt their platform, and protest marches are not intended to be rallying cries for marching on city hall or the state capitols to ‘throw the bums out’, but to mobilize enough voters to show up on election day to throw the bums out.
You see the American-style revolution playing out in Wisconsin - what is the main tactic of the progressives there? To gather enough petitions to hold a recall election as soon as possible, and to challenge the new laws through the courts.
On the other side, the Christian Right gained political weight not by advocating revolution but by electing their candidates - starting with school boards, then state legislatures, and then national offices.
And we have had successful ballot box revolutions in America - candidates elected on the platform that they will change the status quo and dramatically reform government. And they do happen about once a generation - on the national level you have seen Andrew Jackson, Lincoln, Teddy and Franklin Roosevelt, and LBJ.
Obama was elected with the hopes he would be in their mold, but has demonstrated the unfortunate tendency to be far more moderate than what his supporters had hoped for - which he paid for in the midterm election. My wag is that if he had strongly pursued the agenda he campaigned on, the left would have had his back last November, but he did not, and so they did not. Unless he decides to become more assertive, I think there is a good chance he will be a one-term president. His style suggests more assertiveness is not likely. He wants to govern from the middle - which only alienates both sides and the middle is the quietest voice in America and thus very hard to maintain support from.
The other major problem is that successful ballot box revolutions rarely happen through the existing parties. And only by alienating existing parts of their coalitions. If the new part is stronger than the old part, no problem, but the realignments are always messy. Democrats gained the civil rights coalition, but lost the South. Republicans are being dominated by the Christian Right and the Tea Party, but are losing the soccer moms and independents as well as alienating possible coalition members. (I think most Hispanics and Asians would tend to support traditional conservative values - pro-family, pro-business, smaller government, but as long as the rhetoric on the right keeps bashing immigrants and pushing evangelical Protestant Christianity as a litmus test, those groups will remain independent.)
So is America reaching the “breaking point” to support another party to support a “revolution”? Not on the left. The Greens may be a viable alternative if they can get their house in order and attract enough progressive Democrats to jump ship, but they are still reeling from costing Gore the election in 2000 and they still have trouble creating a coherent platform that appeals to progressive voters, let alone independents, (quick - what do Greens stand for?). The Democratic Party might be the vehicle if progressives can wrest control from the neo-liberals and “Third Way” politicians. If the Republicans keep overplaying their hand as they are currently doing, that might drive enough progressives to take control of the Democratic Party. It could also drive enough of them away from the Democratic Party and the left is split in two (or more), and the Republicans continue to reach a plurality at the ballot box which is enough for a majority in the legislatures.
If unemployment hits 10-15% and stays there, and we lose another $10 trillion in household wealth, and inflation starts to rise (a gallon of gas and milk both hit $5/gallon and stay there) while wages remain flat or decline further, then the door is open for a new player if they can provide a coherent platform to address those issues.
But the brunt of the pain will still be felt by two groups - the urban poor and the rural poor, which still have very different views on political and social issues. Until that divide can be bridged, and I am skeptical it can be, any chance of a revolution - ballot box or otherwise - will be slim, and the status quo of ineffective leadership will continue until we reach catastrophic failure - which would entail a complete economic meltdown and collapse of the federal government. At that point I could see the United States dissolving as states decide to go their own way, more than a national revolution. Given our current course, I would say that is about ten to twenty years away with the predominately urban states seceding, and the rural states forming a smaller Union (with a very weak federal government.)