Most of the time Congress is in the hands of the opposition. The present situation with both branches of Congress and the White House in Republican hands is rare. Throw in the Supreme Court and the Media, and just hope it's a fleeting moment.

(a) The liberals generally dominate the Dem Party, but there are centrist who so far have been unsuccessful in taking the power away;
(b) The conservatives generally dominate the Rep Party, but there are centrist who so far have been unsuccessful in taking the power away;

2.  (a) Some libertarians abandon the Libertarian Party and join the Republicans to change the party

(b) Some Greens abandon the Green party and join the Democrats to change the party

3.  Or if they stay in the party, the Lib or the Green swing the results in close elections.

So there are different ways to influence the politics, but its much harder for the small parties in the USA than in Europe.  

Winner-takes-all system is prohibitive in that respect.

4.  There is also a problem of gerrymandering  which is another topic, but very important in the US politics. For example, in 2004, none of the seats in the California congressional (federal), and California State Senate and California State Assembly changed hands.  The Dems and the Rep got together in a backroom deal and designed their own safe districts, and each party gets elected in its own safe seat without any competition.  Sad. Travesty. Tragic. Comical.

Also note that despite the desire of some of us for a truly liberal Democratic party, the fact is that the Dems don't support a whole range of things that are supported by conservatives in other countries, like Canada. Stuff like single payer health, strong environmental policies (remember Kyoto was rejected 99:1 in the Senate), secular politics, etc., that are routine in Europe are not even under consideration here. America is simply not on the same page as Europe in many ways: What's the "center" here is off in the weeds in Europe, and vice versa.

No party with 45% of the vote is going to "hand over control" to a lesser party (in your example, maximum 10%) with which it may form a coalition.

To those who see the advantage of FPTP as promoting rapid change and decisive governance (an argument only relevant to parliamentary systems, and not to the US with its separation of powers) I would ask two things:

1. Is it good to get rapid change that has been endorsed by much less than 50% of the electorate? Perhaps we should consider major changes in the policy realm that are endorsed by 40-some percent (or even less) as less than desirable.

2. Can't a single-party government just as easily resist needed change as promote it? The Muldoon government in New Zealand before 1984 is a good example. It ignored the swelling deficits and inefficiencies in the economy, letting the crisis worsen, and it had twice been outvoted by Labour. It did not even represent a plurality of the NZ electorate.

But in most multiparty systems--and I think this applies to Germany and New Zealand, as well as Norway (which also just had a very close election)--even the little parties have constituencies that are more fluid at the same time that they have principled policy stances. If such a party demands too much it risks losing voters, possibly to one of the bigger parties, or another small party that is not seen as trying to hold everyone else "hostage."

And if there is only one major party or bloc that the small party can credibly make a coalition with, then it is much more constrained in what it can demand in exchange for its support. Otherwise it risks a very bad outcome for its voters: the seating of a government from the other side.