>I'm not a US citizen, so what I think about Obama really doesn't matter....
I am a US citizen and my opinion matters as much as yours does.
I am a US citizen and my opinion matters as much as yours does.
This way, someone could vote for a third option knowing that it won't help the opposing party.
American voters don't elect the President, anyway, they elect Presidential electors; if you had more proportional (compared to FPTP single member districts) representation for the Congress [1], and PR within each states slate of electors for the Electoral College (for the EC, party-list proportional could work rather than candidate-centered), and then change the rules for the EC so that the whole EC meets physically and has a set time to elect a President and VP by majority otherwise those elections fall to the House and Senate, you'd neatly solve the problem of duopoly.
[1] Without abandoning candidate-centered elections and equal representation in the Senate, my preferred method of doing this for the Congress would be to add senators so that each state had 3 instead of 2, elect a slate of 3 Senators from each State every 6 years using Single Transferrable Vote, and also elect the House by STV every 2 years, dividing states up into House districts as follows:
If a state has 1-5 seats in the House, it has one district;
If a state 6 or more seats in the House, it is divided as evenly as possible in to districts of not less than 3 and not more than 5 members.
If by 2-turn you mean majority-runoff, that still has perverse incentives against voting for other than the more preferred major party candidate, though slightly less so than does plurality.
Highly less, I would say.
When the two most voted candidates go to the second round, you may vote for a third candidate knowing that, in case this preferred one fails, there is still the chance to vote against the one you hate more in the second round.
In a single-run voting, people are more compelled to vote against the candidate the hate more, instead of voting for the one they like more.
Not really. Its not like majority-runoff isn't a commonly used (in the US, even), highly studied election method, known to have very similar effects in terms of reinforcing duopoly to plurality.