We're not in the US sense. Our third parties can ally to play "kingmaker." Parties like the Socialists, the Communists, New Komeito, and the Japan Life First Party, and these new nationalist parties (Restoration and Sunrise) often when enough of a vote to get seats. Unlike the US system, where Congress critters get directly elected by first past the pole in their districts, in Japan (without going into several of the peculiarities because, let me put it this way: I took an entire semester CLASS on Japanese elections) seats are doled out by how the parties do percentage wise. So let's say the DPJ gets 35%, the LDP gets 25%, Socialists get 10%, the nationalists get 20% and the Communists get 5%, all other parties share the remaining 5%. There are 300 percentage seats. There are also district seats which operate more like US congressional districts and are first past the pole, 180 of those seats exist. This is the guarantee an area like my own, which is heavily Communist, gets a Communist representative, even if in the nation as a whole, the Communist vote is very small. In this case, the smaller parties can choose to go with the DPJ or the LDP and insist on some pretty major legislative deals in order to help them form a Coalition Government. This wasn't necessary in the previous government because the DPJ had a super majority at 306 seats out of 480. However, with the amount of "a pox on both your houses" I am hearing from the Japanese electorate in regards to both the DPJ and the LDP, it is completely possible for third parties to have quite a lot of power come December.
Here was the breakdown of the House of Representatives (which as the lower house, forms the Government and selects a Prime Minister) as of yesterday:
DPJ/Club of Independents (306)
JCP (Communists) (9)
SDP/Shimin Rengō (Socialists) (6)
former "Hiranuma group" (2)