Debate in the assembly of the country can often be adversarial and not constructive, sometimes revolving around narrowly perceived policy ideas, rather than larger political issues. Sometimes adversarial politics can lead to the opposition disagreeing with everything the government proposes (and vice versa) for the sake of disagreeing. This can lead to important legislation, especially reforms, being blocked that may be beneficial for the country.

The system is more easily corrupted by campaign contributions since there are fewer players to donate to.

Multiple points of view improve the chances that the "right answer" is found.

If the opposition party is weak, a dominant-party system may develop.

Candidates are motivated to run negative campaigns, pointing out the flaws in the "other person" (usually the leader of the other party) and staking out only those positions that are necessary to differentiate themselves from their primary opponent and not constructive or beneficial to citizens.

In an effort to attract voters, each party will adopt planks of the other party's platform, leading to the appearance in some skeptics' minds of a one-party system. Examples include the American notion of a "Republicrat."

-Two parties can more easily collude to ensure a result that is desirable to them but undesirable to the voters. Since no other party is an option, neither is the classic recourse of democracy-"vote the bums out", as in this scenario both parties "in power" are the bums. This also reduces recourse for the wrongdoing to be exposed (as there is no one in power with the clout and wish to do so), and creates a cycle that feeds on itself.

-Bribes (campaign contributions) go further when there are only two sets of palms to grease.

-Makes it likely that the "check and balance" system integral to constitutional democracy will be corrupted by having large numbers of representatives of a single party at all levels.

-Proportional representation would better reflect the "will of all the people", including the minorities, and prevent current problems such as gerrymandering and total exclusion of third parties.

During the 1992 Presidential elections, Ross Perot received 17% of the national vote; a major rebellion of U.S. voters against the two party system. However, when all was said and done that 17% was left with no voice in congress whatsoever. A constitutional amendment instituting a proportional element to our democratic system would a good thing, and would break down the two party system.