There are 3 branches of government in the us. The presidency, the senate, and the congress.

The democrats ave a majority in the senate and hold the presidency, the republicans have a majority in congress. In order for anything to become law in America, a bill needs to pass both houses, and be signed off by the presidency.

The founding fathers set it up like this way to ensure a system of meritocracy, and avoid implementing European style monarchies and church rulers that they'd fled from over here.

That's basically it in a nutshell. The tea party caucus is a minority of the minority, yet they are able to have the most say.

There are enough votes in the whole of congress to pass a bill, apart from 2 reasons:

Republicans now insist on adhering to the Hastert rule, which insists they will only bring a vote to the floor, if a majority of the majority support it. SO whilst a sane bill would pass in congress with both republican and democrat votes, because a majority of republicans don't support it, they refuse to bring it to the floor for a vote.

Also, the day before they shut the government down, they changed the rules of congress so that only the leader of the majority could bring bills to the floor. Prior to that, a democratic congressmen would be allowed to call for a vote, but they changed the rules, knowing what they were about to do.