because
Most Americans are Democrats, followed by Republicans, Independents and other political parties.
It's important for political parties to have a majority in both the legislative houses, because America parties have become extremely partisan. Members of Congress are more likely to vote along with their party's political views rather than whether or not a bill is actually good for the country. They also tend to vote the opposite of the rival party, except in very rare cases. As a result, a majority in both houses, can lead to all that party's political bills being passed.
Republicans.
In the 111th Congress both are Democratic Senators.
The Democratic party, which accounts for about 34-36% of American political party affiliation.
The individual political parties (Democrat OR Republican) do not pass laws. Laws are passed by the entire Congress made up of members of both political parties.
It forces both parties to keep an open mind to others views and forces a more balanced america and control majority tranny.
From 1994 until 2006, the Republican Party held a majority in both Houses of Congress. In 2006, that changed when the Democrats took the majority in both Houses.
A political
Both parties were satisfied with the Compromise of 1850.
a two thirds majority vote in both chambers of congress
Republican in both houses. Trent Lott was majority leader in Congress and Republicans had 55-45 majority in Senate.