Why do the political parties choose who will run for president? why not the people?

Is the United States a democracy?

http://www.thisnation.com/question/011.html

The Pledge of Allegiance
includes the phrase: "and to the republic for which it stands." Is the
United States of America a republic? I always thought it was a
democracy? What's the difference between the two?

The United States is, indeed, a republic, not a democracy.
Accurately defined, a democracy is a form of government in which the
people decide policy matters directly--through town hall meetings or by
voting on ballot initiatives and referendums. A republic, on the other
hand, is a system in which the people choose representatives who, in
turn, make policy decisions on their behalf. The Framers of the
Constitution were altogether fearful of pure democracy. Everything they
read and studied taught them that pure democracies "have ever been
spectacles of turbulence and contention; have ever been found
incompatible with personal security or the rights of property; and have
in general been as short in their lives as they have been violent in
their deaths" (Federalist No. 10).

By popular usage, however, the word "democracy" come to mean a
form of government in which the government derives its power from the
people and is accountable to them for the use of that power. In this
sense the United States might accurately be called a democracy. However,
there are examples of "pure democracy" at work in the United States
today that would probably trouble the Framers of the Constitution if
they were still alive to see them. Many states allow for policy
questions to be decided directly by the people by voting on ballot
initiatives or referendums. (Initiatives originate with, or are initiated by, the people while referendums originate with, or are referred
to the people by, a state's legislative body.) That the Constitution
does not provide for national ballot initiatives or referendums is
indicative of the Framers' opposition to such mechanisms. They were not
confident that the people had the time, wisdom or level-headedness to
make complex decisions, such as those that are often presented on
ballots on election day.

Writing of the merits of a republican or representative form of
government, James Madison observed that one of the most important
differences between a democracy and a republic is "the delegation of the
government [in a republic] to a small number of citizens elected by the
rest." The primary effect of such a scheme, Madison continued, was to:

. . . refine and enlarge the public views by passing
them through the medium of a chosen body of citizens whose wisdom may
best discern the true interest of their country and whose patriotism and
love of justice will be least likely to sacrifice it to temporary or
partial considerations. Under such a regulation it may well happen that
the public voice, pronounced by the representatives of the people, will
be more consonant to the public good than if pronounced by the people
themselves, convened for the same purpose (Federalist No. 10).

Later, Madison elaborated on the importance of "refining and enlarging the public views" through a scheme of representation:

There are particular moments in public affairs when the
people, stimulated by some irregular passion, or some illicit advantage,
or misled by the artful misrepresentations of interested men, may call
for measures which they themselves will afterwards be most ready to
lament and condemn. In these critical moments, how salutary will be the
interference of some temperate and respectable body of citizens, in
order to check the misguided career and to suspend the blow meditated by
the people against themselves, until reason, justice and truth can
regain their authority over the public mind(Federalist No. 63).

In the strictest sense of the word, the system of government
established by the Constitution was never intended to be a "democracy."
This is evident not only in the wording of the Pledge of Allegiance
but in the Constitution itself which declares that "The United States
shall guarantee to every State in this Union a Republican Form of
Government" (Article IV,
Section 4). Moreover, the scheme of representation and the various
mechanisms for selecting representatives established by the Constitution
were clearly intended to produce a republic, not a democracy.

To the extent that the United States of America has
moved away from its republican roots and become more "democratic," it
has strayed from the intentions of the Constitution's authors. Whether
or not the trend toward more direct democracy would be smiled upon by
the Framers depends on the answer to another question. Are the American
people today sufficiently better informed and otherwise equipped to be
wise and prudent democratic citizens than were American citizens in the
late 1700s? By all accounts, the answer to this second question is an
emphatic "no."