Answer:
Democrats believe in government programs and helping the lower class via taxes from the upper and middle classes. Republicans believe in a hands off approach to the economy. They believe in capitalism and earning what you work for.
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First of all, it is important to understand the concept of the "bell shaped curve." Popularity wins elections. Popularity resides close to the center of the bell shaped curve, which is to say, the distribution of opinions. Some voters are on the extreme right, tending towards Fascism. Some voters are on the extreme left, tending towards Communism. But elections are won near the middle of the bell shaped curve, which is where most voters cluster. Therefore, to win elections, both the Democrats and Republicans need to appeal towards the middle.
This is not to say that there are no differences between Democrats and Republicans; but it means that both parties are constrained by having to win enough votes to be elected, which means appealing to the middle, and not being so different that "mass appeal" is lost.
There are differences even within the Democratic Party and within the Republican Party. And the same principle applies, regarding the need to appeal to the center of the bell shaped curve.
The largest landslides occur in elections in which the losing candidate strays too far from the middle. For example, Barry Goldwater the Republican lost in a landslide because he was too far to the right. George McGovern the Democrat lost in a landslide because he was too far to the left.
One common misconception is that Democrats want Big Government while Republicans do not want Big Government. The fact is that both Republicans and Democrats want Big Government, but they simply want a different kind of Big Government. Democrats want Big Government that focuses on the phrase in the Preamble to the Constitution "promote the general Welfare," so they want government programs that provide jobs, promote education, protect the environment, and provide health care. Republicans want Big Government that focuses on the phrase in the Preamble to the Constitution "provide for the common defence," so they want government programs that are Police and Military in nature.