Anyway, ultimately it boils down to this.
The Democrats have long had as their power base labor unions and the lower classes. Dating all the way back to Jefferson and the Democratic-Republican party, they favored the lower classes.
The Republicans have long had as their power base bankers, merchants, industrialists and the monied interests, this too dates back to the same time period with the Federalists and Alexander Hamilton.
There’s competing ideologies in the United States. One values the entrepreneur who makes himself a millionaire because they fuel innovation, the economy, and represent the “American Dream.” On the other hand there’s the ideology that private citizens shouldn’t be permitted vast fortunes, that they should be confiscated and redistributed to the poor. People ascribing to that ideology are going to viscerally oppose the wealthy and support Democrats and oppose Republicans.
Both parties are incredibly diverse, and both parties are polarized when it comes to national governance. For over 150 years the Democrats were painted as the party of slavery and the Confederacy even though a huge percentage of them were strong advocates of Civil Rights and did not support slavery (looking further back.)
There’s something like 537 elected offices in the Federal government and the handful of men and women who hold them tend to shape national thought on what a party is. With each election cycle a slightly different incarnation of the GOP and a slightly different incarnation of the Democrats come to power. The current President for example has little in common with Dwight Eisenhower just as Clinton had little in common with Jimmy Carter.
Both parties have clear lines in the sand drawn and clear differences, but at the same time both parties are “big tent” parties. Over a million Democrats in Texas voted for John Kerry in 2004, and traditionally liberal states have, if not always Republicans in Federal office many Republicans in state legislatures and sometimes the Governor’s mansions. New England, California, and Texas republicans are all different creatures just as Massachusetts Democrats are a far cry from Tennessee Dems.
It makes it difficult to clearly label either party on a wide range of issues (Harry Reid is pro-life, by the way.) Likewise, our government is structured to force compromise. It’s very rare in the history of this country that one party controls the Presidency and both Houses of Congress with commanding majorities. Only when a single party controls the White House, the House of Representatives, and the Senate, can it truly implement its policies without serious compromise. But even then, it can’t really do that, because party discipline in the United States is notoriously weak compared to many other representative governments around the world. Democrats and Republicans both often do not side with their party’s legislative initiatives. This is why sometimes to the casual observer it can be even more difficult to distinguish between the two sides, even when one side is in power and the other is not, compromises have to be made so final legislation tends to have some influence from both parties in it.