The Democratic Party has rejected one of the main ideas embodied in the Constitution by the Founding Fathers. In Federalist Paper 10 James Madison argued that the US government should be a republic, not a democracy. A republic acts through representatives of the people, not by direct votes of the people themselves. In defending this position Madison says:
A republic, by which I mean a government in which the scheme of representation takes place, opens a different prospect, and promises the cure for which we are seeking. Let us examine the points in which it varies from pure democracy, and we shall comprehend both the nature of the cure and the efficacy which it must derive from the Union.
The two great points of difference between a democracy and a republic are: first, the delegation of the government, in the latter, to a small number of citizens elected by the rest; secondly, the greater number of citizens, and greater sphere of country, over which the latter may be extended.
The effect of the first difference is, on the one hand, 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 purpose. On the other hand, the effect may be inverted. Men of factious tempers, of local prejudices, or of sinister designs, may, by intrigue, by corruption, or by other means, first obtain the suffrages, and then betray the interests, of the people. The question resulting is, whether small or extensive republics are more favorable to the election of proper guardians of the public weal; and it is clearly decided in favor of the latter….
Excerpt From: Alexander Hamilton, James Madison & John Jay. “The Federalist.” Apple Books. https://itunes.apple.com/us/book/the-federalist/id809256982?mt=11
The Constitution did not originally say who was qualified to vote for the representatives of the republic; it left it to the states to decide who was eligible to vote. Most states allowed only white, male, adult, property owners to vote for representatives. This was far from a democracy where everyone had a say in the government, as is the case in some New England town meetings. Over the years the right to vote has been greatly expanded by amendments to the Constitution. This expansion has created some of the very problems foreseen by the Founding Fathers. The Democratic Party believes it can get its representatives elected by promising free stuff, particularly to blacks, Hispanics, and recent immigrants, but also to whites, e.g., Medicare and Social Security. Typically, it has made wider public provision of healthcare the focus of Democratic campaigns in the midterm elections.
One of Madison's arguments was that representative government would make it harder for special interests to influence the government, because the representatives would have a broad, diverse constituency. In practice today, however, gerrymandering and lobbying have undermined this principle. Congressional districts are not diverse, and the huge amounts of money controlled by the lobbyists give them inordinate power over the wishes of ordinary citizens. The result has been a distortion that benefits both ends of the population spectrum. Poorer voters get more government benefits because Democrats pander to their demands, and richer voters get more government benefits because their lobbyists bribe lawmakers to give them. The middle class essentially gets left out. Their votes are not for sale, but they can't afford to buy politicians.
Madison's response would probably be that the elected representatives should be people of high moral character and intelligence who would serve the country's interest, rather than a few of their constituents, but this does not seem to be the case today, with a few exceptions.
Stopping immigration weakens the Democratic approach of winning over poorer voters with government benefits. There are remedies for limiting the influence of wealth in the Republican Party, such as higher income and inheritance taxes, and limits on campaign contributions, perhaps requiring that all campaigns must be limited to public funding; however, I don't see any movement toward these reforms.