The election of Barack Obama in 2008 marked the culmination of a push by the left wing of the Democratic Party to move the party to the left. Milestones in this push included the candidacy of Howard Dean, the rise of the "Netroots" on-line community, the closing of the moderate Democrat Leadership Council, the ejection from the party of office-holders such as Joe Lieberman, and the re-branding of the left as "progressive" instead of "liberal".
That last one is important. While they like to recall the Progressives from the early 20th century, the term went out of fashion after the 1948 election when it was used as a front for communists.
If Obama wins a second term then things will continue as they have for the last two years. His slogan is "Forward." (or more recently "Forward!") but his real goal is "Entrenchment!".
Obama believes that the fiscal cliff gives him the upper hand in negotiations with Congress. He is willing to let all of the various tax cuts expire and all of the budget cuts go into effect. He believes that this will make the Republicans so desperate to make a deal that they will have to accept any terms that he dictates. These will start with tax increases on the rich. He may add in other portions of his agenda that failed during his first term.
It is unclear if this tactic will succeed. The important thing is that he is willing to hold the economic health of the nation hostage (a term he used two years ago to describe the Republicans).
Regardless of his relations with the Republicans, Obama has shown that he is willing to ignore Congress and legislate through executive order. This may permanently damage the balance between the three branches of government.
Obama has promised the Russians that he will have more flexibility during his second term in negotiating arms reductions.
He is a follower of Paul Krugman who believes that the solution to a slow economy and a budget deficit is more spending and a higher deficit. This reasoning says that if the government pumps enough money into the economy then it will eventually pick up so much that the deficit can be eliminated. The fact that this has only been accomplished in conjunction with a war that destroyed Europe's industrial capacity escapes them.
Obama has a European view of the relationship between government and its people. In this view, rights come from the government and group rights can take priority over individual rights. Religion is harmful and should be limited to private observances. The economy must be carefully managed by the government and steps must be taken to equalize results. Rich people represents a failure of government.
The fact that most of Europe is in deep financial trouble is irrelevant and can be blamed on misguided attempts at austerity. This feeds back into the Krugman view that government can never spent too much.
If Obama is defeated then none of this will happen. What is more, it will be generations before the Democrats go so far to the left again. That is what happened after McGovern was trounced in 1972. For that matter, gun control has been off the table for Democrats since the 2000 election. The widespread thinking is that Gore lost because of gun control legislation passed under Clinton (with Gore providing the tie-breaking vote in the Senate). That cost him Tennessee which would have given him the election.
If Obama loses then the Democrats will back away from a radical agenda and moderate their position. If they win they will double-down on it.