Generally speaking it's each party's platform of items they're either in favor of or are against. Traditionally it's the ideology people go with. A pro-life person rarely vote for a pro-choice candidate.
One exception would be horrible economy.
Above all else people tend to vote their pocketbook.
Another is when a candidate is a "popular native". (Example California is considered a "blue state" and yet it's former Governor Ronald Reagan won the state when was elected and re-elected president.)
Democrats often view the federal government as being a major part of the solution to citizen's needs while Republicans view government as the problem and would like to see cut down in size.
However today we're witnessing a departure. For instance there are many people who consider themselves to be Republicans and they are supporting Donald Trump even though the his ideas oppose many conservative stances. He said for instance he's in favor of universal healthcare, and also believes we would have been better off leaving Saddam Hussein in power, POWs like Senator McCain aren't war heroes, George W. Bush (failed) to keep us safe on 9/11 and he even skipped one of the Republican debates.
All of these statements/actions would have been political suicide for any Republican candidate in the past.
The old mantra was cut taxes, reduce government regulation. and cut spending on entitlements as the way to solve all of our problems.
Last but not least politics in the U.S. is very much like a sporting event. Both parties host major rallies and the supporters for each side oftentimes behave like rabid fans. There are slogans and chants. Some folks relish the animosity and "differences" to the point of insulting one another based upon their support.