If you go back to the 1993-1994 election cycle when Democrats had the majority in Congress and were very popular in public polling data, the GOP powers-that-be (headed by religious-extremist theocratic-COUP-seeking Heritage Foundation) followed the principles of an often-quoted Newt Gingrich hero, German philosopher Nietzsche, to artificially "demonize" an envied segment of a population---the Jews in pre-Nazi Germany, the popular Liberals within the Democratic party here in the U.S. The religious-extremist fundamentalists here in the U.S. had seized control of the Republican party (fanatical Paul Weyrich, the hissy-snake Pat Robertson and his "Christian Coalition," and their right-wing ilk) during an out-of-it (Alzheimer's-afflicted) Reagan presidency, and the Heritage Foundation joined forces with ambitious ethically challenged Newt Gingrich to set up a carefully planned THEOCRATIC COUP from inside the political system (read Michelle Goldberg, 2006: "Kingdom Coming: The Rise of 'Christian' Nationalism").
In order for this plot of a THEOCRATIC COUP to move forward, however, the popular party of the people (the Democrats) had to be subjected to a carefully planned smear campaign, and the GOP spin doctors were scripted to use buzz words built upon the Nietzsche demonization principles, such as "tax and spend Liberals," or "Liberal media" or "welfare queens"---all designed to market the idea in the minds of the American people that being Liberal was somehow "un-American" (never true, but the PR firm hired by the GOP, Hill & Knowlton, and the many tentacles of the COUP-seeking biblical-literalist Heritage Foundation peddled the lie quite effectively). Divide and conquer...that was the plan.
Larry Flynt (2004), trying to help the Clintons under constant bombardment of propaganda from this right-wing extremist takeover group, uncovered the Heritage Foundation and Newt Gingrich connection, cited in "Sex, Lies & Politics: The Naked Truth." Harper editor Jeff Sharlet (2008; 2010) went undercover inside the Republican CULT of many names (The Family, a.k.a., Youth With A Mission, YWAM, The Federation, C Street, etc.) and then wrote two books well worth reading to find definitive answers to your timely question: "The Family: The Secret Fundamentalism at the Heart of American Power" and "C Street: The Threat to American Democracy."
And another perspective on why "Liberal" was so demonized comes from former Conservative spin doctor and "attack dog" David Brock (2002), "Blinded by the Right: The Conscience of an Ex-Conservative," or (2004), "The Republican Noise Machine: Right-Wing Media and How It Corrupts Democracy," or his most recent publication (2015), "Killing the Messenger: The Right-Wing Plot to Derail Hillary and Hijack Your Government."
Republican Americans, as a rule, have simply been misled for years by a hidden-agenda smear campaign and began to believe the bombardment of negativity that was used by party leaders to hijack the American government. Faux "news" (the sue-to-lie foreigner-owned propaganda spreading media outlet) is part of the problem, and so is hate radio. A steady diet of crap painted "nutritious and delicious" can warp minds.