What are some examples of prominent American politicians in the twentieth and twenty-first centuries - I am talking about high-profile ones, elected officials like Congressmen, Senators, or Governors - who have, over the course of their career, switched from liberal to conservative? I don’t mean merely switched parties, I mean fundamentally switched ideologies?
I feel like an idiot for not knowing.
I can’t think of any off the top of my head. Former Alabama governor George Wallace comes to mind, however – I’m not sure that one can say that he changed his political orientation entirely, but he had been a hard-line segregationist during the Civil Rights movement of the 1960s. Then, in the late 1970s, he publicly disavowed segregation, and his actions during that era, and asked for forgiveness from African-Americans.
Wouldn’t Reagan count? Democrat, union sympathiser, became the conservative icon.
There’s also the question of whether the politician themself changed, or if the background changed around them. There are some who continued to hold the same positions after those positions changed from being regarded as conservative to liberal, or vice-versa.
He’s probably a pretty good example (and I’d forgotten that he’d been a liberal early on). He never held, or ran for, office as a liberal or Democrat, though it looks like he was politically active in supporting liberal candidates and causes through the 1940s, before becoming a conservative in the early 1950s.
He was the first former union organiser elected President. 
I’m not sure how well thought out or strongly held Donald Trump’s positions have ever been, but at various times in the past (in addition to being a registered Democrat) he’s been on the record as “very” pro-choice, in favor of universal health care, in favor of drug legalization, etc.
Defining liberal and conservative becomes pretty important for the question. Shifts in party can be relatively small movements or represent shifts in the party not in the individual making a change. Some of those shifts can also be on issues that don’t really break cleanly along general liberal-conservative lines even though liberal or conservative parties have shaped them as areas of contention.
Defining the question is really important because there’s a body of research that shows a genetic component to political attitudes and behaviors. (Cite.)
The genetic predispositions for political attitudes, let alone early life socialization, aren’t going to be hard to fight against along the course of a lifetime’s addult experiences. If the question is about large shifts in the most basic questions about political disposition, finding examples is going to be tough. We just aren’t that fluid.
So you mean besides our current president?
Elizabeth Warren was a republican in the early 90s.
Without getting into “debate” category, I think it’s exceptionally difficult to ascertain our current president’s actual political ideology, or the line between it and simple self-interest. I also do not view him as a politician, despite the fact that he is currently the president.
I suppose former Senator Robert Byrd of West Virginia might qualify.
He was a war hawk in the Vietnam era, but later opposed the war in Iraq (and was against creation of the Dept. of Homeland Security for giving the executive branch too much power).
This doesn’t equate to “conservative”, but in his earlier days Byrd was virulently racist, organizing a Ku Klux Klan chapter (serving as Exalted Cyclops), and filibustering the 1964 Civil Rights Act. Later he supported the 1968 Civil Rights Act and eventually got a 100% rating on Senate votes from the NAACP.
He consistently was heavily pro-pork (legislation), which is neither conservative nor liberal.
And George H.W. Bush was President in the early 90s, someone who would not be considered a conservative today.
Don’t forget Earl Warren, who as attorney general and then governor of California urged the internment of Japanese-Americans, and was Thomas Dewey’s running mate in 1948. As Chief Justice he presided over the Supreme Court’s most liberal era.
One of those would be Sen. Sam Ervin of North Carolina. He was an ardent civil libertarian who helped to bring down both Joseph McCarthy and Richard Nixon, resisted incursions on Fourth Amendment rights and opposed a constitutional amendment for prayer in public schools. At the same time he opposed civil rights legislation and liberalization of immigration policies.
Basically, to people who define their politics by 1960s issues, Ervin is a conservative. To those who define them by 1970s issues, he’s a liberal.
Others in the “depends on the era” might include Daniel Patrick Moynihan, Scoop Jackson, J. William Fulbright, and even Hubert Humphrey. (One Republican strategist reportedly said Humphrey was, “a man ten years ahead of his time, but his time was 1948.”)
Hugo Black was a KKK member who became a liberal member of the Supreme Court.
Strom Thurmond was a Democrat who famously led a revolt against support for civil rights in the 1940’s, instead choosing to lead the pro-segregationist wing, popularly called the Dixiecrats. He later became a Republican in the 60’s.
He doesn’t really apply; the labels changed, he stayed the same.
John Anderson, a Republican congressman best known for running for President as an independent in 1980. As the linked article notes he started as conservative but by the time 1980 rolled around he was the most liberal of the three major candidates. For example, he won 7% of the overall vote, but 11% of liberals voted for him (only 4% of conservatives).
Wasn’t Truman in the KKK in his youth?
No.