If you read the statements that most legislators put out before voting, they almost always are "yes, but" or "no but". This means that just looking at the voting record, you will see what appear to be flip flops. In some cases, it is that the country, including the legislators moved on the issue; sometimes the bill itself shifts eliminating the "but" in either direction to the point that the legislator - on a different bill shifts; only occasionally are shifts purely political triangulating.
Trump, though has the advantage of no real record in the form of votes or actions taken. All that exists are his comments over time. This makes it harder to claim that he shifted positions than someone who cast a vote that will be in the Congressional record for life. Consider that Howard Dean, in fall 2002, made some relatively aggressive comments on Saddam and spoke of the resolution he would want - that was every bit as bad, if not worse, than the actual words of the IRW - while saying he would vote for Biden/Lugar at the point when it was the SFRC alternative that later lost out to the IWR. By 2003, he spoke as having been completely against the war. Although I did see links to the Face the Nation comments on a potential resolution on Daily Kos and DU, but it and other fall 2002 Sunday shows were NOT included on Dean's web site and those of his advocates and neither the media or Kerry campaign tried to claim that Dean's positions were more complex than the Trippi narrative.
However, though the charge of flip flopping is regularly made, it is hard to find any real time when it really worked. I know that many will cite both 2004 and 2012. While it is true that both used flip flopping, I don't think that is why those elections went as they did.
In fact, in 2004, Kerry had been a Senator for 20 years. On many many issues, early speeches he gave could have been reused years later - just changing any popular culture references. He actually was one of the more consistent people in terms of his values and goals.
So, why did the Republicans use that. One reason was that Kerry in responding to questions at a campaign stop was heckled with people asking repeated questions about the "vote" on funding the wars. At that point, the question was aimed at claiming that Kerry voted against giving the troops fighting overseas as much money as the generals said was needed. The first time, Kerry's answer was complete and very good - he spoke of there being 2 votes - one on a version that he backed where the cost was balanced by rolling back the not yet implemented tax cuts on the top 1 or 2%. Bush said he would veto that version and supported a version where the cost was just added to the deficit, that Kerry voted against. He explained the latter vote was a protest vote about not paying for it - but that in his Senate speech he spoke of it as a protest knowing it would pass. No more than 5 minutes later, he was asked the same thing again - and his answer was to say that he had just answered that - and then in spectacularly unfortunate shorthand said he voted "for it before he voted against it". ANY opposition campaign would have used that gift. The media made that gaffe much worse by acting like they could not understand Kerry's honest explanation.
However, even with that annoying ad, 2004 likely came down to too many people still rallying around Bush, who was the President when we were attacked. The NYT after the election actually sent a team of reporters to Ohio to find out why they voted as they did. Sadly, they found that many people actually did agree with Kerry on most issues, but some actually worried that he would have moral qualms about things like torture etc which Bush and Cheney would not -- and at that point, in their own fear, they voted for Bush/Cheney. They voted for Bush/Cheney for the reasons that most drove us to hate Bush/Cheney. Simply put, the country was not ready to reject the B/C mindset.
In 2012, the charges of flip flopping for Romney were mostly in the primaries. To win the primaries, he had to reject the moderate governor that he had been. In the general election, he shifted back somewhat to that earlier persona, but I think the problem was that even the moderate governor (with no primary shifts) could not have built a case for removing President Obama.
The fact is that in 2012, the economy had improved enough that people agreed that it had improved since Obama took office and was going in the right direct, but not fast enough and not for all people. A Bain capital man was not what the people in greatest need saw as the solution - far from it. On foreign policy, most looming problems were not seen by the voting public. It was easy for Biden to summarize things as "Osama Bin Laden is dead and GM is alive".
In fact, Romney made things worse after Benghazi - by jumping on the administration within hours of the news and attacking the US embassy in Egypt for a statement speaking against the film. It looked completely not Presidential, opportunist and mean spirited. It also ignored that there always is a tendency to rally around the President when there are attacks.
Romney did not lose because he flip flopped, but because he never made a good case why he would be better than Obama on either foreign policy or the economy - especially foreign policy.
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