I can't speak for Lincoln and Trump, but Churchill and Hitler have a little more in common than basic organic functions. I am currently reading
this article on the BBC, and it certainly brings up a few things to think about.
In regards to racial hierarchy and eugenics...
In regards to use of gas...
In regards to the treatment of Jews...
That's just part of the whole article, but these are some key points that one could use to argue the case that Churchill is somewhat comparable to Hitler. I will let you read the whole article yourself and make your own choice as to what arguments you think hold the most ground, but there's no doubt he's a controversial figure for some people.
Anyone arguing Churchill was similar to Hitler in any meaningful way needs to check their history. Churchill wasn't a fascist, a national socialist, nor a dictator.
In 50 or 100 years time people will look back on our views today with very different eyes. All of the views discussed in the article seem unpalletable by our modern standards but would not have been seen as particularly extreme or even unusual in Churchill's time. Eugenics was a popular theory during the first part of the 20th century, including in the UK and US, and was by no means exclusive to Nazi Germany.
The use of gas seems to be concerned with tear gas rather than (lethal) mustard gas (Churchill would have been well aware of the difference having lived through WW1).
Even the cited attack on Churchill's anti Semitsm merely notes he was "no stranger to the latent anti-Semitsm
of his generation and class" (my emphasis).
Churchill was a high born Victorian, conservative politician and member of the British political establishment during the early part of the 20th century. He would have undoubtedly held views that today we would consider mysogynistic, racist, and just about every other ist and ism under the sun.
As I said earlier Oswald Mosley is a far better example of a contemporary aristocratic British politician with striking similarities to (and publicly expressed support for) Hitler.
As for a spivvy Labour candidate trying to score political points by attacking Churchill in 2015, well that speaks volumes about his own ignorance and how far the Labour Party has fallen.