Donald Trump exhibited his seeming obsession with size again in a pre-Super Bowl interview with Fox News in which he attacked Democratic presidential candidate Mike Bloomberg over his height - and alleges the former New York mayor is getting special treatment while also subbing a leading Democratic presidential candidate "a communist."

The Fox host asked Mr Trump if the impeachment saga will become a "campaign issue" or if he will "let it go" after the Senate holds its final votes later this week.

"I'd like to [let it go], but it's pretty hard when you think about it because it's been such a witch hunt," he said. "They (Democrats) don't care about fairness or lies. ... They just want to win."

The president flatly called Vermont Senator Bernie Sanders, who has surged to a lead in polls ahead of Monday night's Iowa caucuses, "a communist" after for months criticising him with words like "socialist" and "radical." Previewing a potential general election attack line, Mr Trump brought up Mr Sanders' trip to Moscow as a younger man with his wife. "It's wonderful. I'm not knocking it," the president said, seeming to do just that in an attempt to plant seeds of doubt in voters' minds about Mr Sanders.

Mr Trump, as a candidate in 2016, bragged about the size of his hands and on his first day in office he deployed then-White House press secretary Sean Spicer to the James A. Brady Briefing Room to contend his inauguration crowd was the biggest in US history even though pictures showed it clearly was not. Now, he is fixated on the 5'8'' billionaire-turned-politician.

"I just think of little. Now he wants a box for the debates to stand on. Okay, it's okay, there's nothing wrong. You could be short, why should he get a box to stand on, OK? He wants a box for the debates," Mr Trump said to Fox News commentator and host Sean Hannity.

"Why should he be entitled to that? Does that mean everyone else gets a box?" Mr Trump said in an interview that was taped Saturday at his Florida Mar-a-Lago resort, where he is spending Super Bowl weekend ahead of his Tuesday night State of the Union address and expected Wednesday acquittal on two impeachment articles by the Senate.

Mr Bloomberg holds a 5.2 percentage-point lead over the president in a hypothetical general election race, according to an average of several polls by RealClearPolitics.

Ever eager to describe himself and his supporters as victims of conspiracies by the media and Democrats, Mr Trump blamed them for polls that show him trailing or narrowly leading the top Democratic presidential candidates. "I should be up by 25 points," he told Hannity. "What they've really done ... they don't want to report that the economy is good."

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Mr Trump went so far as to suggest Mr Bloomberg's height is somehow allowing him special treatment in the Democratic primary.

"[Democratic senator from New Jersey] Cory Booker and all these people couldn't get any of the things that Bloomberg is getting now," he continued. "I think it's very unfair for the Democrats."

As he had for all the leading 2020 Democrats, Mr Trump claimed he would welcome a one-on-one general election race against his fellow-New Yorker.

"But I would love to run against Bloomberg. I would love it," Mr Trump added in an interview that will air on the Fox broadcast network during its Super Bowl pre-game show at 3pm EST (8pm GMT.)

The pre-big game interview is not the first time in recent days Mr has poked fun at Mr Bloomberg's physical stature.

"Mini Mike is now negotiating both to get on the Democratic Primary debate state, and to have the right to stand on boxes, or a lift, during the debates" he tweeted Saturday, using his latest derisive nickname to try and discredit a political foe. "This is sometimes done, but really not fair!"

Mr Trump also alleged on Twitter that Bloomberg and the Democratic National Committee are trying to "rig the election" against Sanders, who has surged ahead in polling before Monday night's Iowa caucuses.

"I will stand on my accomplishments and what I've done to bring this country together and get things done," Mr Bloomberg said at a campaign stop in Los Angeles. "I've been doing it for a long time. I stand twice as tall as he does on the stage that matters.

"This is what happens when someone like me suddenly rises in the polls. All of a sudden, the other candidates get scared, and I think Donald Trump knows that I can beat him," Mr Bloomberg said. "And I think that's why he comes back with those kinds of comments."