Maybe it was the eye-popping FEC data about Mike Bloomberg's Q4 spending. Or a rivalry over their Super Bowl ads. Or a change to Democrats' rules that may soon allow Bloomberg to participate in the primary debates.

In any case, President Trump raged overnight on Twitter, primarily going after the height of the 5-foot-8 billionaire who's running as a Democrat. And Bloomberg's campaign shot back, hitting Trump for his weight and hue.

Driving the news: Trump tweeted that if Bloomberg qualifies for the next Democratic debate, he'd try to "stand on boxes, or a lift." Trump also accused Bloomberg of getting the DNC "to rig the election against Crazy Bernie," and he vented more in a Sean Hannity interview previewed by Fox News.

Democrats have done away with a required fundraising threshold, which Bloomberg would not meet because he is self-funding. But the new rules require a 10% polling average that Bloomberg hasn't yet met.

The other side: Bloomberg first fired back through his campaign's national press secretary, Julie Wood: "The president is lying. He is a pathological liar who lies about everything: his fake hair, his obesity, and his spray-on tan."

Later, Bloomberg responded directly: "I will stand on my accomplishments of what I’ve done to bring this country together and get things done. I’ve been doing it for a long time. I stand twice as tall as he does on the stage, on the stage that matters."

While skipping the first four states to run an unconventional national campaign, Bloomberg will be jetting around California while the rest of his opponents are in Iowa.

"Iowa and New Hampshire have 65 delegates," said Bloomberg's national spokesperson Galia Slayen. "California, Michigan and Pennsylvania have 726."

I'll be joining Bloomberg on Monday, for stops in Sacramento, Fresno and Compton. On Tuesday, Bloomberg plans to campaign in Michigan and Pennsylvania.

Behind the scenes: Three of President Trump's advisers told me that Trump seems to view Bloomberg as a serious problem. Trump takes Bloomberg more seriously than some of his advisers, including the Trump campaign team, sources with direct knowledge told me.

A senior administration official, who told the president that Bloomberg has no hope of winning the Democratic nomination, said Trump replied: "You're underestimating him."

"He [Trump] thinks that money goes a long way," the official told me.

Another Trump adviser said: "He [Trump] takes money seriously. He's a businessman."

Trump has been lashing out at Bloomberg’s constant TV ads, despite some aides advising him to ignore the billionaire who is still polling well below Bernie Sanders and Joe Biden. Trump began his series of tweets about Bloomberg at 12:10 am today.

Kellyanne Conway, counselor to President Trump, said of Bloomberg: "His outsized wealth and outsized ego are matched by his overwrought jealousy and spitefulness towards the president. Jealousy is a dangerous motivator for people, leading them to confuse with a sugar high that money can buy with substance that voters demand to hear."

Bloomberg adviser Howard Wolfson responded to Conway's quote: "History will call to account every single person within this administration who has enabled and abetted this lawless and dangerous president."

responded to Conway's quote: "History will call to account every single person within this administration who has enabled and abetted this lawless and dangerous president." "Mike got into this race with the singular goal of defeating Donald Trump and a strategy of contrasting his record of accomplishment with Trump's lies and broken promises," Wolfson added. "Clearly it's working."

Conway responded to Wolfson: “Howard and I have already made history together. His work for Hillary and my work against Hillary helped keep her out of the White House.”

Wolfson, who worked for Clinton’s losing 2008 presidential campaign but not her losing 2016 campaign, replied, again, to Conway: “And in less than a year we will make history again when Kellyanne and Mr. Trump will have to stand and watch Mike take the oath of office.”

At this point, I ended my role as quote mediator.

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