Apple remains under scrutiny from Wall Street as President Donald Trump's threat of new tariffs on iPhones piled on to the stock's recent downward spiral.

Apple shares closed down 0.2 percent on Tuesday, bringing the stock's loss since the start of November to more than 20 percent.

Trump's threat of a 10 percent tariff on iPhones "could simply be a negotiating tactic ahead of the G20 Summit later this week," UBS said in a note. The firm estimates that a tariff of that magnitude would represent a $1.5 billion hit to Apple's earnings. If Trump ups the rate to 25 percent, as he's threatened, UBS says "the impact would be ~$3.8B or ~$0.83 in EPS."

Baird commented in a note: "When it rains it pours, Trump piles on."

Bernstein's Tony Sacconaghi explained Apple's revenue stream on CNBC'S "Squawk Box," saying that "25 percent of Apple's revenue, call it $50 billion, would be subject to a 10 or 25 percent tariff."