A Wall Street chief strategist says US stocks have 'considerable upside' from current levels — and could climb as much as 20% in a best-case scenario

· *Stocks have upside from current levels even given the size and scope of the market rally from March lows, according to Leuthold Group. *
· *"If a new bull market has begun, despite the size and speed of this year's rally, there is still favorable upside potential over the next couple of years," said wrote Leuthold...