Here's when a Trump upset becomes a real possibility, according to Wall Street

A voting sign sits on a table as an early voter walks out of the Fairfield County Board of Elections Office in Lancaster, Ohio, on Saturday, Oct. 29, 2016.
Ty Wright | Bloomberg | Getty Images

With polls tightening after the new FBI Clinton email revelations last week, FBR Capital Markets gave clients a statistical road map Tuesday for the election and said caution was warranted given Donald's Trump recent surge.

Based on various strategist reports over recent months, the ideal scenario in Wall Street's eyes is for Hillary Clinton to win the presidency but Republicans to keep control of Congress. FBR sees real risk to that scenario arising in the last week.

1) Presidential race:

The firm said Donald Trump needs to be within 2.5 percentage points in national polling to get into the historical margin of error for presidential polls. Tuesday, FiveThirtyEight has the national popular spread narrowing to just 4.1 percentage points.

Source: FiveThirtyEight

"Clinton is still well-positioned to win the 270 Electoral College votes necessary to win the presidency, but Donald Trump is narrowing the gap in the final week before Election Day," analyst Benjamin Salisbury wrote in the note to clients Tuesday.

FBR found previous polling overstated margins in elections by up to 2.5 percentage points. In the 2012 election, Republican Mitt Romney's final voting result was 2.5 percentage points under the last polling average. In 2008 and 2004, Republicans outperformed the polls by 0.1 percentage point and 1.0 percentage point, respectively, while in 2000 the party underperformed by 2.2 percentage points.