This election is a Tossup, regardless of Brexit or current polls.
How has it ever been a tossup?
1. Structural Democratic advantage in the EC
2. Clinton's massive money + infrastructure advantage over Trump
3. Polls showing Hillary leading in virtually all the polls, many times by landslide margins. On RCP, Trump has only led or tied in 11 or 12 polls out of too many to count since he announced
4. Fundamentals are not all that favorable to Trump right now
5. Demographics of the electorate were already unfavorable to Republicans before the election even started
And no this isn't a snide comment, I am genuinely curious why you believe this is currently a tossup.
I'll take a crack at answering this.
1. Trump has identified issues that people are very concerned about, namely jobs and national security; if he can stay on message (granted, a very big "if"), Trump will do well on these issues
2. As Brexit demonstrates, there is legitimate concern over relinquishing a nation's sovereignty in the name of some globalized ideal, and I think that concern is being felt by citizens of the United States as well as those of the UK
3. Hillary Clinton represents the epitome of establishment politics, and this cycle there is a definite anti-establishment sentiment being expressed by the electorate, especially within the block of younger voters
4. Despite her having held the offices of Senator from New York and Secretary of State, supporters of Hillary Clinton still can't point to any firm legislative or foreign policy accomplishments of hers
5. There is still the little matter of the FBI investigation into Mrs. Clinton's use of a private email serve, which I do not believe we have heard the last of
While I agree with Virginia's five points, I do not believe they remove the legitimacy of these five counter-points, the consequence of which supports TN volunteer's statement that this election can be considered a tossup, at least for right now.