What does “tightening” mean in the phrase “tightening election polls”?

Question

The team, led by Vincent Chalgneau, the bank’s head of fixed-income strategy, and Guy Stear, its head of emerging markets, concluded that a Trump victory would likely hurt emerging-market currencies as investors brace for more protectionist trade policies in the U.S. Indeed, many emerging-market currencies are already responding to tightening election polls, it appears. The correlation can be seen in the graph below:

source: http://www.marketwatch.com/story/heres-how-the-us-election-might-impact-the-currency-market-2016-09-09

What does "tightening" mean in the phrase "tightening election polls"?

Answer 1

It simply means that the difference/distance between the competitors/contenders is narrowing down, getting smaller.