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