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Visits to Trump-branded hotels, casinos and golf courses slipped further in September, suggesting that the Republican candidate's tumultuous political campaign could be putting pressure on his companies.

Donald Trump plays a round of golf after the opening of The Trump International Golf Links Course in Balmedie, Scotland. Getty Images

According to new analysis by Foursquare, which looked at visits from its more than 50 million users, the share of customer traffic going to Trump-branded properties fell 16.5 percent in September compared with a year ago earlier.

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The decline follows a 7.1 percent year-over-year dip in August, a 14 percent drop in July, and a 17 percent decline in June, Foursquare said.

Related: Trump's Presidential Campaign Has Crushed Traffic to his Hotels and Casinos

Yet the data does not capture the month of October, when a video was released showing Trump making lewd comments about women. It was also pulled prior to women coming forward with accusations that the Republican candidate had inappropriately touched them. Trump has vehemently denied those claims.

Trump's privately held company, the Trump Organization, doesn't release sales numbers and declined to comment on Foursquare's figures. Yet the report adds to previous analyses showing that Trump's businesses have started to soften amid his run for president. An earlier analysis by online travel site Hipmunk found that bookings to Trump hotels fell more than 50 percent in the first half of the year.

Granted, the outside estimates are only a partial snapshot. In his May financial filing to the Federal Election Commission, Trump reported revenues at his businesses increased $190 million, to $557 million, over the prior year.

His residential real estate portfolio seems to be holding up, though the lead times for buying and selling apartments are much longer. An analysis by Zillow's StreetEasy found that the 16 Trump-branded buildings in Manhattan saw price increases of 5.5 percent in August over the prior year. Prices in the broader Manhattan market grew just 1.7 percent over that period.