"I think the race is close, it's competitive. We believe it's going to tighten even further," Clinton campaign manager Robby Mook said on MSNBC's "Morning Joe."

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Mook argued that "we don't build a big ground [game] and build all the infrastructure we have here at headquarters and around the country because we think the race is going to be easy. We build that because we think it's going to be hard."

Clinton leads the GOP nominee by just 2 points in a four-way matchup against Trump and third-party presidential nominees in the RealClearPolitics index of national polls. She led by an average of more than 6 points last month. Still, she's led in several crucial battleground states.

Mook on Thursday slammed Trump for continuing to insist in a forum the previous night that he has a secret plan to defeat the Islamic State in Iraq and Syria, suggesting Trump may not share details in the presidential debates starting later this month.

Trump said Wednesday night that he didn't want to "broadcast" his intentions to the enemy.