I don't see how they can "just drop it". They'd be handing the election to Trump if they admit defeat.
Think of the campaign meme: One man against the DNC, the vast majority of the media, operatives from the CIA, FBI, FISA, DOJ, the State Department, and various foreign agencies, with an assist from the sitting POTUS, and they came up empty.
The most thoroughly investigated and vetted man in the history of the world, and he's CLEAN.
Who could run against that, and win?
No, they'll have to keep the circus going until the election, in some fashion or another. More allegations will surface, more whistles will blow, more dirt will be invented. They're stuck with it. I think Pelosi saw this coming, hence her apprehension...
.......and they still couldnt get 2016 Hillary into the Oval Office.......
They can drop it by saying our inquiry is over and we want to taker Trump down at the ballot Box to truly humiliate him and his supporters (left unsaid: because CWII is a war we just can’t win with the SJW and snow flakes who support us).