Donald Trump is watching his support erode in one of the most important battleground states

Donald Trump's poll numbers are plummeting in a state that's crucial to him winning the presidency.

In Pennsylvania, a state both Trump and Democratic nominee Hillary Clinton are targeting heavily ahead of November, the past four polls featured on RealClearPolitics have shown the Manhattan billionaire trailing the former secretary of state by 10 or 11 points.

It's a huge dip from earlier in the cycle. Between May 8 and July 11, RealClearPolitics highlighted seven Pennsylvania polls. Just one showed a race that was separated by more than 4 points, and five of the seven surveys showed the race to be within 2 points in either direction.

The shift has drastically changed the polling average in the state. Trump has gone from trailing Clinton by less than a percentage point in late June to currently trailing her by 9.2 points in the Keystone State.

In the polling average, Clinton is now at her highest level of support in the state throughout the entire general election cycle, polling at just a hair under 50%.

The gap Trump now faces in a state that last voted for a Republican presidential nominee in 1988 could prove insurmountable.

"Closing a 9-point gap is not an easy thing to do in a state as Democratic as Pennsylvania," said Jim Lee of Susquehanna Polling and Research, one of the four polling firms that recently found Clinton to hold either a 10- or 11-point lead.