(CNN) Republicans are in the final hours of their frantic, all-out bid to save a Pennsylvania congressional district in the heart of Trump country from falling into Democratic hands in Tuesday's special election.

Democrat Conor Lamb, a 33-year-old Marine veteran and former prosecutor, holds a slight lead among likely voters over Republican state Rep. Rick Saccone, a Monmouth University poll released on the eve of the 18th District election shows.

It's left the GOP bracing for what would be its most embarrassing -- and ominous -- loss since Donald Trump took office. Republicans have spent more than $10 million to prevent a defeat in the district, which Trump won in 2016 by 20 percentage points.

If Lamb wins, or even comes close, it will be a signal that Republicans are in danger even in districts where Trump was won handily in 2016, and raise Democratic hopes of capturing the House and Senate this fall. The race would also serve as a model for Democrats running in deep-red districts across the country.

Read More