Republicans in Washington are coming to grips with the increasing likelihood that Donald Trumpwill be their presidential nominee. It's a scenario few would have taken seriously last summer, when Trump jumped into the race against what Republicans believed was their strongest presidential field in decades. But the real estate mogul has nearly vanquished them all, winning three out of the first four contests by double-digit margins while leaving his closest remaining rivals -- Sens. Marco Rubio (Fla.) and Ted Cruz (Texas) -- in the dust. With surveys putting him ahead in most of the states set to vote in next...