Wisconsin Gov. Scott Walker became the second major candidate to depart the crowded Republican presidential field, bowing out Monday because he no longer saw a path to victory after a series of early stumbles and financial woes.

In his announcement, Mr. Walker took aim at one of the candidates responsible for his demise, real-estate mogul Donald Trump, a political novice who has overshadowed more-experienced candidates since midsummer. Mr. Walker urged other Republicans to drop out so the party could concentrate its resources...