Now that Republican Ted Cruz has chosen Carly Fiorina as his running mate, the next step will be for the remaining candidates to finds their own.
For Donald Trump and Hillary Clinton, who lead the Republican and Democratic parties, it will depend on which advantages they expect a running mate to deliver.
It's possible for Clinton to choose from either Ohio Senator Sherrod Brown, or Senator Elizabeth Warren of Massachusetts.
Brown appeals to the populist wing of the Democratic Party and may persuade Bernie Sanders supporters to fall in line behind a Clinton/Brown ticket, while Warren is already being considered closely, Clinton's advisers say.
Party unity may be of the utmost concern when it comes to vice presidential selection on the GOP side.
Trump may consider former presidential candidate and Cruz supporter Scott Walker, or a running mate with political experience who can steer his policies toward legislative passage and Republican orthodoxy.