Trump has made his pick for VP. Marco Rubio and Doug Burgum ...

MILWAUKEE — Former President Donald Trump has made his pick for vice president, according to two senior campaign officials, and will announce it at 3:30 p.m. CT. The running mate will appear when the vice presidential nomination process starts.

Trump, the presumptive Republican nominee, had been focused on three finalists: Sen. JD Vance of Ohio, Sen. Marco Rubio of Florida and Gov. Doug Burgum of North Dakota. But the search process has been closely guarded by the former president and top advisers, with everything from timing to the identity of his choice largely up to Trump himself.

Four sources told NBC News that Rubio has been told he's out, and three sources said Burgum has also been notified.

Rubio was told early Monday afternoon, just hours before the pick was to be made public, that he would not be the selection.

"Rubio got the 'not you' call," a source familiar told NBC News.

A source familiar said that part of the reason Rubio is not the pick has to do with uncertainty over the issues on the residency requirements — since Trump and Rubio are residents of the same state, which creates complications under the Constitution — that could not be resolved to satisfaction.

A source involved with the process said the Trump and Rubio teams spent the last 10 days getting a wide range of legal opinions related to the residency issue and both sides came to the conclusion that they were not 100% confident that if he were chosen that it would withstand a legal challenge.  

They were concerned about going through the process of uprooting Rubio's family and moving to satisfy the residency requirement only to have it tossed aside through a legal challenge. 

The assassination attempt against Trump at his Saturday rally in Butler, Pennsylvania, could have altered the timing or sent the search in other directions. But a Monday announcement coincides with his long-preferred timeline of introducing his choice at the convention.

In the days before the Pennsylvania rally, Trump had a series of final check-ins with the finalists, three sources familiar with the discussions told NBC News. 

Vance and Rubio met with Trump in person at his Mar-a-Lago resort, a source familiar with those meetings said. Burgum had a discussion with Trump by telephone, two sources familiar with that conversation said. ABC News was first to report on the meetings.