Former President Donald Trump‘s campaign has begun the process of formally requesting information from a small handful of potential running mates, ABC News has learned, marking a significant escalation in the process that will result in Trump choosing a candidate for vice president.

Among those who have been asked by the campaign for vetting paperwork include North Dakota Gov. Doug Burgum and Sens. Marco Rubio and J.D. Vance, sources tell ABC News. The quiet outreach by campaign officials intensified last month, while Trump was consumed with his criminal hush-money trial, the sources said.

Others are likely to be considered for the No. 2 job, but the initial asks reflect who tops Trump’s list at this point. Campaign officials say the final decision will be Trump’s.

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