Donald Trump’s search for a running mate is intensifying, as the former president awaits sentencing on felony charges and prepares for next month’s Republican National Convention.

Vice presidential contenders recently received vetting materials, five sources familiar with the process told NBC News.

Trump’s search, according to one source, is heavily concentrated on four top prospects: North Dakota Gov. Doug Burgum and Sens. Marco Rubio of Florida, Tim Scott of South Carolina and JD Vance of Ohio. Another source described a three-way competition involving Burgum, Rubio and Vance.

It’s unclear, though, who all has been asked to provide vetting details that could rule them in or out. Burgum, who has been spending more time with Trump in recent weeks, is among those who have received a request, said one source familiar with the ask. Advisers to Burgum and other would-be running mates declined or did not respond to requests for comment this week.

Sources plugged into conversations about the search cautioned that Trump is working from a fluid shortlist that at times includes more than a half-dozen names. Additions, subtractions and the emergence of dark-horse candidates remain possible.

Others who have been mentioned as prospective running mates include South Dakota Gov. Kristi Noem, Reps. Elise Stefanik of New York and Byron Donalds of Florida and Ben Carson, who served as the Trump administration’s Housing and Urban Development secretary.

Trump and his advisers have kept a close hold on the search, which until recently had been relatively quiet, as Trump stood trial on 34 felony counts of falsifying business records related to a hush money payment to an adult film actor. Trump has said in interviews that a decision on his running mate is not likely until closer to the convention, which opens July 15 in Milwaukee. In 2016, he announced then-Indiana Gov. Mike Pence as his running mate days before that year’s nominating convention in Cleveland.

Last month, NBC News reported that the Trump campaign had yet to move beyond initial deep dives on prospective running mates; at the time, no questionnaires that might have helped cull the field had been sent. That changed over the last week, as the case neared its finish, with a jury finding Trump guilty on all counts.

The trial became an unofficial audition stage for VP contenders. Both Burgum and Vance, as well as longer-shot prospects like Donalds, joined Trump at the courthouse and have defended him frequently on TV.

Asked Friday in an interview with CNN’s Wolf Blitzer if he was interested in running on a ticket with a “convicted felon,” Vance pushed back.

“Well, Wolf, the entire purpose of this trial was to allow the media and the Democrats to say exactly that,” Vance said. “This was never about justice. This was about plastering ‘convicted felon’ all over the airwaves, when in reality the only thing Donald Trump is guilty of is being in the courtroom of a political sham trial.”

Burgum, who was with Trump at the courthouse the day of the verdict, quickly pivoted to flattery in an interview that night on Newsmax when asked if he was being vetted for vice president.

“There’s a lot of buzz around that,” Burgum replied. “I just say President Trump is so strong right now, he could get elected without a vice president.”

This article was originally published on NBCNews.com