Republican presidential candidate Donald Trump has narrowed his potential running mates to a “very small” list, his campaign manager Corey Lewandowski said Sunday.

Speaking to “Fox News Sunday”, Lewandowski said the vetting team has “started to shrink down that pool.”

“I can say that the list is very small and we have a very good understanding of who those handful of individuals will be and ultimately the decision will be made by Mr. Trump,” he told the network, adding that the final decision will be up to Trump, though aides are now identifying the strengths and weaknesses of each option.

“What we are doing right now is making sure that we’ve done our due diligence so that when he chooses the person he wants, we can give him the pros and cons of each of those individuals…. so we feel very good where we are in the process,” Lewandowski added.

The real estate mogul was officially crowned as the Republican presidential candidate for 2016 last week, after he passed the 1,237 threshold needed to secure the nomination.

With his last major rival Senator Ted Cruz (R-TX) dropping out of the race, Trump was left almost assured of victory, but the new count makes it official and removes the possibility of a turnabout in a contested Republican convention in July.

With regards to his running mate, Trump recently said he has not ruled out New Jersey Governor Chris Christie for the job.

Christie was in the running for the Republican presidential nomination but suspended his campaign after a disappointing loss in New Hampshire.

He later offered an endorsement to Trump, saying there is no one better prepared to lead the country or defeat former Secretary of State Hillary Clinton should she become the Democratic nominee.

Another name that was brought up was that of Florida Senator Marco Rubio, Trump’s former primary rival, who has emphatically ruled out that possibility, though he did say last week that he would attend the Republican convention and was willing to speak on Trump’s behalf.