Senior adviser for the Trump campaign Jason Miller said Monday that former President Trump is narrowing down candidates for his running mate and is close to making his final decision, perhaps as soon as this week. 

Trump's vice presidential pick has gained increasing importance as multiple commentators and Biden allies in the media have called on the president to withdraw from the race, calling into question whether Vice President Kamala Harris will step in as the nominee of the Democratic Party. 

"It's almost an embarrassment of riches that there are so many good people," Miller said on "Fox & Friends" Monday of Trump's choices for running mate. "And what President Trump has said is that whoever he does pick needs to be able to step in and do the job on day one." 

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Miller added that Trump has many "great choices" and that the final decision could happen any time between now and the Republican convention starting on July 15. 

When asked if Sen. Marco Rubio, R-Fla., Sen. J.D. Vance, R-Ohio, or North Dakota Gov. Doug Burgum are the most serious candidates to be Trump's running mate, Miller said that "President Trump has discussed even more names than that." 

"What we don't know is who President Trump and his running mate will be running against," Miller said. "Ultimately, with all the talk this morning about Joe Biden and whether or not he can hang on in the presidential spot." 

"I look ahead as a campaign strategist to what does that vice presidential debate look like," Miller said, referencing calls from within the media for Biden to withdraw from the race. "We don't know if that's going to be Kamala, or maybe they swap her out for someone who's even more liberal, more extreme, although that might be tough to do," Miller said.

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Miller also said that Biden campaign efforts in states like Florida, North Carolina and Georgia are a "waste of time." 

"I think it's doing Joe Biden a disservice for two reasons," he said. "No. 1 is it's highlighting the fact that Joe Biden cannot do three campaign stops in a day." 

"President Trump right now is leading in New Jersey," Miller said. "He's leading in New Hampshire. He's leading in Minnesota. While Democrats might want to talk about going on offense. They're really on their heels in even blue states."

The Trump and Biden campaigns did not immediately respond to requests for comment from Fox News Digital.