"I am deeply, deeply concerned about Trump's effort to undermine American democracy by defunding the Postal Service," Sanders told Stephanopoulos.
In response, Miller said, "I think it's a little disingenuous for Bernie and the rest of the Democrats to be making so much noise on this. They weren't trying to do it in February or March or April or May, and this is something that just is popping up now."
But Stephanopoulos pushed back noting House Democrats passed a $25 billion emergency funding bill for the Postal Service earlier in the spring. He also pointed out that it's not just Democrats raising concerns.
On Friday, Sen. Mitt Romney, R-Utah, the 2012 Republican Party presidential nominee, said, "I don't know of any evidence that voting by mail would increase voter fraud."
In response, Miller said "Mitt Romney is wrong. … Mitt Romney should take a look at Paterson, New Jersey, where upwards of a quarter of all the votes were thrown out."
Both Attorney General William Barr and Trump have pointed to reports of alleged fraud in a special election in Paterson this past May, where several city officials have faced voter fraud charges and the county elections board rejected nearly 20% of mail-in ballots submitted.
Election security experts have argued that the Paterson election, by contrast, shows that safeguards to protect against fraud actually worked effectively given the alleged fraud was quickly reported out to state officials.
Miller also pointed to the tight Democratic primary race in New York between Rep. Carolyn Maloney and challenger Suraj Patel, where absentee ballots took weeks to count. Maloney eventually claimed victory earlier this month after the New York City Board of Elections certified the results, which showed her leading by 3,700 votes.
"I guess Carolyn Maloney is the winner of that. We're still not sure," Miller said.
Stephanopoulos challenged him and said, "But let me just stop right there, because when the votes were thrown out, that shows that the system is working. It shows that the votes that could have been fraudulent."
Miller responded, "But no, but to carry on for six weeks afterwards, and we're still not sure -- I mean, did Carolyn Maloney really win that race? I mean, her opponent doesn't think so."
Stephanopoulos also pressed Miller on Trump's remarks this week refusing to clearly state whether presumptive Democratic vice-presidential nominee Sen. Kamala Harris is eligible to serve as president. Harris, the first woman of color on a major political party general election ticket, was born in California, to immigrant parents. The false and racist theory -- written in an op-ed by John Eastman, a conservative attorney who ran against Harris in 2010 for California attorney general -- has gained traction in some right-wing circles since Joe Biden announced her as his running mate.
Law experts have said these questions about Harris' eligibility are "100% bogus" and Harris, born in Oakland in 1964, is a U.S. citizen eligible to serve as president or vice president.
Asked about the theory during a press conference Saturday, the president said, "So, I have nothing to do with that. I read about it. ...The lawyer happens to be brilliant lawyer, as you probably know. He wrote an article saying there could be a problem. It's not something that I'm going to be pursuing."
Stephanopoulos asked Miller on Sunday why the president couldn't unequivocally say that Harris is eligible to run.
He replied, "Well, he did. He said last night at his press conference -- said it's not something that the campaign is pursuing."
Stephanopoulos pushed back, "But he did not say she's eligible to run for president."
"It's not something that we're pursuing, not something that we're interested in," Miller repeated. "And in our opinion, it is case closed. End of story. And the only folks who keep bringing it up are the media."
But Stephanopoulos noted that Trump campaign legal adviser Jenna Ellis retweeted the article and raised the question about Harris' eligibility.
"She wasn't speaking for the campaign. I am," Miller replied.
Stephanopoulos also pressed Miller on the state of the Trump campaign as national polling averages show Joe Biden leads Trump by about eight points.
"We're actually in a better place than we were four years ago, so I like it," Miller replied. "All the trend lines are looking very positive, particularly in the battlegrounds. We're either leading or within the margin of error of every state that we need to get to 270."
Stephanopoulos pushed Miller on this point, noting that Trump trails Biden in several key states, such as Florida, North Carolina, Michigan, Pennsylvania, Wisconsin and Minnesota.
"What are you seeing that we're not seeing?" Stephanopoulos challenged.
"The media pollsters have not corrected their models since 2016. And the same way that they undercounted Trump voters and they missed so much, nothing fundamentally has changed," Miller replied. "So what we're looking at are models that were correct in 2016. We believe it will be correct in 2020. We feel good about where we are."