Sen. Tim Kaine, D-Va., is still going after Donald Trump for refusing to release his tax returns, and suggested Tuesday that even Richard Nixon had stricter ethical standards than the current GOP nominee.

"Candidates for forty years — both parties, this is not a partisan statement — have released their tax returns. Richard Nixon released his tax returns," the vice presidential nominee said at a campaign stop in Lancaster, Pa.

As the crowd laughed and cheered, Kaine added: "If you can't come up to the ethical standard of Richard Nixon, you should not be within ten time zones of being commander in chief!"

The Virginia senator's remarks about Trump's tax returns come as part of a larger effort by the Clinton campaign to cast a shadow over the Republican candidate's personal finances, business connections and health.

On the tax front, the Clinton campaign has been beating that drum for some time now.

The Democratic presidential candidate released her 2015 returns earlier this month, and challenged Trump to do the same. Clinton's tax filings showed her family paid an effective federal tax rate of 34.2 percent in 2015. The same tax returns also showed Bill and Hillary Clinton's adjusted gross income last year was $10.6 million.

The Clintons have released tax filings dating back as far as 1977. Kaine and his wife this year released 10 years' worth of tax returns.

Trump has refused all throughout the 2016 election cycle to release his tax information, and he has avoided doing so by issuing a number of excuses, including that he is under audit and that there is "nothing to learn from them."

Trump's refusal to release his tax returns has drawn criticism not just from Democrats, but also from conservatives, including Sen. Ted Cruz, R-Texas, former Massachusetts Gov. Mitt Romney and political analyst George Will, who've suggested the GOP nominee has something to hide.

On Tuesday, along with questioning Trump's taxes, Kaine also suggested the GOP nominee may not be as physically fit as he claims.

Casting doubt on Trump's health is a relatively new strategy for the Clinton campaign, and one that follows on the Republican candidate and his supporters doing the same to the Democratic nominee.

Voters want and deserve to know if candidates are in "good enough health to perform their duties," Kaine said Tuesday at a separate campaign stop in Erie, Pa.

"We've never had a candidate quite like Donald Trump. He has no comparable public record, and so we know less about him than we know about other candidates in modern history. And that makes these questions … about his health even more urgent than usual," he said.

The Virginia senator added, "Donald Trump thus far has utterly failed to answer basic questions that American voters have a right to know."

The only medical record that Trump has made publicly available is a hastily written letter from his physician claiming the Republican nominee would "be the healthiest individual ever elected to the presidency."

Trump has said he'd release more information if Clinton releases additional medical documents. Her campaign has dismissed this challenge, saying that the Republican candidate has failed to come close to matching the level of information that Clinton has already made publicly available.