Here's the full statement — a press release, which I received as email:
On January 6, I said voting to reject the states’ electors was a dangerous precedent we should not set. Likewise, impeaching a former President who is now a private citizen would be equally unwise. The impeachment power can be turned into a political weapon, especially if it is primarily used to disqualify an individual citizen from running for public office. My Democratic colleagues would have rightfully objected to Republicans – when they controlled Congress – using the impeachment power to disqualify former Secretary of State Hillary Clinton from running for president in 2016 because of her email controversy. The great hallmark of our Democratic Republic is self-government, and I have faith in the American people to assess the qualifications of presidential candidates and make an informed decision themselves, just as they have done every four years since George Washington was elected as our first president. Congress should not dictate to the American people who they can and cannot vote for.
In support of that argument, it's extremely important to remember that there is a "fundamental principle of our representative democracy . . . 'that the people should choose whom they please to govern them.'" I'm quoting the Supreme Court case rejecting term limits for members of Congress, which was quoting a case about Congress's power to exclude someone the people have elected. The internal quote — "the people should choose whom they please to govern them" — comes from Alexander Hamilton, arguing in favor of ratifying the Constitution:
