Former President Bill Clinton speaks at a campaign stop for his wife, Hillary Clinton, on Wednesday, at the Green Street Community Center in Concord, N.H. (John Minchillo/Associated Press)

Bill Clinton looks “smaller and his voice sounds weaker” these days. Fans are “wondering if he has lost some of that old Clinton magic.” Even his friends fret that he might “look zippier if he ate the occasional cheeseburger.” That is not a right-wing take down of the former president, but the New York Times.

The Times and other political observers strain for explanations and rationalizations. He’s mellowed. He’s matured. He’s still a great advocate. Maybe there is something to those rebuttals. But still, this is not the Bill Clinton we saw in 2008, let alone 1998. Several things are arguably going on here.

This ad, shot ahead of Bill Clinton's 1976 run for Arkansas attorney general, shows the political couple in their Fayetteville, Ark., home. (Clinton House Museum-Fayetteville, Arkansas)

First, Donald Trump did, it seems, knock Bill and Hillary off balance. Trump said what many voters dimly remembered (or never knew because they were too young in the 1990s to have been following the news): Bill Clinton was a womanizer, a personal time bomb and an adept liar. His wife was at his side, excusing his conduct and attacking critics, including young women. Trump’s comparison of Bill Clinton and Bill Cosby was uncomfortable, to say the least. All of that has very possibly led to a more subdued, less aggressive Bill Clinton. He’s been smacked on the nose and now seems consciously trying to stay out of the fray.

Second, the Bill Clinton of 1992 was about where Ohio Gov. John Kasich is today (except on abortion) — fiscally prudent, a believer in the safety net, tough on crime, generally pro-business and cautious on foreign policy, although very much in the bipartisan tradition of American foreign policy. That, unfortunately, is not in style in the Democratic Party these days. His wife is getting all she can handle from socialist Sen. Bernie Sanders (I-Vt.). Hillary is running left these days, away from Bill and even herself. Under the circumstance, having her husband around, the embodiment of the “third way” and the Democratic Leadership Council, is, well, awkward. One could understand how he’d be tentative, even clumsy in navigating between his own views and record, and hers.

And finally, by today’s standards Bill is not old, but he sure is older. The once-rascal president, the brash pol who could talk for hours and wanted to charm every voter will be 70 years old in August. He has been out of office for 17 years. His vegan diet makes him look gaunt. His voice sounds raspy. In his AARP phase he underscores how long both he and Hillary have been around. Indeed, if Hillary Clinton is the nominee and faces one of the younger Republicans, the Clintons will be running against someone much closer to Chelsea’s age (35) than to theirs.

In the primary, Hillary need not worry about the age issue, as she competes against the more-senior Sanders (who makes even Bill Clinton look young). But in a general election, the Clintons will encounter voters who don’t really recall either she or Bill in their pre-President Obama days. Nostalgia is not a presidential message.

Age — and the inevitable sense one’s best days are behind and not ahead — was a problem for Sen. John McCain (R-Ariz.) in 2008, and it will be for Clinton or Sanders, whichever one is the nominee. Bill Clinton — because the difference in his vitality from years gone by is so stark — has just reminded even sympathetic observers of the passage of time. In the general election, however, the voters will see it day after day. No wonder the Clintons do not want to face Sen. Marco Rubio (R-Fla.) and his “New American Century” message in the worst way.

The Washington Post's Tom Hamburger explains how Bill and Hillary Clinton's bond as a political team formed over time, starting with his 1974 race. (Sarah Parnass/The Washington Post)