A few weeks ago, when Biden's campaign appeared doomed, he agreed to some national television interviews, in what seemed to be a last ditch effort. He struggled through an awkward session with Chris Wallace on "Fox News Sunday," having to explain one clip in which he said he was running for the "senate," and another in which he falsely claimed to have been "arrested" in South Africa. He ended the interview by calling Wallace "Chuck" (as in NBC's Chuck Todd, with whom he spoke by satellite earlier that morning).

Biden won in South Carolina by a bigger margin than seemed possible and has roared ahead ever since. But with his prospects going from dim to glowing, his strategy has shifted back to duck and cover.

To be clear, Bernie Sanders has never enjoyed sit-down interviews either and seems only to have increased his availability in inverse proportion to his prospects in the race. But Sanders has always delivered lengthy stump speeches, and he maintains a far more demanding schedule of events than does the former vice president. Referring to Biden's truncated speeches, Sanders said at a Fox town hall, "I think that the American people at this incredibly complicated and difficult moment in our history are entitled to thoughtful answers to the crisis we face."

Meanwhile, opponents have started rolling out video mash-ups of Biden's flubs. They're easy to come by, and don't require editing tricks to make the case that Biden has apparently lost some mental dexterity.

The clips are all over the internet. "We choose truth over facts!" Biden told supporters in Des Moines. "Poor kids are just as bright and talented as white kids," he said in a speech to the Asian and Latino Coalition. "150 million people have been killed (by gun violence) since 2007," he stated in the Charleston debate.

To Biden's supporters these are just "gaffes": harmless flubs that are merely examples of "Joe being Joe." To his detractors, however, they are warning signs of something more serious. When he draws a blank on the Declaration of Independence, "We hold these truths to be self evident, all men and women created by the, you know the thing," as happened in Texas recently, alarm bells ring.

As troubling as these verbal mistakes are, it is even more worrisome to review transcripts of Biden's answers in debates. Watching the video his remarks seem reasonable; you're able to get the point. But a close reading of the text reveals that Biden is often disjointed. He relies on subtle distractions — like saying "my time is up," or "I've said too much" — to cover for the fact that he has suffered an apparent mental block.

After Biden's weak showings in Iowa and New Hampshire, I wrote in USA Today that he should consider withdrawing from the race as a patriotic gesture to help the party win in November. Although he has since had remarkable success and climbed into the lead, I still believe withdrawing would have been the wisest course at the time. Democrats might be willing to look at Biden's candidacy with blinders, but Republicans won't.

Consider these two observations: (1) "It is a legitimate question whether or not the 77-year-old Biden has the stamina and the strength, the mental acumen and the focus required to serve in what is the most difficult job in the world." And, (2) "There are a lot of people who are concerned about Joe Biden's ability to carry the ball all the way across the end line without fumbling."

The first remark was made last week on Fox News Channel by Sean Hannity. The second was made last September on CNN by New Jersey Sen. Cory Booker. Hannity isn't changing his tune; he's a staunch Trump supporter. Booker, however, is now a Biden backer and no longer willing to question his mental fitness.

The nightmare scenario is that Biden prevails at the convention and then stumbles badly in the campaign between July and November. No amount of glowing endorsements, illustrious past accomplishments, or even an ambitious selection of a running mate will save Biden if it becomes clear to voters that the Democratic candidate has, to use the metaphor favored by overly kind commentators, lost a few feet off his fastball.

With only two Democrats left in the race (three, if you count Tulsi Gabbard, who most don't), it's no longer logical to ask Biden to withdraw. But it is reasonable to ask him to put himself out there, on TV and on the stump, so voters can decide if he can really go the distance.

The March 15 debate — Biden v. Sanders — should be telling. I'm hoping the better man wins on the issues, not due to unforced lapses. But sadly, like the pipes under my kitchen sink, these guys are not getting better with age.

(c) Peter Funt. This column originally appeared in USA Today.