My trick knee is throbbing again.

It’s telling me that Vice President Joe Biden is going to run a third time for president of the United States. It’s also telling me he likely should forgo the 2016 and call it a career.

Truth is, I don’t have an actual trick knee. But if I did it would suggest that the vice president needs to think as deeply about this possible campaign as he has thought about any key political decision he’s ever had to make.

One analysis suggests a Biden candidacy depends on an implosion by Hillary Rodham Clinton. The e-mail controversy keeps nipping at her. Will it forestall her expected nomination? Is the vice president the person to carry a similar message — whatever it is — forward onto the campaign trail?

I happen to like and admire the vice president. I believe he and the president have formed a true friendship; I also believe President Obama’s relationship with Hillary Clinton is, well, not nearly as warm.

But warm-and-fuzzy relationships with an incumbent president aren’t enough.

Clinton is going to remain a formidable opponent for anyone — be they Democrat or Republican. As someone noted last night on MSNBC, which political demographic group does Biden take away from Clinton?

The vice president has run twice already for the White House. His 1988 campaign cratered over reports that he was lifting statements from a British pol and using them in his own stump speeches. His 2008 campaign ran into a buzzsaw operated by a young U.S. senator from Illinois, Barack H. Obama.

That ol’ trick knee is telling me he doesn’t want to lose a third time.

As much as many of us out here would like to see him run, my hunch is that the vice president is going to call it career.