Saul Loeb/AFP/Getty Images

In the wake of criticisms about Vice President Biden's "chains" comment - and calls from Republicans such as Sen. John McCain, R-Ariz., and Sarah Palin for President Obama to replace him on the ticket - the president last night personally sent a tweet of support for his Number Two.

"Today marks four years since I chose @JoeBiden to be my running mate," read the tweet. "Here's to an outstanding Vice President and a great friend. -bo."

The "-bo" signifies that the tweet was sent personally by President Obama.

Last week, former Virginia Governor Doug Wilder, a Democrat, assailed Biden's comments to a Virginia audience with many African-Americans about how Republicans "want y'all back in chains." Wilder called the remarks "divisive" and "uncalled for."

On Sunday, Obama Deputy Campaign Manager Stephanie Cutter said Biden was "absolutely not" a liability.

-Jake Tapper

*This post has been updated,.

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