Here’s a crucial lesson to be learned by all those new to PR and politics: If you see an opening, exploit it–and if your message resonates, don’t be afraid to stick with it. President Obama has clearly taken this advice to heart; he hit “Late Night with David Letterman” last night and, of course, made a point of mentioning Mitt Romney’s recently released “47%” video.

Letterman gave Obama plenty of room to accuse the former Massachusetts governor of “writing off a big chunk of the country” while noting that the president has to “work for everyone, not just for some.” That’s what we call staying on message.

Of course, there’s always the danger that the many millions of Americans who don’t salivate over political news will get sick of hearing about this latest campaign snafu and tune out any related comments.

Question for PR pros: How does one locate the line between repetition and overkill? For example, does President Obama need to worry about exploiting this new opening a little too aggressively?