A day after taking heat at a GOP debate on the issue of illegal immigration, Newt Gingrich's campaign fought back, accusing Mitt Romney of flip-flopping on the issue and pointing reporters to a 2007 video of Romney supporting that argument.

The debate dustup centered on Gingrich's assertion that while illegals who have recently come to the United States should be deported, others who have been here for many years should be allowed to stay legally, but not obtain citizenship.

Both Romney and Michele Bachmann quickly labeled that amnesty.

Gingrich acknowledged last night he would be attacked for his position, but argued it is the right thing to do.

"I'm prepared to take the heat for saying, let's be humane," the former U.S. House Speaker said.

Now Gingrich is going on the offensive, using Twitter to tweak Romney.

"Here's a trip down memory lane," Gingrich tweeted with a link to a 2007 "Meet the Press" video.  So what's your position on citizenship for illegals again? 

You can judge for yourself whether Romney used to have a different position in 2007 than in 2011.  Back then, legal status - even citizenship was okay, but now it would be amnesty.

"My own view is, consistent with what you saw in the Lowell Sun, that those people who have come here illegally, and are in this country, the 12 million or so that are here illegally, should be able to sign up for permanent residency or citizenship," Romney said.

Here's a thought - Romney has always seemed vulnerable on the "flip-flop" issue, but it hasn't really stuck.

Will part of Gingrich's ascension to the top of the GOP field mean sharper attacks on Romney highlighting those policy changes over the years?

There is a 200 page playbook out there of opposition research on Romney that was done by the campaign of Sen. John McCain (R-AZ) four years ago.

Something to consider.