WASHINGTON -- The most recent entry in the National Republican Senatorial Committee's $2.5 million campaign against Democrat Michelle Nunn is on the topic of immigration.

Its math: Nunn + Obama = amnesty.

The problem: That "amnesty" was the comprehensive "Gang of Eight" immigration reform bill, co-written by possible 2016 presidential contender Sen. Marco Rubio, R-Fla. It cleared the Senate last year with 14 Republicans in favor, including 2008 presidential nominee Sen. John McCain, R-Ariz.

In other words: Nunn + Rubio +McCain = amnesty, too.

Rubio in particular worked hard in those heady days of spring 2013 trying to convince conservatives that a 13-year path to citizenship requiring fines and back taxes is not "amnesty." He got enough Senate Republicans on board, but House Republicans have not touched it.

There was debate at the time about how much the NRSC, which has the job of electing and defending Republican senators, should press the bill as a weakness for vulnerable Democrats. Spokesman Brad Dayspring tweeted in June 2013: "NRSC doesn't plan a major focus on immigration or to engage in a national effort around the legislation."

It is worth noting that the ad above came from the NRSC's independent expenditure unit. It draws from NRSC funds, but due to campaign finance laws, the spending cannot be coordinated with campaigns -- so much of the committee, including the communications shop, had no hand in developing the message.

Immigration does provide a contrast in the Georgia race. Nunn has voiced support for the Gang of Eight bill. Sens. Johnny Isakson and Saxby Chambliss voted against the bill, even though the lame-duck Chambliss was courted as a possible swing vote.

And David Perdue has been outspoken against it -- including slamming Rep. Jack Kingston during the runoff for being endorsed by the "pro-amnesty" U.S. Chamber of Commerce. The Chamber lobbied for the bill, even as Kingston said he was against it.