One of the most entertaining (and horrifying) things about this long Republican primary is watching the candidates attack each other using tactics that they'd previously mostly reserved for Democrats. It's not only that Newt Gingrich is going after Mitt Romney using the arguments from the left (he's anti-immigrant! he's a One Percenter!), but both candidate's drawn-out, bare-knuckle approach that usually comes into play across parties deeper into the general election.

Here are some accusations being traded within the Republican party that are making the primary so aggressive:

1. Moral bankruptcy

In previous campaigns, Newt Gingrich was a pioneer in taking out his opponents by portraying them as twisted and corrupt. He infamously sent out a memo in 1990 on how to use the right words --"sick," "anti-flag," etc -- could be used to portray Democrats as outside of the mainstream. In 1996, Gingrich said on Meet the Press, "I had a senior law enforcement official tell me that in his judgment up to a quarter of the White House staff, when they first came in, had used drugs in the last four or five years." In 1994, Gingrich said of his election goal, "It was to portray Clinton Democrats as the enemy of normal Americans."

This tactic is now being used by Romney against Gingrich. The Washington Post's Greg Sargent posts this flyer, at left, mailed to voters in Florida by the Romney campaign that points to Gingrich's "well of sleaze." Utah Rep. Jason Chaffetz, a Romney backer, told Politico's Alex Isenstadt and Jake Sherman Friday that the idea of Gingrich winning the Republican nomination "scares me to death... Newt Gingrich is an unreliable leader. He’s prone to becoming unhinged. He’s been mired in scandal in his personal and professional life. And he is a consummate D.C. insider." New Jersey Gov. Chris Christie called Gingrich an embarrassment. During Thursday night's debate, Romney called Gingrich's claim that he was anti-immigrant "repulsive."

Gingrich has a tougher time making this case against Romney, who has been married only once. But that doesn't stop him from trying. In Thursday's debate, Gingrich portrayed Romney as greedy and depraved, saying Romney had invested in firms that profited from foreclosing on Floridians. "So maybe Governor Romney in the spirit of openness should tell us how much money he's made off of how many households that have been foreclosed by his investments?" he asked.

2. Corruption

Gingrich took down Democratic incumbents by accusing them of violating ethics rules. He brought ethics charges against Speaker Jim Wright in 1988 (Wright resigned). He was involved in the House postage scandal that brought down Dan Rostenkowski. He pushed for an investigation into the House banking scandal, in which members of Congress -- including Gingrich himself -- bounced checks from their House bank accounts. And, of course, he led the impeachment of President Clinton. Now Romney is portraying Gingrich as a corrupt creature of Washington. On Gingrich's consulting work for Freddie Mac, Romney said at the debate, "You can call it whatever you like -- I call it influence peddling. It is not right. It is not right." The reverse side of the mailer Sargent posted, at right, calls him unethical.

Gingrich has tried to present his eat-the-rich attacks on Romney's business career as a question of ethics. Romney was "looting companies," Gingrich said. "It’s not fine if the person who is rich manipulates the system, gets away with all the cash and leaves behind the human beings," he said earlier this month. This time, "the system" is finance, instead of Washington. "Romney owes all of us a press conference where he explains what happened to the companies that went bankrupt and why Bain made so much money out of companies that were going bankrupt."

3. Elitism

Since the Nixon era, Republicans have argued that uppity liberals want to impose their rules on hardworking, upstanding "traditional" families. It worked against John Kerry in 2004. Barack Obama played into this one in 2008, when he said the working class bitterly clung to guns and God, which offered quite the opening for Sarah Palin. Gingrich said of Romney this week, "I think you have to live in a world of Swiss bank accounts and Cayman Island accounts and automatic -- you know, $20 million a year of no work -- to have a fantasy this far from reality." Worse, Gingrich said in a campaign speech, Romney thinks you're a moron. Referring to Romney's questioning of his Reaganite credentials, Gingrich said, "This is the kind of gall they have to think we're so stupid and we're so timid... The message we should give Romney is, 'We aren't that stupid and you aren't that clever.'"

So far, Romney hasn't tried this one yet. In fact, he makes it pretty clear that he does think he's better -- than Gingrich, at least. Romney called Gingrich zany, and at the debate, he scoffed, "I spent 25 years in business. If I had a business executive come to me and say they wanted to spend a few hundred billion dollars to put a colony on the moon, I'd say, 'You're fired.'"

What's been the result? Well, just as they work against Democrats, these tactics work against Republicans, too. On Friday, a poll from NBC News/The Wall Street Journal showed that Romney has a net unfavorable rating -- a rarity at this stage in the election. "All of the GOP candidates are a net-negative in favorability ratings, with Santorum getting the best marks -- 26 percent positive, 27 percent negative," NBC's Domenico Montanaro reports. "Romney scores 31-36, and it’s worth noting that Bob Dole, John McCain, and George W. Bush were all net-positives at the same time in their fights for the nomination. The exception of a recent major party nominee being a net-negative at this point -- John Kerry, who was 22-26 in January 2004."