If you thought you were living through a particularly nasty presidential primary season, turns out you were right.

Four years ago, just 6 percent of campaign advertising in the GOP primaries amounted to attacks on other Republicans; in this election, that figure has shot up to more than 50 percent, according to an analysis of advertising trends.

And the negative ads are not just more frequent — they also appear to be more vitriolic.

In 2008, one of harshest ads Mitt Romney ran ahead of the Iowa caucuses criticized the immigration position of Sen. John McCain (Ariz.), but only after calling him “an honorable man.”

In 2012, such a nicety seems quaint.

Romney’s campaign began running an ad Friday in Michigan showing a limp body sinking in murky water while a narrator intones: “America is drowning in national debt, yet Rick Santorum supported billions in earmarks.”

All of this invective is flowing in an election season when Republicans had hoped to train their resources on beating President Obama. Candidates typically save their sharpest attacks for the general-election campaign, largely sparing their fellow party members.

But a wildly unpredictable GOP nomination battle has upended that plan and dissolved the truce. It is happening largely because of new rules governing campaign money. Also, this race has a different dynamic: a front-runner who lacks a prohibitive lead.

Once the tone of the race turned negative, it stayed that way. One Ron Paul campaign ad calls Newt Gingrich a “serial hypocrite.” Another spot, from a group backing Romney, asks, “Haven’t we had enough mistakes” from Gingrich?

A group supporting Gingrich accuses Romney of being a “corporate raider” and shows footage of an elderly woman saying, “I feel that is the man who destroyed us.” Another spot from the group accuses Romney of making “blood money” from a company that was found guilty of bilking the government for Medicare payments.

Romney and the groups backing him have led the trend, spending two-thirds of their money on negative ads. Gingrich and the Winning Our Future PAC backing him have spent half of their funds on spots attacking other Republicans. Santorum and the PACs behind him have devoted one out of four dollars to attack ads.

Winning Our Future spokesman Rick Tyler said his group’s message was positive until it was forced to counter Romney’s “scorched earth” strategy.

“When this whole campaign started, the Republicans were very enthusiastic,” Tyler said. Romney’s approach, he said, is also depressing turnout: “By the time he’s done, there will be no one left to vote against Barack Obama.”

The Romney and Gingrich campaigns did not respond to requests for comment, and Restore Our Future, the largest super PAC supporting Romney, declined to comment.

Party strategists point to million-dollar political contributions to super PACs as part of the reason for the negativity.

Data show that super PACs, which have run more advertising than the campaigns themselves, have spent 72 percent of their money on negative ads. The figure for campaigns is 27 percent, according to a Washington Post analysis of data from Kantar Media/Campaign Media Analysis Group, which tracks television advertising across the country. (For this article, ads were considered negative if they mentioned another GOP candidate.)

Super PACs can accept corporate money and personal checks above the $2,500 limit on donations to campaigns, but they are prohibited from coordinating ads with the campaigns they are trying to help. That makes it harder for the PAC spots to feature candidates. And because stock footage and voice-overs can be dull, the result has been more negative ads.

“Super PACs are left with no good choices,” said Brad Todd, a longtime GOP adman who worked for Romney and the party in 2008 but is unaffiliated in this contest. “If they didn’t run comparison or contrast ads, they would have some very boring television.”

Super PACs don’t have to follow the “stand by your ad” provision in campaign law, which requires candidates to state clearly that they approve of an ad’s message. Candidates, in fact, have tried to deflect blame for the contest’s tone, saying they don’t have control over the spots run by the PACs.

But many strategists have noted that the groups, typically run by former aides to the candidates, can easily glean cues from the campaigns.

“There are so many forms of communication other than picking up the phone and saying, ‘We like that negative ad — double the buy,’ ” said one Republican media consultant who worked for a candidate no longer in the race. “It’s not that hard to have a symbiotic relationship.”

Studies of advertising before the 2008 race are not directly comparable but generally show that the Democratic primaries in 2004 and 2008 were also less negative than this year’s contest.

Even before the Iowa caucuses on Jan. 3, the race turned negative. The first ad from Restore Our Future, the super PAC backing Romney, said that “Newt Gingrich has more baggage than the airlines.”

That, several GOP strategists said, signaled to the other candidates that they would need to go negative.

“I think that staying positive through Iowa, through $3.5 million of negative attacks, proved you either have to unilaterally disarm and leave the race or you have to at least bring up your competitor’s record,” Gingrich said at a debate last month.

Romney’s campaign and the super PACs behind him have also dominated the airspace, running more than half of all the advertising in the race.

“Mitt Romney has resorted to a carpet-bombing strategy that helped him win some early primaries,” said Mark McKinnon, a political strategist for McCain and George W. Bush, “but his favorable impression among independents has collapsed. Seems likely there is some correlation.”

Obama’s advisers are looking on with glee as the Republicans bash one another. Romney in particular has been thrown off his strategy of attacking the president as he moves to take down other Republicans.

“He spent last year saying he was just going to make a general-election argument, and I don’t think anyone would say that’s what he was doing for the past three months,” said one Obama adviser. “He’s been trying to get to the right of Gingrich and Santorum.”