The race for the Republican presidential nomination has come a long way since early September, when Rick Perry was the clear frontrunner. In the two months since, the Texas governor has seen his support collapse, both nationally and in key early states, and by some measures, Perry is now running fourth.

And yet, he’s still the candidate the current frontrunner is most worried about.

Mitt Romney may be skipping some major G.O.P. events in Iowa this week, but he’s still got his eye on someone who isn’t: his rival Rick Perry.

According to The Associated Press, Mr. Romney’s campaign on Thursday paid for automated phone messages to Iowans that assert that the Texas governor has fostered illegal immigration.

The attack message may signal that Mr. Romney still views Mr. Perry, despite his recent slide in the polls, as his main threat in Iowa, which will hold its first-in-the-nation caucuses on Jan. 3.

In these new robocalls, Iowans are told that Perry “opposes a border fence” and “signed the bill to make Texas the first state in the nation to grant in-state tuition discounts to illegal immigrants.” It’s not clear how much money the Romney campaign is investing in these robocalls, but the former governor’s aides have confirmed that the calls are theirs.

Arguably more interesting than the calls themselves is the strategy behind them — it seems that Perry still has the capacity to worry Romney, and that it’s the Texas governor, and not Herman Cain, that has Team Romney concerned.

Remember, these new robocalls are just the latest in a series of moves. Two weeks ago, the Romney campaign released a video slamming Perry’s debate performances and suggesting the governor is a bit of a dolt. Soon after, Romney’s team used some pretty harsh language to describe Perry, calling him “a petulant little boy.”

And now Romney is using anti-Perry robocalls in a state he’s ostensibly not worried about.

There’s a real risk to this strategy. Sure, Romney’s attacks might weaken Perry’s support even more, but it’s just as likely the moves will backfire. By attacking Perry directly and aggressively — instead of simply ignoring a candidate who appears to be imploding all on his own — the Romney camp isn’t even trying to hide the fact that it considers the Texas governor a serious threat.

Most campaign observers believe there’s one credible candidate standing between Mitt Romney and the Republican nomination, and that’s Rick Perry. In recent weeks, Romney and his team have dropped the pretense, chosen to ignore Cain, and made clear they accept the conventional wisdom as fact.

A few weeks ago, Romney’s team was delighted to highlight the Obama campaign’s criticisms of the former governor, and even put together a video, streaming together Democratic criticism of the Republican frontrunner. It came with a one-word title: “Nervous?

Given the recent offensive against Perry, one might ask Romney the same question.

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Follow Steve on Twitter @stevebenen. Steve Benen is a producer at MSNBC's The Rachel Maddow Show. He was the principal contributor to the Washington Monthly's Political Animal blog from August 2008 until January 2012.