SOMEONE BUY THUNE A CALCULATOR…. Politico reported this week that Sen. John Thune (R-S.D.) is trying to “build up his policy credentials,” in advance of a possible presidential campaign in 2012. With that in mind, the conservative, unaccomplished senator intends to “make a name for himself on budgetary matters.”

He’s off to a rough start.

Sen. John Thune (R-SD) — the fifth highest ranking Republican in the Senate — has a new plan for lowering deficits, and as you might expect from GOP leadership, it involves zero tax hikes. It does however, involve math and, if his appearance on Fox News last night is any indication, Thune finds math rather difficult. There’s really no other way to explain his utter failure to remember the law of diminishing returns when he talked about the benefits of his deficit reduction plan.

Appearing on Fox News, Thune and host Greta Van Susteren discussed the bill’s call for the creation of a Joint Committee on Deficit Reduction, tasked with reducing the deficit 10 percent year over year.

“It would be required to find 10% in savings — 10% of the deficit in savings every budget cycle,” Thune said.

“So in 10 years we wouldn’t have a deficit?” van Sustern asked.

“Theoretically, yes,” Thune replied.

Mathematically, no.

Let’s say the government starts with a $1 trillion budget deficit. If Thune’s committee reduces it by 10%, it would be a $900 billion deficit a year later. The next year, it cuts another 10%. Would that bring it down to $800 billion? No, it’d be $810 billion ($900 billion – 10% = $810 billion). A year later it would be $729 billion, followed by $656 billion, and so on.

Thune thinks this approach would eliminate the deficit in 10 years, but he forgot to do the math, so he’s off by an entire decade*. It’s understandable for van Sustern to mess this up — she’s a Fox News personality — but this is the senator’s own plan, intended to give him credibility in advance of a national campaign.

Someone couldn’t let him borrow a calculator?

Arithmetic aside, if Thune’s idea is part of a larger effort to “build up his policy credentials,” the senator might need a back-up plan. His scheme seems like a pretty thin gimmick — task some committee with coming up with ideas to reduce the deficit without raising taxes. At that point, Congress would be free to ignore the committee’s ideas.

The reason it’s hard to take Thune’s national ambitions seriously is that he doesn’t seem to know anything about anything. Think back over the last couple of years — when was the last time you remember Thune saying or doing something noteworthy? There was that time a year ago when he urged President Obama not to pick a gay Supreme Court nominee, which came soon after his argument that economic recovery efforts are bad because $1 trillion, if stacked by $100 bills, would make a very tall pile.

When Sen. Al Franken (D-Minn.) pushed a measure to protect victims of sexual assault who work for defense contractors — the Jamie Leigh Jones effort — Thune dismissed it as a “Daily Kos-inspired amendment.” Soon after, Thune said a colleague often disagrees with Republicans because he understands policy details.

This guy has presidential ambitions? Please.

* Update: Brian Beutler emails to note that he had a minor math error of his own, and that Thune was off by far more than a decade. Using Thune’s model, it would take 43 years to get deficits down to 1% of current levels, making his observation that much more incorrect. (And as some of you have also noted, if one reduces a debt by 10% a year indefinitely, it’s impossible to ever eliminate the total altogether.)

Steve Benen

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.