Factcheck.org has done some homework on the numbers used by Rudy and Mitt in the latest debate.

In fact, the ensuing back-and-forth left heads spinning even among fiscal experts. "I have watched campaigns for decades and, even by the standards of statistics being misused, [Tuesday] night was excessive," said Michael Widmer, executive director of the nonpartisan Massachusetts Tax Foundation, when called by our bewildered fellow fact-checkers at washingtonpost.com. Giuliani was particularly guilty of this when he accused Romney of raising taxes significantly in Massachusetts:

Giuliani: [T]he point is that you've got to control taxes. But I did it; he didn't. I controlled taxes. I brought taxes down by 17 percent. Under him, taxes went up 11 percent per capita. I led; he lagged.

Hearing this, one could be forgiven for thinking that Romney had raised tax rates substantially during his four-year term as Massachusetts governor. Not true – he tried to lower state income-tax rates three times but was stopped by the Democratic-controlled Legislature...

Giuliani's selective use of statistics threw fuel on the fire as far as Romney was concerned, but he then also made a misleading claim:

Romney: It's baloney. Mayor, you've got to check your facts. No taxes – I did not increase taxes in Massachusetts. I lowered taxes.

Romney is correct, as we noted above, that he did not raise anything he called a tax...

As for the other claims in their furious exchange, we'll take them in turn:

Giuliani: I cut taxes 23 times when I was mayor of New York City. I believe in tax cuts. I believe in being a supply sider. I cut the income tax I think it was 24 percent. We got 42 percent more revenues.… I cut taxes by over $9 billion.

We've seen – and panned – this film before. As we've written, eight of the 23 cuts he takes credit for were initiated at the state level, not by him at all, and a ninth, pushed by the city council, was resisted by Giuliani for some time before he acquiesced. It was the largest income tax cut in history for city residents. So Giuliani can claim personal credit for cutting either 14 or 15 taxes for a total of either $5.4 billion or $8 billion, depending on whether you give him credit for the council-sponsored cut described above – not 23 cuts totaling $9 billion.

During Romney's next at-bat, he said this:

Romney: But if you want to cut taxes, you're going to have to cut spending. … Mayor Giuliani took the line item veto that the president had all the way to the Supreme Court and took it away from the president of the United States. ... I'm in favor of the line-item veto. I exercised it 844 times.

Giuliani did challenge President Bill Clinton on the line-item veto after he used it to cut a provision that could have helped NYC's bottom line. It was declared unconstitutional by the Supreme Court in 1998. Romney is also correct to say that he exercised his state-level line-item veto power 844 times...

The former governor had much more to say:

Romney: He also fought to keep the commuter tax, which was a very substantial tax, a almost $400 million tax on commuters coming into New York. And when it's all said and done, if you're a New York taxpayer, city taxpayer, your state and city tax combined can reach as high as 10 percent. And in our state, if you're a Boston worker, it's going to be more like 5.3 percent.

Giuliani indeed fought like a wolverine when state officials moved to eliminate the city's non-resident income tax. He lost that battle. During 1999, the last year it was in effect, it brought in close to $400 million, according to an estimate by the city's nonpartisan Independent Budget Office, as Romney says.

Romney's also correct about New York City taxes – in fact, at the end of Giuliani's term, combined city and state taxes went as high as 10.5 percent for some residents...Romney's figure for Boston is right as well.