JIM LEHRER: And that brings us to a discussion of negative campaigning. The tone on the Republican side has also very turned harsh. Terence Smith begins our coverage.
SEN. JOHN McCAIN: You're putting out stuff that is unbelievable, George, and it's got to stop.
TERENCE SMITH: As the Republican race has tightened in the three weeks since the New Hampshire primary, the leading candidates have become distinctly less chummy in their references to each other.
SEN. JOHN McCAIN: I will not take the low road to the highest office in this land. I want the presidency in the best way, not the worst way. A clear choice will be offered, a choice between my optimistic and welcoming conservatism and the negative message of fear.
TERENCE SMITH: That was Senator John McCain's concession speech in South Carolina, minus any identifiable concession. Governor George W. Bush had this to say about Senator McCain.
GOV. GEORGE W. BUSH: This is a man who evidently says one thing and does another. He says he's going to take the high road - said he's going to run a positive campaign, and distributes material that he first denied was his, and then puts out stuff that's factually untrue.
TERENCE SMITH: The candidate's surrogates have gone even further down the low road. Anti-McCain messages have been going out to Michigan voters via automated phone calls from evangelist Pat Robertson, a Bush supporter.
PAT ROBERTSON: Tomorrow's Republican primary may determine whether our dream becomes reality or whether the Republican Party will nominate a man who wants to take First Amendment freedoms from citizen groups while he gives unrestricted power to labor unions.
TERENCE SMITH: Bush campaign officials declined responsibility for the calls, and said they had no advance knowledge of them. Meanwhile, McCain supporters put out their own automated phone calls in Michigan attacking Bush for attacking their man.
WOMAN: He's attacking John McCain because John wants to rid Washington of the special interests and return government back to us. Let's show them Michigan is different. Vote for Senator John McCain.
TERENCE SMITH: In interviews the candidates have asserted ownership of the high road.
GOV. GEORGE W. BUSH: The people who actually showed up at the polls said they appreciated my campaign. It was a positive campaign and I stand by the campaign I ran.
SEN. JOHN McCAIN: I am proud of the campaign we ran and we have run so far. There is nothing about this campaign that I'm not proud of.
TERENCE SMITH: And the negative ad war that began in earnest in South Carolina still reverberates in today's important contests. These ads, analysts and exit polls say, helped turn South Carolina sour on John McCain and sweet on George W. Bush.
SEN. JOHN McCAIN: His ad twists the truth like Clinton. We're all pretty tired of that.
GOV. GEORGE W. BUSH: Politics is tough, but when John McCain compared me to Bill Clinton and said I was untrustworthy, that's over the line.
SEN. JOHN McCAIN: For the moment, both candidates have curtailed their more strident ads, but they continue to run their warm and fuzzy introductory ads, putting forward what they hope is their best video foot in these delegate-rich primaries.