There are, I suppose, politicians who have had more dreadful years than Bill Frist, the Tennessee Republican who serves as Senate majority leader. Bill Clinton had his season of Lewinsky. Richard Nixon had Watergate. But rarely has a major politician endured such a spell of relentless day-to-day ugly.

Frist, once esteemed as a citizen-politician, a heart- and lung-transplant surgeon who spent part of each year donating his services in the most desperate parts of Africa, has transformed himself into the ultimate political opportunist. His moves—like last week's ludicrous attempt to hijack the Senate's immigration debate and move it in a punitive, populist direction—have been so clunky that he has lost the respect of his colleagues, especially Republicans. (The Democrats are thrilled by his ineptitude.) "I hear he was a pretty good surgeon," a Republican Senator said last week when I asked how Frist had been as majority leader.

Why would a man universally described as kind and intelligent suddenly start acting like a dodo? Presidential ambition, of course. Frist's descent began a year ago, when he destroyed his reputation for medical probity by announcing, on the Senate floor, that he had seen the videotapes of Terri Schiavo, "and from my standpoint as a physician, I would be very careful before I would come to the floor and say this ... Based on the footage provided me ... she does respond." This was utter nonsense, as subsequent autopsies of Schiavo's brain proved. "He didn't have to go that far," another Republican Senator told me. "He simply could have opposed pulling the plug on Schiavo."

A series of terrible leadership moves have ensued. There was Frist's effort to deploy the "nuclear option" — that is, to perform radical surgery on the Senate's filibuster rules in order to allow votes on President Bush's more extreme judicial appointments. But the nuclear option was thwarted when 14 Senate moderates cut a deal to keep the rules and allow votes on some of the appointees. "We saved him on that," said a G.O.P. staff member involved in the negotiations. "Frist never had the votes he needed for the nuclear option." More recently, Frist has embarrassed himself on the Dubai Ports deal. He was one of the first Republicans to oppose the deal--his opposition made it safe for the rest of the party to buck President Bush--but he immediately retreated after a White House briefing. "As I've gotten more information, I have a greater comfort level," he said. Translation: I shot my mouth off before I knew anything because I wanted to thrill the G.O.P. base.

And now there is immigration. "He forced us to rush a bill," said a Republican member of the Judiciary Committee. "Then he didn't like what we produced"—Frist spent a few days opposing any bill that would include a process to make illegal immigrants legal—"and so he filed his own bill, which is dead on delivery. He's not even part of the real negotiations at this point. It's pretty sad."

Frist will leave the Senate at the end of the year and start his presidential campaign. "He'll disappear," said a Republican consultant. "He's not built for heavy weather. He's just not an instinctive politician. And when you're a light candidate, every maneuver seems naked and tactical. With Frist, it's been college Republican sort of stuff."

In fairness, Frist can deploy some tepid excuses for his behavior: he's new at this game. President Bush drafted him to be majority leader even though Frist had been in the Senate for little more than one term.

Others don't have such excuses. The early stages of the 2008 presidential campaign have been marked by a low-road stampede—even among heavyweight candidates who should know better, like Senators John McCain and Hillary Clinton.

One of the pillars of McCain's reputation for fierce independence has been his insistence on fiscal responsibility, his opposition to the Bush tax cuts. But he recently voted to extend some of those same Bush cuts in capital gains and dividend-income rates. "I've never voted for a tax increase, and that would have been one," he told me last week. Oh, please. It was a vote, in essence, to restore tax rates McCain had previously favored, and he blinked. Clinton diminished herself on the Dubai Ports deal when she said, "Our port security is too important to place in the hands of foreign governments." As Clinton well knows, the responsibility for port security rests with the U.S. government. The deal was about managing the ports, not securing them. Her husband was never so blatantly misleading ... on a matter of public policy.

Both McCain and Clinton have taken courageous positions on important issues, especially the war in Iraq, which makes their cheesy calculations all the more glaring. The public has come to know, and hate, the poll-driven games that politicians play. The litmus test in 2008, I suspect, will be, which candidate is willing to tell me something—anything—that resembles the truth? The John McCain of 2000 was. The question is, Who's going to be the John McCain of 2008?

To see a collection of Klein's recent columns, visit time.com/klein

Commentary: