SIMI VALLEY, California--Here's what I took away from tonight's face-off between the four remaining Republican candidates at the Ronald Reagan Library:

1. The Dynamic: Before South Carolina and Florida, the GOP contest was a four-man muddle. But tonight marked the start of a mano-a-mano head-to-head sprint to finish between McCain and Romney. It's a whole new race. Free to focus their fire on each other, the frontrunner (McCain) and the challenger (Romney) defined the dynamic that will determine the Republican nomination: character vs. conservatism.

Romney wants the contest to center on conservative cred, which is why he unloaded on McCain for sponsoring "a number of pieces of legislation where his views are out of the mainstream, at least in my view, of conservative Republican thought": McCain-Lieberman (cap-and-trade), McCain-Kennedy (immigration) and McCain-Feingold (campaign finance reform). Romney's goal? To convince anti-McCain conservatives to coalesce around his candidacy, providing a powerful brake to Mac's post-Florida momentum.

McCain, on the other hand, wants Super Tuesday voters to compare his character to Romney's. Which is why he took Romney to task for hedging on the surge while the Arizona senator "unequivocally put [his] career and [his] political fortunes on the line... to support" Bush's policy. The implication: that Mitt doesn't have the cojones to lead in a time of terror.

So who's dominating the tug-of-war? I'm going to say McCain. While Romney raised valid questions about his rival's conservative "apostasies"--questions, I should note, that the senator largely dodged (the one on his opposition to the Bush tax cuts was particularly blatant)--it's not like many voters expect the "maverick" McCain to tow toe the party line. Romney is telling people a story they already know. But when McCain questions Romney's character, he's not only drawing a contrast--he's reminding voters that Romney, a moderate back in Massachusetts, has tacked to the right on key conservative issues like abortion, gay rights, immigration and abortion. (Did I mention abortion?) According to McCain, Romney is casting stones from a glass house--and his lack of consistency (read: character) also undermines his conservative cred.

This was Romney's last best chance to reorient the race before Super Tuesday--but he spent most of the night defending or clarifying his positions (especially on Iraq, McCain's strong suit). Not helpful.

2. The Personalities: I thought Anderson Cooper's first question--"In terms of the economy, are Americans better off than they were eight years ago?"--provided a fascinating window on the candidate's contrasting personalities.

Romney basically refused to answer, immediately changing the subject to his record as governor of Massachusetts; it was as if his CPU detected political danger and automatically emitted a distracting flurry of talking points (The rather ineffectual Cooper even called him on the dodge, asking, "Are you running for governor or are you running for president?")

McCain hedged, saying "you could argue that Americans overall are better off" but admitting that "things are tough"; he was the cautious frontrunner struggling not to alienate any potential supporters with divisive "straight talk."

Huckabee scored points on candor--he started his answer with a frank "I don't think we are"--but  distinguished himself by acknowledging the existence of, you know, actual human beings: the folks "driving trucks" and struggling to pay for fuel and the "families... that don't have a paycheck." That sort of empathy--even when it borders on sappy--is at the heart of his appeal.

And Ron Paul was the only one who sounded like a person instead of a politician--no small feat for a presidential candidate. "No, no, we're not better off," he said. "We're worse off, but it's partially this administration's fault and it's the Congress."

3. The Petulance: What was up with Huckabee using all of his air time... to whine about not getting enough air time? "I'd appreciate maybe a question that we could talk about that would involve some of us down here at the end," he said when offered the floor, then repeated the complaint a few minutes later: "a while ago, you said you were going to shower me with questions, and I think then you turned the spigot off." It's a valid concern, but Huckabee had to know that the moderators would focus on McCain and Romney. C'est la vie CNN. He was so insistent, in fact, that it seemed inauthentic--like a strategy for earning sympathy instead of actual chagrin. Huckabee's usually even-keeled; this made him look small. And McCain came close to crossing the line a couple of times, too--especially when he "praised" Romney as a "fine man" by saying, "I think he managed companies, and he bought, and he sold, and sometimes people lost their jobs." Ick. Too many moves like that and McCain looks like a typical pol--which completely undermines his appeal.

4. Ron Paul: My favorite moment of the night? When Paul tsk-tsked Romney and McCain for sniping over Iraq strategy--an area where their positions are largely indistinguishable. "I find it rather silly, because they're arguing technicalities of a policy they both agree with," he said. "They agreed with going in; they agreed for staying, agreed for staying how many years?  And these are technicalities.  We should be debating foreign policy." The other candidates--not to mention reporters--often dismiss the Good Doctor, but as the field narrows, I have to say: it's fun to watch him give them a hard time. He was certainly talking sense tonight.