Warner, a pro-business Democrat who was hawkish on defense issues, and Bayh, a popular two-term governor from a heavily Republican state who chaired the centrist Democratic Leadership Council, won little support from a party that is now in the grip of the anti-war left.

"Without them in the race, it leaves us without diversity on the campaign trial. That also leaves us in a circumstance where the remaining candidates will be more liberal than the mainstream voters and will not be challenged as aggressively if Bayh or Warner were in the race," Penny told me.

Democratic campaign strategist Alan Secrest also acknowledged their withdrawal "probably does leave the field somewhat more liberal." But "I don't think it's fair to say they left the race having concluded that a centrist cannot win," he added.

"It wasn't ideology that drove Warner and Bayh out of the race. It was the personalities in the race they were up against -- personalities like Clinton and Obama that often overcome ideology," he said.

However, another Democratic campaign adviser cautioned that, as left wing as the field is now, "the conservative nature of the electorate has not changed. It's still relatively conservative. People like Obama and Clinton know where the boundaries are in the electorate. I don't think it means racing off the cliff to the left. It means appealing to the center once they win the nomination."

Even so, he added, "In the short term, the pack is more liberal than it was." That is demonstrably clear right now. Al Gore is more fiercely liberal on economic and national-security issues than he was in 2000. All the others are also playing deep into left field. Kerry's score is 86 percent. Connecticut Sen. Chris Dodd and Delaware Democrat Joe Biden score 79 percent and 77 percent, respectively. Ohio Rep. Dennis Kucinich is nearly off the charts at 92.5 percent.

Compared to these and other contenders, Bayh's more centrist score was a milder 61.9 percent.

So the lesson in 2008 for discerning centrist Democratic voters is this: Beware of all that moderate-sounding rhetoric from the candidates. Check out their voting records first.