Tonight, the Bush campaign will take the unprecedented step of having a sitting senator of the opposition party, Georgia Democrat Zell Miller, give the keynote address at the Republican National Convention.

Twelve years ago, Mr. Miller stood at a podium in the same arena and gave the Democratic keynote address for Bill Clinton, a speech ghostwritten by current “Crossfire” host Paul Begala. The subtext is now even thicker; Mr. Miller’s speech is intended to send the signal to swing voters that the Democratic Party has drifted to the left, counter to the Kerry’s campaign’s strenuous efforts to appear centrist.

In a counter-vailing action, the Kerry campaign last week released a compelling new advertisement featuring retired General Merrill McPeak, who served as Air Force chief of staff during the first Gulf War under President Bush’s father. In the ad, General Mc-Peak addresses the camera directly and states, “I voted for George W. Bush in 2000.This year, I’m voting for John Kerry,” adding, “Nothing is more important to me than protecting America.” The ad reinforces Mr. Kerry’s attempts to reclaim credibility on military issues from the Republicans, answering the damaging Swift-boat ads, and echoing the high-profile defection of Chairman of the Joint Chiefs of Staff Admiral William J. Crowe to Mr. Clinton’s campaign in 1992.

The script is being replayed because it works. Call them traitors or courageous individuals standing up for their beliefs in the face of former colleagues, but swing voters respond best to one of their own. It’s a lesson that Ronald Reagan perfected and rode to two terms in the White House.

It’s often forgotten, but conservative icon Reagan was a registered Democrat until he was more than 40 years old. He built his political career on appealing to the swing voters who became known as “Reagan Democrats,” using his own political evolution to send the message that it was okay to cross party lines. ” I didn’t leave the Democratic Party,” Reagan would say, “the Democratic Party left me.”

Reagan deployed unexpected surrogates to reinforce his efforts, such as Ralph Abernathy and liberal Democrat Eugene McCarthy in the election of 1980. These cross-party endorsements caused disaffected Democrats to reassess their reflexive votes. In 1984, the Gipper returned to his playbook by tapping United Nations ambassador and self-described “lifelong Democrat” Jeane Kirkpatrick to speak at the Republican National Convention.

And while many Southern Democrats re-registered as Republican because of the Reagan Revolution, today only 14% of the electorate say they always support the candidates of a single party. The ranks of independent voters are swelling at the expense of rigid party loyalty. The influence of cross-party endorsements is even more powerful in the effort to win over the battleground states that will determine the winner of this year’s election.

Accordingly, the Bush campaign is trotting out an impressive list of Democrats who are supporting the president for re-election. Among these are Mayor Koch and the temporary crown jewel, Mr. Miller.

Earlier this year, Mr. Miller published his manifesto, “A National Party No More: The Conscience of a Conservative Democrat,” and declared his intention to support Mr. Bush for re-election. But the maverick Southern senator whose nickname is “Zig Zag Zell” has come under fire from former aides who attribute his defection to opportunism rather than a constant stand of principle in the face of a shifting political tide.

One of these, Ed Kilgore, the current policy director of the centrist Democratic Leadership Council and the federal state relations director for Mr. Miller from 1992-94, says that his former boss “went out of his way to abandon his party at a really crucial moment in 2001 when he was an original co-sponsor of Bush’s tax bill, undercutting the moderate Democrats who were fighting for a compromise.” The senator, says Mr. Kilgore, “keeps claiming he’s trying to get the Democrats’ attention, but you don’t get anyone’s attention by leaving the room.”

In all fairness, it is hard to see the current Democrats as significantly further to the left of the party of Bill Clinton that Mr. Miller so heartily endorsed in 1992. Instead, it seems more likely that it is Mr. Miller who has moved to the right. Viewers tonight will get to decide for themselves whether “Zig Zag Zell” is really guilty of the dreaded flip-flop.

In the meantime, Democrats have yet to come up with as high-ranking a Republican elected official who will cross party lines to endorse Mr. Kerry. Instead, they prepared to raise the name of 1980s business icon Lee Iacocca and Michael Forbes, the brother of a former Republican presidential candidate, Steve Forbes. Still, polls show that 5% to 8% of Bush voters in 2000 intend to vote for Mr. Kerry in this year’s election – a troubling sign for a campaign that fell more than 500,000 votes short of winning the popular vote in the last election. Documentary filmmaker Errol Morris has devoted himself to making a series of advertisements featuring such voters explaining their reasons for their disaffection. Democrats hope that in the absence of Republican elected officials, such grass-roots defections will send a similar message to undecided swing voters, who will determine the outcome of this election.

In the wake of the Reagan Revolution, the electoral map shifted toward the Republicans. Democrats know they have plenty of ground to make up, but the absence of an animating celebrity who appeals across party lines – such as last night’s prime-time star, Governor Schwarzenegger of California – makes the case more difficult to make. Breaking out the tar and feathers won’t win over swing voters at this late date. The parties will have to do what they like least: appeal to reason and trust the voters.