The darkest moment of the 2012 campaign for President Obama was the first of his three debates with Mitt Romney, the Republican nominee. Obama was rusty and under-prepared, which contributed to his poor showing that night, but a big reason he lost the plot so badly is that the Mitt Romney he had prepared to debate was a composite of public statements, briefing papers, and other documentation from the past. The Romney who showed up was a shapeshifter adapting to his immediate circumstances.

So when Obama attacked Romney, accurately, for proposing to cut taxes on the affluent so dramatically that the middle class would have to pick up the tab, Romney simply and dishonestly denied this was the case.

Obama pointed out the discrepancy, but by that point the debate might as well have been over. Or at least it had transformed into something other than a debate. The shared premise disappeared, and the vast majority of people watching had no way of knowing who was right and who was wrong and how brazenly Romney had lied.

In the days afterward, Romney struggled badly to defend his tax plan, but by wide acclaim, he outperformed Obama that night. Romney lost the election, but Republicans apparently decided that his debate tactics were terrific, because they mimicked them repeatedly during the third primary debate Wednesday night. This presents a huge challenge to political reporters and, in a different way, to the Clinton campaign. The Republican policy agenda has been radical for a long time, and Republicans have mastered all the buzzwords meant to elide this fact. Their tax plans aren’t regressive, but engines of economic growth. They don’t plan to phase out the existing Medicare program and replace it with subsidized private insurance, but to “save Medicare for future generations.” Now, though, Republicans seem content to simply disclaim the contents of their own white papers.

The exchange that most resembled the Romney-Obama row transpired between Marco Rubio and moderator John Harwood.