The Post-Bloomberg Republican primary debate tomorrow will feature Herman Cain and Mitt Romney in the center of the stage, side-by-side. That is a function of the polling numbers released today that show Romney leading with 24 percent of those voters leaning Republican, Cain with 16 percent, Texas Gov. Rick Perry down to 13 percent and everyone else below 7 percent. Jon Huntsman is dead last with 0 percent, suggesting John Weaver has taken large sums of consultant fees based on the false suggestion that there is a Huntsman electorate out there.

I asked the pollsters what is going on here. The Post’s pollsters point to the flip in support from Perry to Cain. On September 1, 45 percent of “very conservative” voters backed Perry with 16 percent for Romney and 6 percent for Cain. Now, Cain has 22 percent of those voters, Romney has 19 percent and Perry has 18 percent. Likewise, among strong Tea Party supporters Perry went from 45 to 10 percent, Romney went from 15 to 19 percent while Cain went from 5 to 30 percent. Meanwhile, Rep. Michele Bachmann’s support among very conservative voters and Tea Partyers has collapsed since July, going from 26 to 7 percent and 28 to 9 percent respectively.

What is striking is the degree to which Romney has remained almost perfectly constant. He leads (as he has since July) among conservative and Tea Party voters while holding on to just under 20 percent of the very conservative and strong Tea Partyers. Others have risen or fallen, but none has stayed close to the top for very long. Moreover, Perry, Bachmann and Cain have all failed to breakout beyond the hardcore base. There is now a very short window before the January Iowa, New Hampshire, Nevada, South Carolina and Florida contests for one of Romney’s competitors to figure out how to put all the pieces together.

That will necessitate one of them crushing the other anti-Romney contenders and then attracting voters beyond their hardcore base. It is possible, but increasingly difficult. It does however mean that Perry, Cain and Bachmann, if they want to move up, will need to take on one another, even more so than Romney, whose base of support seems virtually impervious to the assaults from the right flank.