New numbers from Gallup and the Pew Research Center showing the presidential contest tied among all voters in recent days are sure to buoy Republican hopes that Mitt Romney did more than win a debate last week. But the newly released data also undercut a persistent criticism of election polls: that there is a “true” measure of partisan identification -- and its malicious corollary, that pollsters are manipulating reality.

In the two new national polls, President Obama and Romney are now tied among all registered voters. In Gallup, this is change from a five-point Obama edge in the three days leading up to the Denver debate; for Pew, it is a shift from a nine-point advantage for Obama in mid-September.

So who moved in Romney’s direction?

Well, not political independents, for one. There was no meaningful change in their support for Obama or Romney in either poll.

All of the change in both polls came from the composition of each sample. In pre-debate interviews by Gallup, self-identified Democrats outnumbered Republicans by five percentage points, according to Gallup’s Jeff Jones. By contrast, in the three days following the debate, the balance shifted in a GOP direction, with 34 percent of registered voters identifying as Republicans (two points up from pre-debate), 33 percent as Democrats (four points down).

For Pew, a nine-point Democratic advantage in mid-September is now plus one percentage point for the GOP. (The turnabout in “likely voters" was even more dramatic, shifting from Democrats up 10 to Republicans up five.)

The recent hullabaloo about adjusting samples (or, in the parlance of pollsters "weighting") to pre-determined partisan splits misses that movements in party identification can be just as real as movements in voting intentions.

These polls show more people are now identifying as Republicans in the wake of Romney's debate performance. Just as they showed a clear Democratic tilt earlier in the fall, and in the immediate aftermath of Romney's "47 percent" comments. Shifts happen in the electorate. The only constant is change.

If either Pew or Gallup kept their “sample balance” constant, there would be far less of a shift in their headline numbers. Poll watchers need to closely assess both trend-lines as the election draws near.