The difference between sampling likely and registered voters gets made plain in the latest CNN/Time Magazine poll in Nevada, among other states, where the results of the former show Sharron Angle taking her first slight edge over Harry Reid, 42/41 in a virtual tie.  Time breaks down the internals, but avoids one key observation on the top-line results:

Most striking is how closely fought the Nevada Senate race remains. The poll in that race, which pits the most powerful Democrat in the Senate against former state assembly member Angle — shows that Angle is capturing 42% of likely voters polled to Reid’s 41%, a statistical dead heat. Angle’s standing against Reid comes despite the fact that there’s an official Tea Party candidate on the ballot who might be expected to siphon away votes from her. But Scott Ashjian is backed by only 5% of those polled.

There is a striking gender difference among those polled in Nevada. Women go for Reid by a 51-33 margin, while men back Angle, 49-32. With both parties’ bases mobilized and overwhelmingly backing their nominee, the critical ground will be fought over independent swing voters. Angle wins here, collecting 40% compared to Reid’s 33%. Among self-described moderates, Reid holds sway, 57-23, while Angle has a big margin among conservatives, 72-13. Reid wins among those who attended college (44-40) as well as those making less than $50,000 a year (45-31). But Angle prevails among those with no college experience (44-37). They’re evenly divided among voters making more than $50,000.

The key observation?  Take a look at the numbers from the two sample types in the survey:

• Registered voters: Reid 42, Angle 34
• Likely voters: Angle 42, Reid 41

There are two implications from these numbers.  First, and obviously, Angle supporters are more enthused and active.  Harry Reid’s supporters are not all that inclined to show up on Election Day.  That bodes well for Angle as voters make up their minds in the next seven weeks.  But even among all registered voters in a CNN/Time poll, Harry Reid can’t get above 42% support.  He’s a multi-term incumbent who has been in state politics for 40 years, and seven weeks from an election only four in ten voters can muster a commitment to his re-election.

This election is not a referendum on Sharron Angle.  It’s a referendum on Harry Reid, and no matter how much he’d rather put Angle under the microscope, it’s going to continue to be a referendum on Reid and the Democratic agenda.  This race may look tied now, but if Angle continues to put the focus on Reid and doesn’t make any serious errors, the undecideds will either choose Angle or their armchairs on November 2nd.