With three weeks to go before the midterms, both Rasmussen and Gallup have their final peek at the composition of the likely voters that will decide who goes to the 112th Congress, and … nothing much has changed over the last few weeks.  In fact, at Gallup nothing has changed since it started tracking likely voters in this cycle, with both of its two models showing the same wide Republican advantages as before.  This rebuts the meme that Democrats have rallied the forces and generated new enthusiasm for their candidates, and the models show why:

Republicans maintain a substantial advantage over Democrats among likely voters in Gallup’s generic ballot for Congress — in both lower- and higher-turnout scenarios — fueled in part by the GOP’s strong showing among independents.

Gallup’s latest election update shows that if all registered voters were to turn out, 44% of voters would favor the Democratic candidate in their district and 47% would favor the Republican candidate. The race has been close since the beginning of September, suggesting there has been little structural change in Americans’ broad voting intentions in recent weeks.

Among voters Gallup estimates to be most likely to vote at this point under either a higher- or lower-turnout scenario, Republicans maintain substantial double-digit advantages. In Gallup’s higher-turnout scenario, Republicans lead 53% to 41%. In Gallup’s lower-turnout scenario, Republicans lead 56% to 39%. These likely voter estimates are based on respondents’ answers to seven turnout questions, with the results used to assign a “likelihood to vote” score to each registered voter and, in turn, to create hypothetical models of the electorate based on various turnout scenarios.

In addition to turnout, independents’ voting intentions are a critical determinant of the midterm election outcome — particularly relevant, given that more than 90% of Democrats and 90% of Republicans say they will vote for their party’s candidate in the elections. At this point, independents tilt strongly toward the Republican candidate in their district, helping shift the race in the GOP’s direction.

The issue in this election is not that Democrats aren’t voting for Democrats.  It’s that independents have fled Democrats in large numbers and are motivated to come to the polls this year.  Independents give the GOP a ten-point gap in the broader registered-voter sample, but that turns into a gap of more than 20 points in either turnout model for likely voters — 54/33 in the higher-turnout model, and 57/32 in the lower-turnout model, the one that uses the normal 40% overall midterm turnout.

At Rasmussen, the numbers have changed a bit over the last three weeks, but in favor of the Republicans:

With just three weeks to go until Election Day, Republicans hold an eight-point lead on the Generic Congressional Ballot.

Polling for the week ending Sunday, October 10, shows that 47% of Likely Voters would vote for their district’s Republican congressional candidate, while 39% prefer the Democrat.

The Republican advantage comes from a number of factors. One is the fact that midterm elections typically feature an older electorate with a smaller share of minority voters. Additionally, in 2010, there is clearly an enthusiasm gap favoring the GOP.

Due to these and other factors, the data projects that 35% of voters this year will be Republicans, while 33% will be Democrats. In the previous midterm election of 2006, the Democrats had a two-percentage point advantage. Unaffiliated voters strongly favored Democrats in 2006 and strongly favor Republicans this year.

Note that the composition of likely voters will not differ widely from the overall composition of the electorate.  Democrats have an advantage between 1-3 points in party affiliation in the general population, but that reverses to a two-point GOP advantage among those likely to vote in these midterms.  If that 3-5 point swing was the only difference in the electorate, Democrats wouldn’t face a wave of the magnitude we’re seeing.  The difference is that their radical agenda and profligate spending has thoroughly discredited them not with their base but with everybody else.

Given these numbers, an appeal for more Democrats to come to the polls may help make a couple of races a little more close, but it won’t help stem the tide.  In fact, as Barack Obama and the Democratic leadership become more strident and polarizing in their appeal to the base, they may wind up seeing that backfire as they alienate even more of the independents over the next three weeks.