I've had my ups and downs with Gallup (one might say the relationship is like the plot of the Gallup generic ballot itself!), but since they've been solidly running registered voter surveys again, the numbers have looked reasonable.
But now they're shocking us from the other direction by showing the Republicans having their largest ever lead in the generic ballot survey, showing a larger edge than even today's Rasmussen's GOP +6 (notwithstanding the alleged House Effect of the latter).
This Gallup result is so large, I had to see what it shows in the Swingometer. As always, I boil it down to two party results. In 2008 we had a 56 D - 44 R split, and this Gallup simplifies to a 45 D - 55 R split. So the swing is from a D+12 to an R+10, or a 22 point swing.
So right now, that means Gallup of all polls, using Registered Voters, is projecting in the Swingometer a 60 seat Republican gain for a 238 R-197 D majority. The last time an election took the Democrats that low was the election of 1946, saith Wikipedia. Election night in 2004 took them to 202 for the second lowest.
Rasmussen today, by contrast, shows only a 20 point swing, a 57 seat Republican gain, and a 235 R - 200 D majority, still lower than an election since Truman has taken the House Democrats. If I then take the mean of these two and double weight the Rasmussen Likely Voter poll, I get R+58, the new projection.