3. Other Republicans are shifting to the right.

At one point in the last few months I thought Walker had the best chance of winning the nomination. Then he showed what kind of voters he was trying to attract by taking ultra-conservative positions on national policy issues. Very conservative voters were already impressed by Walker’s record of standing up to intense union opposition, and many would have supported him anyway. By shifting to the right on immigration, foreign policy and social issues, Walker has made himself look more conservative and less attractive to voters who weren’t already inclined to support him.

With other Republicans moving rightward, there’s a vacuum in the middle of the electorate — one that Bush is well-placed to fill.