The Bloomberg survey gives Bush a 46% favorable rating, while Romney is at 43%. To be fair, Bush got higher ratings than Vice President Joe Biden (42%) and the Republican Party (41%).
From the Dallas Morning News:
The survey shows first and foremost that Romney is struggling in his effort to unseat President Barack Obama — a fact highlighted by several other polls conducted ahead of the first presidential debate next Wednesday.
But it also shows how Bush — following the trend of other former presidents — has seen his popularity improve since leaving the White House, when his favorable numbers in some surveys hovered around the mid-30s.
The ratings are an improvement for Bush from even earlier this year, when a CNN/ORC poll found that Bush ranked as the least popular living ex-president. And as Ari Fleischer, Bush's White House press secretary from 2001 to 2003, noted on Twitter Thursday, the numbers are the former president's "best level in years."
That being said, it seems unlikely there will be any change in the public relationship between Romney and Bush — a setup that has included the former president skipping the Republican National Convention and Romney rarely mentioning Bush on the campaign trail.

