Nothing about this is surprising. Clinton remains the overwhelming favorite to win the nomination, and it's no shock that Republicans are interested in scoring a few early hits on the probable Democratic nominee.
Anyone who believes that Clinton's "Wall Street cash" won't be a major attack theme for Republicans in the general election is kidding themselves. Particularly if Trump is the nominee. Sanders' apparent success with this message only demonstrates its potential. And you don't need to be to Clinton's left to make this populist line of attack work.
They're not running it just to "score early hits" though. If they were, they'd be running it in Ohio, Florida, etc. Instead they're running it in just Iowa. It's clearly meant to prop up the Sanders campaign. Any other benefit is secondary.
To political junkies, it was obvious what McCaskill was doing in 2012. But her ads weren't aimed at political junkies. And they worked. Rove is trying the same exact thing.