Rep. Steve King (R-Iowa) is hitting back against American Crossroads President Steven Law in a new fundraising e-mail, declaring: "Nobody can bully me out of running for Senate."

King, a deeply conservative congressman with a tendency to make controversial utterances, has come under scrutiny from Republican strategists who worry he will make a bid for retiring Democratic Sen. Tom Harkin's seat.

"We’re concerned about Steve King’s Todd Akin problem,” Law told the New York Times in a story published over the weekend. Law is leading a new effort to recruit electable GOP Senate candidates and protect them from the ones viewed as less electable.

"I’m no stranger to outlandish attacks like this. They said I couldn’t win in 2012 -- the entire political machine was against me -- but I soundly defeated my opponent by 8 percentage points," King wrote in the fundraising e-mail, according to the Des Moines Register.

King's e-mail illustrates the double-edged sword of the Republican establishment taking a more active role in Senate nominating contests. One the one hand, it may help diminish the chances of weak Republicans being nominated. On the other, conservatives will use the threat of interference to further fire up their own bases.

So far, no Republicans have entered the race for Harkin's seat. King is clearly thinking about it, and another name that has surfaced is Rep. Tom Latham, a more moderate Republican whom some strategists view as a better fit for the general election.

On the Democratic side, Rep. Bruce Braley declared his intention to run Thursday. Braley, a strong fundraiser, is viewed as a top Democratic recruit for the race.