MADISON, Wis. -

Gov. Scott Walker won't say which of the four Republicans running for U.S. Senate he will vote for, but he said they would be well served to focus on the differences between one another and Democrat Tammy Baldwin.

Walker has said he plans to be a referee in the Senate race. When asked Tuesday if he's seen anything out of bounds yet, Walker said "I haven't dropped a flag yet but it's getting pretty close."

The campaign has turned increasingly negative as the Aug. 14 primary nears.

Walker said he hasn't seen a "blatant lie" from anyone yet, but "the question is interpretation."

Republicans running are former Gov. Tommy Thompson, former U.S. Rep. Mark Neumann, state Assembly Speaker Jeff Fitzgerald and hedge fund manager Eric Hovde.