Here’s another sign that some folks may be getting mighty nervous about the Dem drives to recall Wisconsin GOP state senators: I’m told that a major national Republican polling firm is in the field in the state testing some hard-core anti-union messages, including ones about how pro-union forces targeted a GOP state senator with death threats. It’s another sign that the Wisconsin standoff has escalated further into a protracted, high-stakes national war.

The GOP firm, Public Opinion Strategies, is doing a poll to gauge public sentiment about the effort to recall State Senator Alberta Darling, a top target of Democrats, a source familiar with the poll confirms. It’s unclear who’s paying for the poll, but it’s clearly being paid for either by Darling herself (her campaign didn’t return calls) or by outside conservative groups that are now spending money in the state to defend embattled GOPers being targeted by the recall drives.

I was unable to obtain the exact wording of the poll, but the source, who is familiar with the line of questioning, confirmed to me that it was testing messages asking whether recipients’ opinions would be changed by the following:

* The fact that union sympathizers have given Darling death threats

* The fact that the recall is being funded by out-of-state unions

* The fact that the highest paid public employee in Madison is a bus driver who made nearly $160,000 a year

* The fact that unions used collective bargaining to try to get Viagra

Again, these are approximations, but they’re telling nonetheless. The bus driver in question, of course, is the famed Madison driver who earned roughly $160,000 in 2009 thanks to union-negotiated rules allowing drivers with the most seniority to snap up lots of overtime — rules that have now been changed. The Viagra reference is to a recent effort — which has since been dropped — by Milwaukee teachers who went to court to get health coverage for the drug.

The poll amounts to a fairly standard testing of negative messages — but the harshness of the anti-union messages being tested suggests that some of the GOP senators and their defenders may know they’ve got an extraordinarily tough fight on their hands. Indeed, in another sign of just how high the stakes have gotten, another recall target, GOP state senator Randy Hopper, has also recruited national campaign talent to help fend off the recall drive.

Officials at Public Opinion Stratgies and Venture Data, the calling firm contracted to do the poll, declined comment.

All this comes as a district judge issued a temporary restraining order on Scott Walker’s anti-bargaining rights proposal, citing possible violations of the state’s open meeting law. Wisconsin Reupblicans will appeal the ruling, and may try to pass the law through the legislature again, but this could only enable Dems to stoke passions further, giving the recall drives more momentum.