Despite any issue about cause and effect the partisan connection matters. In 2012 a Midwest union leader said to me that Indiana taught him something. When Pubs took over all branches in 2011 the unions weren't surprised that hostile legislation came up. But when they went to lobby, the unions realized they had not a single sympathetic ear to their concerns on the Pub side. They had piled all their donations to the Dems for a decade and now the Pubs could care less about their issues and specifically about any money going to the unions. I'm not surprised that congressional Pubs feel the same way about PP.
At risk of derailing this thread, I think this is a very good point to illustrate the partisan polarization of interest groups regarding issues, like labor and abortion.
In addition to this, I think part of this is also the fact that since the late-1960s/early-1970s, unionized labor simply hasn't had the political power that it did from the 1930s - 1960s as the economy transitioned from manufacturing to services.
As the number of voters who were union members entered a steep decline, there simply was no need for Republicans to campaign and govern on providing a business-labor balance (as opposed to the stridently pro-labor Democrats), when they could simply do away with union support and focus on non-union white collar service industry workers (plenty of non-manufacturing blue collar jobs could also be included here) who simply don't care about unions.
America's economy has changed and it's simply not necessary for Republicans to try to win labor votes, and the decline has reached a point that Democrats have only given sparing attention to labor over the past 25 years. Organized labor has reached the point where they have to go with the lukewarm Democrats over the GOP that simply doesn't care about their interests and votes, so all their donations go to Democrats, and in turn Republicans increasingly turn against them.
EDIT - I have no hard stats for this, but it is also very important to note that the composition of union members has also completely changed, with public-sector unions dominating and private-sector unions virtually non-existent. Private sector unions may have been more open to Republicans striking a balance between business and labor so their employers prosper and distribute the economic gains, but since Republicans get much of their support for cutting government spending and taxes, public sector unions have literally no reason to support Republicans.
IIRC, the racial demographics of public sector and private sector unions is also radically different, with private-sector unions having mostly been made up of white working class people back in the 1930s whose loyalty to Democrats was pretty much entirely based on Democrats being pro-union, but miles apart from the modern Democratic party on cultural and energy policy issues. Due to strong anti-discrimination policies put in place by the government, public sector jobs have often been a path to greater economic prosperity for minorities (especially among the still-highly discriminated against blacks, but also recently-immigrated ethnic whites before our modern image of what it is to be a "White American" formed before the racial tensions of the 1960s) who would otherwise have had limited private-sector opportunities.
So I think it's safe to say that many White Americans had parents who were pro-labor Democrats but culturally conservative from the Great Depression to the 1960s when the service and knowledge economies took over, and the children of them got college degrees or job training , went into occupations where labor was difficult to organize and non-existent, and achieved a comfortable lifestyle, so when the New Deal coalition fell apart, they switched to the GOP in a heartbeat.
As True Federalist mentioned above, not only has abortion has become a partisan issue, but also the link between voting patterns among racial groups is tied to the partys' histories and governing philosophies. To tie this all together, we're seeing an increasingly polarized electorate by race, occupation, and income, with the history of labor being a big factor in how the last 50 years have played out politically.
Sorry for the jumbled, wordy post. Everything connects in the end.
