In the half century since Wisconsin became the first state to give its public workers the right to bargain collectively, government employee unions have mushroomed in size and power — so much so that they now account for more than half of the nation’s union members.

But the legislative push by Wisconsin’s new governor, Scott Walker, a Republican, to slash the collective bargaining rights of his state’s public employees could prove a watershed for public-sector unions, perhaps signaling the beginning of a decline in their power — both at the bargaining table and in politics....

Some Republicans quote President Franklin D. Roosevelt, a Democrat, who bridled at public-sector unionism and once said, “The process of collective bargaining, as usually understood, cannot be transplanted in the public service.”

Republicans say the Democrats have embraced the government employees’ cause because weaker unions would reduce crucial political support for Democratic candidates. Republicans have often denounced what they say is a squalid deal in which public-sector unions spend generously to elect allies to office and then those allies lavish generous wages and benefits on union members.

Ever since Wisconsin gave its government employees the right to bargain in 1959, it has generally been Democrats who have extended that right in other states. In 1962, President John F. Kennedy gave most federal employees the right to unionize and bargain collectively....