Kansas City, Mo. ? Aaron Jones has heard the frustrations as he goes door-to-door among union households — the economy remains poor; issues important to workers are stalled in Congress; Republicans seem more energized this election.

Put mildly: “There is an enthusiasm challenge,” said Jones, the leader of a five-person voter canvassing crew for a local chapter of the Service Employees International Union.

With a growing sense of urgency, labor unions are deploying tens of thousands of activists nationwide in an attempt to counteract the malaise in the rank and file and prevent Democrats from losing their congressional majorities in the middle of President Barack Obama’s term.

Unions expect to spend about $100 million in the weeks leading up to the Nov. 2 election, targeting nearly 100 House races, 18 Senate seats 14 gubernatorial races. Between campaign walks, door knocks, mailings, phone calls and work site leafleting, union officials already have contacted about half of their approximately 17.5 million members nationwide.

But for the first time in many election seasons, the results of the effort are uncertain. The ambivalence — or even opposition — among some workers has Democratic candidates worried. The disaffection of middle class men, who represent a large part of the union workforce, is a major reason that Democrats are braced for potentially serious losses nationwide.

Union households have long been a staple for Democrats. In 2008, Obama drew support from nearly 60 percent of union households compared with a little over 50 percent of the overall electorate, according to an exit poll conducted by Edison Research for The Associated Press and television networks.

But workers have not reaped all the rewards they expected. Unemployment remains above 9.5 percent. States have been slicing public employee pensions and laying off unionized workers to compensate for declining tax revenues. And, despite Democratic control, Congress has been unable to pass legislation that would make it easier for unions to organize workers.