As President Obama begins his push on a host of liberal second-term agenda items, the news that union membership has reached its lowest level since the New Deal is a sobering indicator that a key constituency in the president's coalition is fading, said progressive activists and labor movement experts.

Just 11.3 percent of the workforce now belongs to a union, according to Labor Department figures, meaning the president's base of support is slipping. Unions are a key group for Obama in looming legislative fights on issues like government spending.

"As we are weakened, the ability for us to prop up Democrats shrinks," said Chris Townsend, political action director for the United Electrical Workers. "These folks had better realize there is no more energized constituency to support them than organized labor. It's about time this damn thing got turned around, but I'm not optimistic."

In his inaugural address, the president outlined his liberal vision for the next four years, calling for action on climate change and gun control, while giving a forceful defense of an expansive federal government.

The White House will launch a campaign-style blitz for these initiatives and is banking on union workers to help move public opinion and lawmakers wary of the reforms. But with shrinking state and local government payrolls and the growth in right-to-work states, the labor movement, once the bedrock of Democratic politics, could lose even more political clout.

"It's simple math; the fewer union members there are, the fewer foot soldiers on the ground for the president," said a top United Auto Workers official who led outreach efforts for Obama in Ohio. "Yeah, I know a lot of people are expecting major things in his second term, but seriously, [the decline in union members] is a huge deal for the president, Democrats and what they want to accomplish."

Unions have adjusted their tactics amid declining membership, joining forces with outside groups such as the NAACP and Planned Parenthood for major political efforts. And some argue that new laws in states like Wisconsin have emboldened organized labor, ensuring their viability in future elections.

But just two decades ago, one in every five workers was a union member.

Republicans welcome the decline, saying public employees unions have contributed to bloated budgets, forcing taxpayers to bear the burden of runaway spending. But progressives see the trend of shrinking unions as an augury of reduced political clout.

"It is worrisome; it's not a good sign," said Michael Wasser, a senior policy analyst with the pro-labor group American Rights at Work. "It's far more difficult for workers to unionize now."

Obama has sought to repay unions recently for the hundreds of millions of dollars given to his presidential bids and the thousands of hours spent mobilizing voters on his behalf. He ripped Michigan's right-to-work push during a recent visit to Detroit and made income inequality a centerpiece of his inaugural address.

However, such actions came after the president in his first term didn't press for "card check," which would ease unions' effort to organize by abolishing secret-ballot workplace elections, or visit Wisconsin ahead of the contentious recall election there in which Republican Gov. Scott Walker prevailed.

And some union officials said they were motivated more by fears of a Republican winning the White House than they were endorsing the president and his second-term agenda.

"We've come to a choice where electing the ambivalent is preferable to those who would liquidate us," Townsend explained.