Two of the nation's largest, most influential unions expressed serious reservations Thursday about the Senate health care bill, with the AFL-CIO coalition calling it "inadequate and too tilted toward the insurance industry."

"The absolute refusal of Republicans in the Senate to support health care reform and the hijacking of the bill by defenders of the insurance industry have brought us a Senate bill that is inadequate: It is too kind to the insurance industry," said AFL-CIO President Richard Trumka.

Union opposition presents a political problem for the White House, as Barack Obama relied on the unions' ability to organize and influence voters to build his presidential campaign last year.

In a statement released Thursday, Trumka listed what he saw as the fundamental problem with the Senate version of the health care bill, now that it has been stripped of a public option and a proposed expansion of Medicare benefits.

-- It lacks a public health insurance option, to offer real competition to insurance companies to bring down costs.

-- It fails to make sure employers take responsibility and pay their fair share.

-- It’s funded through a new tax on working families’ health care benefits.

"It doesn’t have to be this way," Seth Michaels wrote on the AFL-CIO blog. "The bill passed by the US House is far better than the Senate’s bill on these and other measures. The House bill finances health care through a small tax on the very wealthiest of earners—those who reaped vast benefits from the Bush tax cuts—and it includes a public health insurance plan and real responsibility for employers."

Meanwhile, the president of the Services Employees International Union issued a "challenge" to President Barack Obama to improve the Senate bill. In a letter to union members, Andy Stern said the president "must remember his own words from the campaign. His call of 'Yes We Can' was not just to us, not just to the millions of people who voted for him, but to himself. We all stood shoulder to shoulder with the president during his hard fought campaign.

"Our challenge to you, to the President, to the Senate and to the House of Representatives is to fight. Now, more than ever, all of us must stand up, remember what health insurance reform is all about, and fight like hell to deliver real and meaningful reform to the American people."

Stern wasn't as strong as the AFL-CIO in his condemnation of the Senate bill, and in fact advocated that the bill be moved forward. But he made it clear the union was unhappy with the the removal of the public option.

"At the very moment that we saw real and meaningful changes within our grasp, one Senator came forward to say 'no we can't.' He can't let the Senate have an up-or-down vote on health insurance reform," Stern wrote, referring to Sen. Joe Lieberman, the Connecticut independent whose declaration that he couldn't support the Medicare buy-in prompted Senate Majority Leader Harry Reid to remove the provision, the last vestige of a public option in the Senate bill.

In expressing discontent with the Senate bill, the unions added themselves to a growing chorus of progressive voices who are turning against the Senate bill. The dissent in the liberal ranks began with former Vermont Gov. Howard Dean's declaration that the bill in its current form is a "dream" for private health insurers and should be killed.

Dean's voice was followed by criticism of the bill from Sen. Russ Feingold, and a declaration from Sen. Bernie Sanders that he won't vote for the bill in its present form, indicating that the concessions to Lieberman did not secure the 60 votes the bill needs to overcome a Republican filibuster.

Dean's vocal opposition to the bill has made him a hero to some progressives. Writing at the Huffington Post, David Sirota declares Dean a "genuine hero" for his opposition to the removal of the public option.

"Here is, in short, a rare movement leader in the age of cynicism showing what a movement can do -- or at minimum, have a realistic shot to do -- when it musters a little bit of courage," Sirota wrote.

Sirota's argument that Dean has transformed himself into a leader of the American progressive movement is strengthened by the fact the White House attacked Dean's criticisms on Wednesday -- indicating the administration sees his as an influential voice.