A GOP senator under fire for blocking a veterans' health care bill that aims to expand mental care and offer home assistance to wounded veterans had a message for his critics: "Show me the money."

Sen. Tom Coburn, an Oklahoma family physician, has blocked the $3.7 billion dollar health bill since May because he says the funding for the expanded care may not exist.

"I'm not opposed to doing what we're doing for vets," he said Monday at a news conference. "I just want to make sure it's paid for."

Thirteen major military and veterans groups have mobilized to try to force Coburn to release his hold on the legislation, and Democrats are blasting him for his opposition.

"I believe when we vote to send our troops to war, we have an obligation," said Sen. Daniel Akaka, D-Hawaii.

Sen. Mark Begich, D-Alaska, called Coburn's roadblock on the benefits "inexcusable."

Coburn, who is known as Dr. No for his opposition to earmarks and spending increases, noted that the bill only covers veterans who have served since the Sept. 11 terrorist attacks, won't even take effect this fiscal year, which began Oct. 1.

In a written statement, Coburn said, "I support many of the goals of this legislation but we simply cannot continue to spend billions on new programs without paying for them. If senators would pay for this program and make a few common sense change this bill could pass the Senate today.

"Unfortunately, bill sponsors are more interested in holding press conferences and playing political games than doing the hard work of legislating.

Coburn expressed concern that the bill discriminates against veterans of World War II, Vietnam and the Persian Gulf War. He said the bill also duplicates some existing programs that aim to help veterans in need of home care.

"The American people and our veterans understand that our spending problem has become a national security problem," he said. "We are borrowing massive sums from potential adversaries and are watching the value of the dollar decline because other nations doubt our ability and willingness to pay off our $12 trillion debt. If we don't start making hard choices we may not have a country left to defend."

The bill would, among other things, provide benefits to caregivers of wounded veterans, offer outreach to homeless veterans and enhance Veterans Affairs medical services.

When asked why the bill is not funded, Sen. Jon Tester, D-Mont., a co-sponsor of the legislation, said, "because the decision was made long before I got here to send troops to Iraq ... and later to Afghanistan. ... War funding was not paid for. ... We have an obligation to those folks."

Sen. Dick Durbin, D-Ill., said the bill is a "lifeline" for many veterans.

"It is not much to provide what they ask for," he said. "Two weeks of respite so they can have a little time off for themselves and hire nurses to fill in."

"He is the only senator objecting," Durbin added. "Because of his objection, veterans go without."

Fox News' Trish Turner contributed to this report.