* 'Risk to our national security,' Lieberman says
* McCain says Obama puts politics ahead of security
* Many worried over looming second round of cuts
WASHINGTON, Feb 14 (Reuters) - Skeptical lawmakers questioned the Pentagon's 2013 spending plan at a Senate hearing on Tuesday, challenging everything from Defense Secretary Leon Panetta's decision to cut 100,000 troops to his proposal to study closure of more U.S. military bases.
"I consider this budget to represent unacceptable risk to our national security," Senator Joe Lieberman said as Panetta appeared on Capitol Hill to defend the proposed spending plan sent to Congress on Monday by President Barack Obama.
Panetta told members of the Senate Armed Services Committee the budget effectively balanced competing strategic objectives while achieving the $487 billion in savings over 10 years required by an August agreement between Obama and Congress.
The defense spending plan includes a $525.4 billion Pentagon base budget, 1 percent less than the amount approved for 2012. It seeks $88.5 billion for U.S. combat operations overseas, a 23 percent drop largely due to the end of U.S. involvement in Iraq.
The budget, which would cut about 100,000 troops, delay or terminate weapons systems and study the possibility of closing more military bases, included enough changes that most lawmakers on the Senate panel could find something to dislike.
"The savings that we are proposing are significant and broad-based and will impact on all 50 states," Panetta said. "This is what Congress mandated on a bipartisan basis, that we reduce the defense budget by almost a half a trillion dollars."
The defense spending plan is part of Obama's overall budget, which was widely seen as an election-year roadmap of the course he would chart in a second term. It included higher taxes for the rich as well as $800 billion in spending to create jobs even as it curbed defense spending after a decade of growth.
Senator John McCain, the panel's senior Republican, charged Obama's defense budget put "short-term political considerations over our long-term national security interests" and expressed concern about cuts in military personnel, aircraft and ships.
"By any objective assessment, the worldwide threats to our nation, our interests and our ideals are not diminishing."
"And yet the defense budget before us would reduce the size of our force," said McCain, Obama's Republican opponent in the 2008 presidential election.
Lieberman, an independent and former Democratic vice presidential candidate, said Panetta had met the spending reductions required in the Budget Control Act passed by Congress last August. But he said those cuts would endanger the country.
"It's hard for me to conclude that there's any reason you would make such a recommendation other than the fact that you're required by law to do it," Lieberman said.
Senator Carl Levin, the panel's Democratic chairman, rejected the notion of another round of base closures just six years after the government agreed to shutter 22 facilities. The process, which is still being wrapped up, has been controversial because much of the expected savings were not realized.
"Before we consider another round of BRAC (Base Realignment and Closure), the department ought to take a hard look at whether further reduction in bases can be made overseas, particularly in Europe," Levin said.
He noted that the budget plan called for removal of four combat brigade teams - about 7,000 soldiers - from Europe but that the United States would still have some 70,000 military personnel on the continent.
Many of the lawmakers worried about a looming second round of defense cuts. Under the Budget Control Act, the Pentagon could be hit with another $600 billion in cuts to projected spending over 10 years unless Congress compromises on ways to further reduce federal spending by $1.2 trillion.
Panetta and General Martin Dempsey, the top U.S. military officer as chairman of the Joint Chiefs of Staff, have warned a second round of cuts would be devastating to the military.