Dec 1 (Reuters) - Republicans and Democrats in the U.S. Congress expressed reservations about President Barack Obama's Afghanistan strategy after he announced a plan to send an additional 30,000 U.S. troops to Afghanistan by next summer and bring some soldiers home in 18 months.

Among the lawmakers' worries are:

COST

Can Washington afford to expand the war in Afghanistan? That question is taking center stage because of growing unease about the U.S. national debt of nearly $12 trillion and demands on the government to help the struggling economy and consider an expensive healthcare overhaul.

Legislative proposals to pay for Obama's escalation in Afghanistan may be targeted by war opponents.

"The cost of this is astronomical," said Rep. Walter Jones, a North Carolina Republican.

A long-term commitment "could cost anywhere from $500 billion to $900 billion over the next decade, which could devour our ability to pay for the actions necessary to rebuild our own economy," said Representative Dave Obey, who chairs the House committee in charge of approving government spending.

Obey, a Democrat, has proposed a war "surtax" to pay for the conflict. But a tax increase is unlikely, especially with midterm elections next year.

EXIT STRATEGY

For months many Democrats urged Obama to offer a timetable for when U.S. military operations in Afghanistan will start to wind down. Some of them welcomed his pledge to start the transfer out of U.S. forces in July of 2011.

But others noted the lack of a final end date.

"I am disappointed by his decision not to offer a timetable for ending our military presence there," Senator Russ Feingold, a liberal Democrat, said.

Republicans criticized Obama's comments about a pullout.

Representative Howard McKeon, the ranking Republican on the House Armed Services Committee, said he found the public announcement of a withdrawal date "disconcerting."

"I don't like having a deadline. You can have one in mind, but why tell the enemy?" McKeon said

AFGHAN ARMY

Many lawmakers think the Afghan army should be shouldering more of the burden of the eight-year-old war, a view championed by the Senate's leading voice on military matters, Armed Services Committee Chairman Carl Levin.

Levin, a Democrat, says the emphasis should be on training Afghan forces, not pouring in more U.S. combat troops.

"It is time to take the training wheels off and let's let a capable Afghan force fight for its own country," Representative Jane Harman, a California Democrat, told CNN.

AFGHAN PRESIDENT KARZAI

Whether they support staying in Afghanistan or not, many lawmakers are dissatisfied with Afghan President Hamid Karzai, saying he has not clamped down on corruption and cronyism.

House Speaker Nancy Pelosi, who opposes upping the ante in Afghanistan, calls Karzai an "unworthy partner" who does not deserve either an increase in U.S. troops or civilian aid. She said on Tuesday that Obama had offered Karzai a chance to prove that he is reliable.

PAKISTAN

Obama said the United States would not tolerate Pakistan allowing its territory to be a safe haven for militants and urged Islamabad to fight the "cancer" of extremism.

But some lawmakers worry that Pakistan, a sanctuary for some militants, could be further destabilized by an expanded war in neighboring Afghanistan. Pakistan's government has nuclear weapons.

Others worry that by putting so much emphasis on Afghanistan the United States is not adequately addressing the problems in Pakistan.

"If our fight is truly with Al Qaeda, then we're in the wrong country. They have moved to Pakistan," said Rep. James McGovern, a Massachusetts Democrat.

ALLIES

Some lawmakers ask why U.S. allies don't step up with more troops for Afghanistan. Others say it is unrealistic to assume they will do so.

"I disagree with the president's two key assumptions: that we can transfer responsibility to Afghanistan after 18 months and that our NATO allies will make a significant contribution," said Senator Arlen Specter, a Democrat.

(Reporting by Susan Cornwell in Washington, editing by Alan Elsner)

((For more on Afghanistan, click on [nAFPAK])) ((susan.cornwell@thomsonreuters.com; +1 202 898 8390; Reuters Messaging: susan.cornwell.reuters.com@reuters.net))

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