WASHINGTON — President Barack Obama demanded Monday that Congress raise the federal debt limit quickly, warning that “Social Security benefits and veterans’ checks will be delayed if they don’t,” and cautioning Republicans not to insist on concessions in exchange.

“They will not collect a ransom in exchange for not crashing the economy,” he said at the final news conference of his first term. “The full faith and credit of the United States of America is not a bargaining chip. And they better decide quickly because time is running short.”

“We are not a deadbeat nation,” he declared.

The president also said he will soon ask Congress to enact new gun legislation in the wake of the shootings a month ago that left 20 elementary students dead in Newtown, Conn. Facing stiff opposition from the National Rifle Association, he conceded lawmakers may not approve everything he asks for.

Among the proposals under consideration are a ban on assault-style weapons and high capacity ammunition magazines. Obama said he would unveil his proposals next week. He was meeting after his news conference with Vice President Joe Biden, his point man in producing gun control measures to present to Congress.

Obama opened his news conference with a statement noting that a vote to increase the debt limit “does not authorize more spending. It simply allows the country to pay for spending that Congress has already agreed to. These are bills we’ve already racked up and we need to pay them.”

Obama said he was willing to consider future deficit cuts, but only if they are done independently from a vote to raise the $16.4 trillion debt limit.

In a blunt rebuttal to Republicans who say they will not agree to any more tax increases, the president said taxes and spending both must be on the table.

He said he is “open to making modest adjustments to programs like Medicare to protect them for future generations,” and wants to close tax loopholes at the same time.

Obama spoke less than a week before his inauguration for a second term, and several days after he signed legislation that narrowly averted a “fiscal cliff” of automatic spending cuts and across-the-board tax increases.

Combined with other bills he signed earlier in the term, he said he and Congress have reduced deficits by about $2.5 trillion over a decade, somewhat less than the $4 trillion he said is necessary to get them down to a manageable size.

“I’m happy to have a conversation about how we reduce our deficits in a sensible way,” he said, but added repeatedly he wasn’t willing to let congressional Republicans use the debt limit as leverage in negotiations over spending cuts.

Failure to raise the debt limit would put the United States into a first-ever default, a step that Obama said could “blow up the economy.”

Congressional Democrats have recently urged the president to lift the debt limit unilaterally. He said — as he has before — that he won’t do it, that Congress had voted for the spending that resulted in federal borrowing, and should now agree to pay the bill.