WASHINGTON (AP) — There hasn't been a government shutdown in nearly two decades, but top lawmakers on Capitol Hill are finding trickier-than-usual obstacles in their path as they try to come up with must-do legislation to keep federal agencies running after Sept. 30.

Conservatives making a last stand against President Barack Obama's new health care law and Senate Democrats' resistance to a $20 billion spending cut wanted by many, if not most, Republicans are two of the major problems confronting House Speaker John Boehner, R-Ohio, and other GOP leaders.

The combustible mix raises the possibility of the first government shutdown since the 1995-96 battle between President Bill Clinton and GOP insurgents led by Speaker Newt Gingrich. Republicans got the worst of that battle and have avoided shutdowns ever since.

The prevailing thinking is that it will all get worked out since leaders in both parties want to avoid a shutdown. But unlike last year, when Congress opted to delay debate over the so-called fiscal cliff until past the election and the December holidays, there has been little negotiation this time. The differences on spending levels also are more troublesome than last year.

At issue is what is normally routine: a plug-the-gap measure known as a continuing resolution to fund the government for a few weeks or months until a deal can be worked out on appropriations bills giving agencies their operating budgets for the full 2014 fiscal year, which begins Oct. 1.

The appropriations process is hopelessly tangled this year, in great part because the Democratic-led Senate and GOP-controlled House are more than $90 billion apart on how much to spend on Cabinet agency operations. And Oct. 1 is deceptively close since Congress takes the month of August off and has a limited schedule in September because of the Jewish holidays.

The ordinary thing to do would be to continue running the government on autopilot at current levels as has been done dozens of times since the 1995-96 debacle.

But many tea party Republicans, spurred on by outside groups like the Heritage Foundation and the Club for Growth, which has a history of backing right-wing challengers against incumbents in GOP primaries, are vowing to oppose any short-term bill for keeping the government open that doesn't block spending on Obama's health care law.

"If you pay for a budget that pays for Obamacare ... you have voted for Obamacare," said Sen. Marco Rubio, R-Fla. "Some will say, 'That is crazy. You are going to shut down the government over Obamacare.' No. What is crazy is moving forward with this."

While some parts of that law already have taken effect, the biggest and most expensive provisions are just around the corner. Consumers can start shopping for coverage under the new law on Oct. 1, but the insurance doesn't actually take effect until Jan. 1. The law is expected to help more than 25 million uninsured people eventually, through expanded Medicaid programs and new tax credits to help middle-class people buy health insurance if they don't get it from their employers.

Consumers will shop for coverage through online marketplaces or exchanges in each state but, largely because of political opposition from Republicans, the federal government will be running the markets or taking the lead in 35 states. Virtually all Americans will be required to carry health insurance or face fines. The idea behind this "individual mandate" is to get more healthy people into the insurance pool. A coverage mandate on larger businesses has been postponed until 2015.

In the past, GOP leaders have beaten back efforts that made averting a government shutdown contingent on stopping funding for Obama's health care law. But conservatives are casting this as a last stand against a law they detest.

"It's spreading. It's kind of getting out beyond just the tea party. It's starting to get to regular people that are very frustrated with Obamacare," Rep. Lee Terry, R-Neb., said. "That's why it's getting some legs here in the House."

Some Republicans are nervous about the effort, fearing it could complicate routine passage of a continuing resolution. And they say it's not a winning strategy anyway because Obama brings both a veto pen and the White House podium to the battle.

"I think it's the dumbest idea I've ever heard," said Sen. Richard Burr, R-N.C. "Some of these guys need to understand that if you shut down the federal government you better have a specific reason to do it that's achievable. ... At some point, you're going to open the federal government back up and Barack Obama's going to be president and he won't have signed a dissolution of the Affordable Care Act."

"Shutting down the government, I think, that's almost never a good tactic," said Rep. Tom Cole, R-Okla., whose views usually reflect those of Boehner. "It wasn't good for us in 1995; it's not going to be good for us in 2013."

In the Senate, Mike Lee, R-Utah, is rounding up fellow conservatives to pledge to oppose any continuing resolution that funds implementation of the health care law. But Democrats seem sure to get enough support to hit the 60-vote threshold needed to advance the measure past conservative opposition.

A separate wrinkle involves what spending levels to set. Democrats insist, at a minimum, that spending should continue at rates consistent with the current $988 billion cap on appropriations for the 2013 budget year ending Sept. 30. But current law, set by the hard-fought 2011 budget and debt deal, sets a lower cap of $967 billion for 2014 as required by automatic budget cuts known as sequestration. That's the level demanded by many Republicans, including Senate Minority Leader Mitch McConnell, R-Ky.

"We made this commitment on a bipartisan basis two years ago, and we intend to keep it," McConnell told reporters this week.

Boehner is facing pressure from conservatives to try to force the $967 billion figure upon Senate Democrats. Their leader, Harry Reid of Nevada, has vowed he won't accept it.