* Boehner says groups lose credibility in opposing deal
* Republicans thwart effort to extend unemployment benefits
* Bill seen winning House passage with bipartisan vote
WASHINGTON, Dec 12 (Reuters) - U.S. House of Representatives Speaker John Boehner erupted in anger at conservative groups who oppose a rare bipartisan two-year budget deal on Thursday as lawmakers prepared to vote on the measure in the lower house.
The Republican Boehner said groups had "lost all credibility" and stepped "over the line" by urging lawmakers to oppose the bill, which would avoid showdowns for about two years and blunt some of the automatic budget cuts known as the "sequester".
The agreement itself appeared to be headed for passage in the Republican-dominated House with a combination of Democratic and Republican votes.
A preliminary procedural vote ran along strict party lines as Democrats complained the bill did not extend long-term unemployment benefits that expire at year's end.
"We are talking about people living on the edge, 1.3 million Americans that will lose unemployment benefits this holiday season. It is cruel, it is morally wrong, it is economically stupid," said Representative Barbara Lee, a California Democrat who had authored a three-month extension for the benefit checks.
Although the procedural vote effectively thwarted her effort, Democrats said they would support the underlying deal, which would mitigate some of the "sequester" cuts by boosting discretionary outlays by $63 billion over two years and provide $23 billion in additional deficit reduction over 10 years.
The remaining question appeared to be how much public strife the budget deal would cause among Republicans at a time when they have been trying to draw attention away from their own ideological split and toward the failings of President Barack Obama's health care law.
Boehner, who has been going along reluctantly but quietly with Tea Party-oriented conservatives on most fiscal issues until now, blasted groups such as the Heritage Foundation, the Club for Growth and Freedom Works for egging on House members to oppose compromise both now and during the fall showdown that led to a 16-day partial government closure.
"They pushed us into this fight to defund Obamacare and to shut down the government. Most of you know, and my members know, that wasn't exactly the strategy that I had in mind," he said during a news conference.
"But, if you'll recall, the day before the government reopened, one of the people at one of these groups stood up and said, 'Well, we never really thought it would work'."
"Are you kidding me?" Boehner said, grabbing the podium and raising his voice for emphasis.
Asked if he was asking the outside groups to stand down, Boehner said, "I don't care what they do ... There just comes a point when some people step over the line."
A source close to Mitch McConnell, the Republican Minority Leader of the U.S. Senate, said he would vote against the measure if and when it arrives in the Senate next week.
McConnell, who faces a challenge from a Tea Party conservative in his bid for re-election from Kentucky, opposes the deal because it would increase the spending level for government programs to about $1.012 trillion this year, up from $967 billion that had been previously scheduled.
In breaking ranks, he joined other Republican senators, including potential presidential contenders Marco Rubio of Florida and fellow Kentuckian Rand Paul.
McConnell is facing a primary re-election challenge from a more conservative candidate who is casting the incumbent as too soft on government spending.
A number of conservatives in the House of Representatives also have said they would vote against the deal, probably enough to require Democratic votes for it to win passage. Boehner wants at least a majority of the 232 Republicans who control the House to support the measure.
Conservative groups kept up their barrage of criticism, with the Heritage Foundation saying the deal's promised savings may never materialize.
"It spends long before it saves. The more you buy, the more you spend. Spending is the now - and the later may never come," the group said in a blog posting.
House Budget Committee Chairman Paul Ryan, who negotiated the deal with his Democratic counterpart, Senate Budget Committee Chairwoman Patty Murray, said he would be able to persuade House Republicans to support the measure.
"A good majority of our colleagues I do believe will be supporting this. So I feel good about where we are in the House Republican caucus," he said on CNBC's "Squawk Box" program.
House Democratic leader Nancy Pelosi said her party would reluctantly support the deal, even though it omitted some Democratic demands, such as extending unemployment insurance.
"I don't think that our members will let this vote go down," Pelosi told reporters. "While they don't like this bill ... it's an ok thing to vote for."