By David Lawder and Richard Cowan

WASHINGTON (Reuters) - Republicans will use their new dominance of Congress to repeal or cut back President Barack Obama's health care reforms, approve the Keystone XL oil pipeline and trim the nation's debt, House of Representatives Speaker John Boehner said on Thursday.

In his first news conference since Republicans seized full control of Congress in Tuesday's elections, Boehner also warned that Obama will jeopardize any hope of passing immigration reforms during the remainder of his presidency if he uses executive action to loosen immigration regulations.

"I've made clear to the president, that if he acts unilaterally, on his own, outside of his authority, he will poison the well and there will be no chance of immigration reform moving in this Congress. It's as simple as that," Boehner said.

Before the end of this year, Obama is expected to use his executive authority to help possibly millions of undocumented residents who have been living in the United States for an extended period, many raising families here. His actions could allow many of them to seek jobs without fear of deportation.

In June 2013, the Senate passed a bipartisan bill to rewrite immigration laws that was later blocked by House conservatives. Obama has since pledged to help some of the nearly 12 million undocumented people in the United States.

After more than 50 failed attempts to repeal or neutralize Obamacare, Boehner said the House would vote again in 2015 to scuttle the law, even if Democrats can still block it in the Senate and it faces a certain veto from Obama.

The House will then move quickly to eliminate individual parts of the law, Boehner said, including a tax on medical devices, a provision that Americans without health insurance must pay a tax penalty and a 15-member healthcare payment advisory board expected to launch next year that has been derided by Republicans as a "death panel."

Boehner said he believes there are some Democrats who would support such alterations to the healthcare law.

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"The American people have made it clear: they're not for Obamacare. Ask all those Democrats who lost their elections Tuesday night. A lot of them voted for Obamacare," he said.

The Ohio Republican said the House also would pass dozens of business-friendly bills aimed at boosting job growth and approve TransCanada's Keystone XL crude oil pipeline, which would connect Canada's oil sands to U.S. Gulf Coast refineries.

White House spokesman Josh Earnest said Obama was committed to his current review process for the pipeline, and noted there was a clearly established precedent requiring a presidential decision on projects crossing international borders.

Obama has said he is studying whether the pipeline would add to greenhouse gas emissions and global warming.

"We haven't seen what Congress is prepared to do, specifically, on this," Earnest said, declining to say whether Obama would veto a potential Keystone bill.

(Additional reporting by Amanda Becker and Jeff Mason; Editing by John Whitesides and Dan Grebler)