Republican Congressman John Boehner has formally replaced Democrat Nancy Pelosi as Speaker of the US House of Representatives following the mid-term elections.
It is the third most senior position in US politics after the President and Vice-President.
The Republican-majority chamber ends four years of Democratic Party control and is expected to challenge President Barack Obama's legislative agenda.
Mr Boehner told the House that there could be no delay in seeking change.
‘We gather here today at a time of great challenges,’ he said in his inaugural address yesterday, citing high unemployment, painful health care costs, and a swelling national debt now at $14 trillion (€10 trillion)
‘Hard work and tough decisions will be required of the 112th Congress. No longer can we fall short. No longer can we kick the can down the road. The people voted to end business as usual, and today we begin carrying out their instructions,’ he said.
Mr Boehner has led House Republicans in opposition to Mr Obama's policies, notably his signature overhaul of US healthcare, which the new speaker has vowed to target with a symbolic repeal vote on 12 January.
As Speaker, and backed by Republican committee chairs, he will enjoy vast powers to shape the agenda in Washington and hamstring the White House through to Mr Obama's re-election campaign in 2012.
But with Democrats still in control of the Senate, and Mr Obama still holding a veto, the Ohio lawmaker will have to harness his considerable deal-making powers to enact key parts of the Republican platform.
Meanwhile, The Congressional Budget Office has warned the republicans in the House of Representatives that the cost to repeal the healthcare law enacted last year would add to the huge federal budget deficits.
In a preliminary estimate of legislation the House is set to begin debating on Friday, the CBO said repealing the law that President Barack Obama and his fellow Democrats enacted would 'increase federal budget deficits over the 2012-2019 period by a total of roughly $145bn (€111bn).' It said that figure would rise to $230bn (€176bn) by 2021.
The CBO also said repeal would result in 32m fewer people having health insurance.
Tomorrow, John Boehner plans to begin a debate on Republican legislation to repeal the healthcare law, which aimed to reduce US medical costs and insure millions of people who currently can not afford coverage.
