Washington: A battle over an oil pipeline has become the opening salvo in a power struggle between newly empowered Republicans and President Barack Obama that could consume his final two years in office and shape his legacy.

The outcome of the dispute over the Keystone XL oil pipeline will test who really is in charge in Washington: The Democratic president or Republicans who won control of the Senate and expanded their House majority in November elections. It will set the tone for fights ahead over Obama's signature initiatives: diminishing carbon emissions, changing immigration policies and protecting his cherished health care law.

Shrugging off a White House veto threat, the House overwhelmingly passed legislation Friday to build the pipeline, which would carry Canadian tar sands oil across the U.S. midsection to refineries on the Gulf of Mexico. It was the 10th vote the House has taken since July 2011 to advance the $7 billion project.

The difference this time is that the measure is virtually certain to clear the Senate—in the first major Republican show of power since the taking control of the upper chamber this week. Two months ago, with the Democrats still in control, the Senate narrowly turned back the legislation.

Obama has not publicly expressed his opinion on the merits of the pipeline project. In threatening the veto, the president cited unfinished studies about the economic and environmental effects of the pipeline.

Environmentalists and other opponents argue that any leaks could contaminate water supplies, and that the project would increase air pollution around refineries and harm wildlife. Republicans, the oil industry and other backers say those fears are exaggerated, and that the pipeline would create jobs and ease American dependence on oil from the Middle East. They note a U.S. State Department report raised no major environmental objections. The pipeline requires Obama's approval because it would cross the U.S.-Canada border.

The pipeline legislation has the support of at least six Senate Democrats, however, the Republicans are unlikely to be able to muster the two-thirds votes needed in each chamber to override a presidential veto. Still, the Republicans are using the measure to portray themselves as championing bipartisan legislation -- 28 House Democrats voted in favor of the pipeline Friday—and Obama as obstructionist.

Also undercutting Obama's stance, the Supreme Court of Nebraska on Friday rejected a lawsuit challenging the pipeline's route, an obstacle the White House had said must be removed.

The pipeline legislation is moving toward the president's desk just ahead of what likely will be the Republicans' next objective: a serious revision of the rules in the Obama's cherished 2010 health care law. Obama has promised to veto that measure, which would alter the definition of fulltime employees from those working 30 hours weekly to 40. Such a change would greatly undercut the number of people eligible for government subsidized health coverage, endangering the program as a whole.

The fights most likely will continue with Republicans trying to block Obama's anti-climate change moves to stop construction of new coal-fired electricity generating plants and force the closure or refitting of existing plants. Republicans argue that such moves against the coal industry would badly damage the economy.

Like health care, Obama is hoping that sharply reducing U.S. carbon emissions will become part of his legacy. And the president seems determined to use his veto powers to protect what he sees as the fulfillment of promises he made when first elected president in 2008.