By Jeff Mason and Steve Holland
WASHINGTON, April 20 (Reuters) - The latest delay to a final decision on the Keystone XL oil pipeline will reinforce a White House strategy to energize President Barack Obama's liberal-leaning base before fall elections in which Democrats risk losing control of the U.S. Senate.
Environmentalists, worried about the project's effect on climate change, have put enormous pressure on the president to reject the pipeline from Canada's oil sands, staging demonstrations outside the White House and protests in states where he travels.
A decision to approve it now could have prompted that vocal group, which was instrumental in electing Obama in 2008 and 2012, to sit out the Nov. 4 congressional elections.
The State Department's announcement on Friday that it would give government agencies more time to study the project was seen by strategists from both parties as a move to prevent that and boost Obama in the eyes of his supporters. Support for the president, or lack of it, is generally reflected in mid-term voter turnout.
Approval of the pipeline would also have risked dampening the enthusiasm of wealthy donors such as billionaire investor Tom Steyer, who is spending tens of millions of dollars to boost environmentally-friendly candidates.
"This is rotten eggs for TransCanada and good news on Good Friday for those who oppose Keystone as not being in our nation's best interest," Steyer said in a statement.
Obama cannot run for re-election again, but the outcome of the congressional elections, particularly control of the Senate, will determine how much of his agenda can be enacted during his final two years in office.
Mobilizing the parts of his base that showed up to vote in 2012, including environmentally-conscious young people, gays and lesbians, Hispanics, and women, is key to helping Democrats in a year when the White House is not up for grabs.
But the delay of TransCanada Corp's proposed project to bring oil from Alberta to the U.S. Gulf Coast does carry risks for Obama's party. Republicans, who argue the pipeline would boost job creation, have used the administration's delays to attack Democrats in conservative-leaning states.
BOOSTING CORE SUPPORTERS
In November, all of the seats in the House of Representatives and one-third of the Senate will be up for election. Republicans already control the House and need a net gain of six seats to take control of the Senate.
In recent weeks, Obama has warned of the risks to the party if disillusioned Democrats do not vote in November, and the White House has taken other actions to rev up core Obama supporters. Those include regulations designed to ensure women working for federal contractors are paid equally for work that is similar to that done by men.
In addition, Obama has sought to stress his healthcare program's success in signing people up for insurance as a win for Democrats. The party was stung by a shaky rollout last fall.
"The biggest problem in any mid-term, especially in a six-year mid-term, is having some level of enthusiasm in the president," said Norm Ornstein, a political expert at the American Enterprise Institute.
"If you have a lot of people who are angry at you or disillusioned and sit on their hands, you're going to have a disaster in November."
Defeating Keystone XL remains a top priority for Obama's base, and delays on the decision have become a common occurrence. In 2011 the administration said it would study a new route for the pipeline, pushing the process past the 2012 presidential election. The following year further delays were announced.
The State Department said the postponement would allow time for the Nebraska Supreme Court to settle a dispute over the proposed path for the pipeline.
A department spokeswoman said politics did not play a role in the decision.
Environmentalists welcomed the latest delay. "Mostly we think this helps us," said Michael Brune, executive director of the environmental organization Sierra Club. "The only thing better than a pipeline that shouldn't be built is to delay it for up to another year or more."
Republicans, meanwhile, indicated the decision would provide fresh fodder for their criticism of Obama over the pipeline. Republicans are looking for new lines of attack on the president after his healthcare law, known as Obamacare, recovered from last year's acute teething problems.
Many Democrats are vulnerable to attack, and some worried that the delay on the pipeline decision could undermine moderates such as Louisiana Senator Mary Landrieu and Arkansas Senator Mark Pryor whose re-election bids will help determine whether Republicans gain control of the Senate.
After the State Department's announcement, Landrieu promised constituents in her energy-producing state that she would wield her power as chair of the Senate Energy Committee to get the Keystone project approved.
Landrieu was one of 11 Democratic senators who urged Obama in a letter a week ago to make a decision on the project by May 31. But even if she and her colleagues joined Republicans to pass a bill compelling Obama to approve Keystone XL, they would have a difficult time reaching the necessary two-thirds majority in the 100-person Senate to override a presidential veto.
"We don't think it's particularly likely," said Christine Tezak, Managing Director at ClearView Energy Partners, LLC about the possibility of Congress forcing the issue.
That in itself left an opening for Republicans to exploit.
"The biggest impact from our perspective is this takes away the argument from red-state Democratic senators that they have influence in Washington to push things through like Keystone," said Tim Miller, executive director of America Rising, a Republican super PAC. (Additional reporting by Richard Cowan, Roberta Rampton, and Valerie Volcovici; Editing by Caren Bohan and Frances Kerry)