By Richard Cowan and Susan Cornwell

PHILADELPHIA (Reuters) - When President Donald Trump was elected last November, Republican lawmakers enthusiastically joined his call to rewrite the tax code and dismantle Obamacare in the first 100 days of his presidency.

But as congressional Republicans gathered for an annual policy retreat in Philadelphia on Wednesday, the 100-day goal morphed into 200 days. As the week wore on, leaders were saying it could take until the end of 2017 - or possibly longer - for passage of final legislation.

Trump had a different idea when he spoke to lawmakers in Philadelphia, telling them: Enough talk. Time to deliver.

The divergent views on the timetable were among many indications of tensions that simmered just below the surface at the three-day Republican retreat.

Before the cameras, Trump and Republicans sought to convey an image of a happy, unified family, playing down differences over tax policy, whether to reinstate torture interrogation techniques and investigating 2016 election fraud.

And clearly there is none of the open warfare that has sometimes erupted among Republicans, such as when Senator Ted Cruz infuriated many of his colleagues by leading a standoff over Obamacare that partially shut down the government in 2013.

But barely visible in Philadelphia, there are potential flashpoints of disagreement within the Republican rank-and-file in Congress as well as between Republican lawmakers and the unorthodox new president.

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These include how and when to replace Obamacare if Republicans succeed in their quest to repeal it; how to revamp the multi-layered tax code, whether to build a wall on the U.S. border with Mexico and the nature of the U.S. relationship with Russia.

When it comes to tax reform, senior congressional aides said the spring of 2018 might be a more likely time than this year for the passage of legislation.

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EXPENSIVE WISH LIST

Republican lawmakers lavished praise on Trump in public. In dozens of interviews, many said they felt he would be an energetic champion of issues they cared about. But some also voiced fears that his big agenda would drive up deficits and said they were still searching for details on his plans.

Several Republican lawmakers and aides said they were wary of the cost of his plan to build a wall on the border with Mexico. Republican leaders have said the wall proposal under discussion would cost $12 billion to $15 billion cost but some congressional aides say it could end up easily topping $20 billion.

Republican Representative Will Hurd, whose Texas district partially borders Mexico, went a step further, calling the wall an ineffective tool for stopping illegal immigration.

Others warned a border adjustment tax on foreign goods to pay for that wall could hurt U.S. companies' profits, raise costs for American consumers and spark retaliation by foreign trading partners.

Some lawmakers also worry that some of their constituents could be at risk of losing healthcare coverage if the push to repeal Obamacare moves too quickly.

Republican Representative Tom Cole said rank-and-file lawmakers have an incentive to fall in line behind Trump.

"You don’t want to be the reason why we weren’t successful in getting these things done," he said.

Still, Cole said Republicans are taking stock of the potential cost of the biggest items on Trump's agenda such as the wall, infrastructure projects, tax cuts and beefing up military spending.

"I think they worry about it," Cole said.

Following Trump's speech to the lawmakers on Thursday, Senator James Risch said that no decisions had been made on the replacement of Obamacare, a complex law that has expanded healthcare insurance to millions of Americans.

"It's going to take a while to resolve it," Risch said. Asked by reporters whether Republicans had a clear idea of what Trump would like to replace Obamacare with, Risch responded, "In detail, no."

(Reporting By Richard Cowan, David Morgan and Susan Cornwell; Editing by Caren Bohan and Alistair Bell)