WASHINGTON (Reuters) - President Donald Trump has launched a charm offensive of the type not seen before in his brief and chaotic tenure, forcefully rallying behind legislation to repeal the Obamacare healthcare law while trying to placate the bill’s opponents. In doing so, the often blustery Trump faces a test of credibility for the voters that catapulted him into office: How does a celebrity outsider, the CEO president, cut deals in Washington? Does the New York businessman live up to the image of dealmaker in chief? Interviews with more than a dozen White House and congressional aides, members of Congress and conservative activists offer a glimpse into his attempts at conducting the most formidable, high-stakes negotiation of his presidency. They show a more circumspect Trump than many see publicly. While they acknowledge he can make his points with a blunt and combustible style, he appears to be doing more listening than talking, they said, trying to appease both supporters and critics by signaling flexibility over legislation that faces criticism on multiple fronts.  Democrats and some influential Republicans say it would rip health insurance away from millions of Americans and increase costs for many others, including voters who helped elect Trump - a problem that could haunt his fellow Republicans in 2018 congressional elections. Conservatives say it does not go far enough in gutting the Affordable Care Act, President Barack Obama’s signature healthcare reform passed by Democrats in 2010. Republicans have long sought to dismantle the law, which they see as government overreach. Trump has called Obamacare a “disaster” and made its repeal and replacement a key campaign pledge. The political stakes are immense for an eight-week-old  presidency marked by instability, infighting, battles with the media, questions over temperament and a stubborn investigation into ties between his campaign and Russian intelligence. “A lot of times you have politicians who gather in a room to pontificate. That’s not why he has gathered people in the room,” a senior White House official said of Trump’s negotiation style this week. “He’s gathered people in to hear their opinions. I think that’s lost a little bit because he does speak so forcefully. He definitely does let them say their piece, and he listens.” The president has reached out to influential conservatives such as U.S. Senator Ted Cruz and groups such as the Heritage Foundation and Americans for Prosperity, which have expressed skepticism about the bill. “He was gregarious, reasonable. He listened. It was a not a lecture,” said Tim Phillips, president of AFP, a group backed by billionaires Charles and David Koch and part of a small group of conservative leaders who met with Trump in the Oval Office on Wednesday night. “He said: ‘This is a negotiation. Let’s figure out ways to make this proposal better,’” Phillips said of Trump.  Trump has indicated he will only go so far to make conservatives happy, insisting the core elements of the bill must remain intact if it has any chance to pass the House of Representatives and then the Senate, both controlled by Republicans. One sticking point involves the use of tax credits to help consumers purchase health insurance, which Trump favors. “He’s made it clear this is the vehicle to finally undo the damage of Obamacare and repeal and replace it,” said another senior White House official. “And if it can be improved in this process, he has encouraged that.” Trump is operating with a razor-thin margin for success. A defection by 20 or so Republicans in the House could sink the bill’s prospects. There is already discontent among some in the Senate, where Republicans hold an even slimmer edge. Democrats and groups such as AARP, which advocates for older Americans, and the American Medical Association have come out strongly against the bill.  Conservatives in the House and advocacy groups opposed to the bill would like to slow the process and rework its fundamentals. They argue the legislation retains basic facets of Obamacare, including federal assistance to purchase health insurance and penalties if coverage lapses.  House Speaker Paul Ryan is hoping to pass the legislation within two weeks so the House can move on to other priorities. That leaves little time for wholesale alterations. Representative Kevin Brady, chairman of the House Ways and Means Committee, said he expected the bill would largely remain in its current form. “I know lots of people have good ideas. That’s terrific. And those will fit in future bills.” The White House has tried to persuade conservatives that the House bill is just the first step in a three-step process, and will soon be joined by a companion bill that would embrace some of their policy priorities. Regulations put in place by Secretary of Health and Human Services Tom Price will also address their concerns, aides said.  The challenge for Trump is whether he can convince enough wary conservatives to back the first step of the plan without being able to guarantee the other phases will come to pass. It could leave them on the record voting for a bill they do not feel adequately dismantles Obamacare. Some conservatives may also not see the point of sticking their necks out backing a bill that may die in the Senate. “This is a futile effort,” said Rachel Bovard, a policy analyst for the Heritage Foundation, which opposes the bill. The White House is aggressively making the case that the House bill is the best chance to do away with Obamacare. Moving too far rightward to placate conservatives could stir up opposition from moderate Republicans and lead to a bill that stokes a powerful backlash among millions of Americans who would lose health insurance - including many Republicans. Many Democrats are already planning to run campaigns on the issue. The White House was busy this week trying to reassure moderate Republicans as well. Vice President Mike Pence is holding meetings in Congress, including with Senate Majority Leader Mitch McConnell, a Republican aide said. Trump’s budget director, Mick Mulvaney, a former House member, has invited conservative lawmakers to dinner at the White House next Tuesday.  Trump plans to leverage the power of his office in another way, making trips to Kentucky and Tennessee in coming days to sell the House bill to the American public.  Earlier this week, Trump welcomed about 30 Republican House members, many of whom said they had never been in the White House before - a contrast in style from Obama, who was often criticized for not attempting to engage more fully with Congress. In the East Room, Trump told them to come back every week.  Grover Norquist, a longtime conservative tax advocate, praised Trump’s strategy, saying: “He is making people feel loved and appreciated and part of the team.”  