(Corrects 6th paragraph to make clear Messer is not chairman of the Republican Study Committee)
By David Morgan and Richard Cowan
WASHINGTON, March 23 (Reuters) - U.S. President Donald Trump made a final push on Thursday to win over skeptical members of his own Republican Party to begin dismantling Obamacare in the House of Representatives or risk failure on one of his top legislative priorities.
The effort is seen by financial markets as a crucial test of Trump's ability to work with Congress to deliver on his agenda, including planned tax cuts. Republican leaders hoped to vote on the healthcare bill on Thursday, the seventh anniversary of former Democratic President Barack Obama signing his healthcare law.
Although Republicans have a majority in the House, Democrats are united against the bill and Trump and House Speaker Paul Ryan, who has championed the bill, can afford to lose only 21 Republican votes. On Thursday morning, MSNBC said a count by NBC News showed that 30 Republicans were planning to vote "no" or leaning that way.
Trump met at the White House on Thursday morning with some of the bill's strongest opponents - members of the conservative House Freedom Caucus who say it does not go far enough to undo Obamacare.
"We're still open for negotiations," Representative Ted Yoho, a member of the group, told CNN before the meeting. "There is still time."
Republican Representative Luke Messer, a member of the House leadership, said after a meeting with Ryan they were still hoping to hold a vote on Thursday but were awaiting the outcome of Trump’s meeting with the Freedom Caucus. Messer said he expected the chamber to go ahead with a planned evening vote.
Trump urged Americans in a tweet early on Thursday to press their representatives to vote for the bill, known formally as the American Health Care Act.
Uncertainty over the bill has rattled financial markets and a failure just two months into Trump's presidency would be a setback for the White House, which as late as Wednesday said there was "no Plan B" for the healthcare measure.
U.S. stock markets have risen steadily in recent months on optimism over a pro-business Trump agenda but fell back sharply on Tuesday as investors worried that failure to push through the healthcare bill could have a knock-on effect on other Trump priorities such as tax cuts and infrastructure spending.
U.S. stocks were higher in late morning trading on Thursday as investors snapped up beaten-down bank stocks but remained cautious because of the healthcare bill.
"Investors are concerned ... if this vote goes poorly, then what are the implications for tax reform and repatriation of offshore capital," said John Traynor, chief investment officer at People's United Bank in Bridgeport, Connecticut.
The normally well-scripted House, which runs on tight rules that lay out the duration of debates on legislation and approximate times for votes, was anything but scripted on Thursday.
The Rules Committee, which is the gatekeeper for all legislation heading to the House floor, had not yet set the parameters of the debate, including any amendments that would be allowed. Republicans still did not know exactly what would be in the legislation.
An aide to a senior moderate House Republican told Reuters that negotiations were continuing and that there were particular concerns about how the legislation handles low-income constituents between the ages of 50 and 64.
If certain provisions were added or removed from the bill to please the House Freedom Caucus, the leadership would lose the support of moderates, the aide said.
"With the HFC (House Freedom Caucus) at the White House now, who knows what they’ll do to the bill," said the aide, who asked not to be identified.
Republican control of the White House, the Senate and the House has given the party a chance to achieve what it has aimed to do for seven years - overturn Obama's 2010 Affordable Care Act.
The law aimed to boost the number of Americans with health insurance through mandates on individuals and employers, and income-based subsidies. Some 20 million Americans gained insurance coverage through the law.
The House replacement plan would rescind the taxes created by Obamacare, repeal a penalty against people who do not buy coverage, slash funding for the Medicaid program for the poor and disabled, and modify tax subsidies that help individuals buy plans.
But while conservative Republicans are pushing for more aggressive cuts to Obamacare mandates, some moderates in the party are worried that the legislation would hit some of their core voters by depriving them of insurance.
Changes to the bill to satisfy conservatives would carry the risk of turning moderates against it, and also of complicating the work of their colleagues in the Senate.
The nonpartisan Congressional Budget Office estimated 14 million people would lose medical coverage under the Republican plan by next year. It also said 24 million fewer people would be insured by 2026.
Before Trump's meeting on Thursday with Freedom Caucus Republicans, the White House sought to convey optimism.
“We are moving full steam ahead," said White House spokeswoman Sarah Sanders. "President and his team talking to members letting them know this is the bill to repeal and replace Obamacare and today is the day to get it done."
Graphic on Obamacare and Republican healthcare bill (tmsnrt.rs/2n0ZMKf)
Graphic on shifting positions in the U.S. Senate on Republican healthcare bill (tmsnrt.rs/2mUE4Xf)
Graphic on poll on Americans' views of the Republican healthcare bill ( tmsnrt.rs/2n7f3e4)
Additional reporting by Roberta Rampton, Susan Cornwell, Jeff Mason, Doina Chiacu, Steve Holland and Richard Cowan; Writing by Frances Kerry; Editing by Michael Perry and Bill Trott