WASHINGTON - US President Donald Trump and congressional leaders are weighing changes to their effort to dismantle the Obamacare health law, a White House spokesman said Tuesday, as Republicans defended their plan following an estimate that it would cause 14 million Americans to lose insurance next year.

Press Secretary Sean Spicer said White House officials and leaders in the Republican-controlled US House of Representatives were considering whether they could tweak the plan, which faces growing doubts within party ranks.

The effort to bolster support came a day after the nonpartisan US Congressional Budget Office (CBO) estimated that by 2026, the plan would boost the number of people without health insurance by 24 million.

The findings make it harder for Republicans to sell the Republican president's first major piece of legislation and deliver on a longtime pledge to undo Democratic former president Barack Obama's signature domestic policy achievement.

Trump was set to speak by phone to House Speaker Paul Ryan and Majority Leader Kevin McCarthy, and he has met with numerous Republican lawmakers in recent weeks.

He spoke to managed care company Anthem Inc Chief Executive Joseph Swedish earlier on Tuesday.

"Part of the reason we are engaging with these individuals is to hear their ideas," Spicer said. "We are obviously in talks with House leadership about the contents."

Republicans' plan to replace the 2010 Affordable Care Act, popularly known as Obamacare, would repeal a requirement that every American has health insurance and slash spending on the Medicaid program for the poor and disabled.

It would also revamp tax subsidies that help people buy insurance if they do not get it from work. The amounts would be based on age, not income.

Democrats say the Republican plan would hurt the elderly, the poor and working families but give tax cuts to the rich. Doctors, hospitals and advocates for patients and senior citizens also have panned it.

Spicer said the plan would give Americans more choice in insurance. He said the CBO struggles to estimate insurance coverage. Before the 2010 law passed, the office projected many more people would buy individual insurance plans through new online marketplaces than actually did.

The White House has, however, promoted the CBO's estimate of the plan's effect on federal deficits.

The report said it would cause deficits to fall by $337 billion between 2017 and 2026.

Moderates, particularly in the US Senate, worry about causing people to lose coverage, while conservatives do not think the bill goes far enough in undoing Obamacare.

House leaders have resisted a conservative push to bring a quicker end to Obamacare's expansion of Medicaid. But they are considering changes to tax credits to help older, lower-income Americans.

The CBO said that group would see higher insurance premiums under the Republican plan, which would let insurers charge older enrollees more than they can under Obamacare.