President Donald Trump is confident a bill replacing key parts of Obamacare will be approved Thursday by the House of Representatives despite some Republicans promising to vote "no," the White House said Wednesday as it rejected calls to delay the vote.

"Yes," said White House spokesman Sean Spicer when asked at a news conference if Trump believes the American Health Care Act will pass the House on Thursday.

"This is the vote. This is the time to act," Spicer said. "This vote needs to happen."

"This is the only train leaving the station," he said of the bill.

Spicer also brushed aside suggestions that the bill is headed for failure in its present form because of the fact that at least 25 Republicans, if not more, could buck their own party's leadership and vote "no" on the plan.

"The count keeps getting stronger for us," Spicer said.

"Member by member we keep seeing tremendous support flow in our direction," he said. "I'm optimistic in the sense of what we're seeing … and the amount of votes that are flowing our way and not the other way."

Spicer's sunny predictions came after the House Freedom Caucus, which is made up of more-conservative members of the House, said that there are more than 25 of its members who oppose the bill.

A spokeswoman for the caucus also suggested that the legislative process to repeal and replace Obamacare should start over.

The bill is expected to need 215 votes to pass, given the planned absence of a Democratic member who will be attending his wife's funeral.

Republican leaders, as a result, cannot afford to lose more than 22 votes from their own caucus, because Democrats are expected to vote uniformly against the bill.

In recent days, Trump, Vice President Mike Pence, and other members of the administration have lobbied conservatives in the House to support the bill. GOP leaders also have made changes to the plan to make it more palatable to conservatives.

On Tuesday, Trump took the unusual step of personally appealing to Republicans on Capitol Hill to back the legislation. He warned that many of them risk losing their seats in Congress next year if they vote against the bill.

"He's not there to threaten them," Spicer said Wednesday of Trump. "He's there to explain the political landscape to them."

Conservative opposition to the bill is based on the belief that it does not do enough to repeal Obamacare, and would continue, albeit to a reduced extent, federal subsidies to help people buy individual health plans.

If the bill does manage to get passed by the full House on Thursday, it could face even tougher opposition in the Senate, where both conservative and moderate members of the GOP caucus have problems with the plan. Moderates are concerned that millions more people will become uninsured if the bill is signed into law.