The White House has a message for Republicans who oppose Obamacare: This is the best chance you'll get.

Pockets of the GOP who want to repeal and replace the Affordable Care Act oppose the plan backed by President Donald Trump and House Republican leaders. The bill, the American Health Care Act, faces hurdles in passing without changes, especially in a Senate in which three GOP defections kill it.

Press secretary Sean Spicer contended Tuesday that lawmakers fighting the current bill may not get a better chance to repeal President Barack Obama's landmark law.

"This is the only vehicle that seeks to achieve what people on our side of the aisle have been talking about since 2010. This is it," Spicer told reporters at his daily briefing. "If we don't get this through, the goal of repealing Obamacare and instituting a system that will be patient-centered is going to be unbelievably difficult."

For the bill to pass the House, where it already cleared hurdles in two key committees, its supporters need to convince skeptical conservatives, who have said it too closely resembles Obamacare. Spicer said Tuesday that the White House is helping with a "manager's amendment" to appease conservatives. One possible provision is moving the phasing out of Medicaid expansion to the beginning of 2018, rather than 2020, according to Politico.

The plan faces an even tougher time in the Senate, where several moderate GOP senators have already raised concerns about the Medicaid plans and where conservative Sen. Rand Paul of Kentucky has dubbed it "Obamacare Lite." Spicer said the White House is working with both chambers of Congress to resolve concerns and hear ideas.

Some Republican senators, including Ted Cruz of Texas, will attend a meeting at the White House Tuesday in an effort to compromise on the bill, NBC News reported, citing two sources familiar with the matter.

A score from the nonpartisan Congressional Budget Office has added to some senators' qualms. The report estimated that 24 million more people will be uninsured under the plan in 2026 than under current law. While the federal deficit is expected to shrink, older, poorer Americans are estimated to take the worst hit from the changes.

While Spicer said the White House does not have its own estimate of coverage changes, he contended that the CBO is "historically wrong." He said the score did not include the two remaining "phases" of health-care reform Republicans have promised, which he argued would increase consumer choices and drive down costs.