By MJ Lee and Stephen Collinson

CNN

WASHINGTON (CNN) -- Republicans opened their effort to quickly drive a bill to repeal Obamacare through Congress in simultaneous committee sessions Wednesday that could extend late into the night and possibly into the morning.

Kevin Brady, chairman of the House Ways and Means Committee, said that the Affordable Care Act was collapsing but that "relief is on the way" as his markup got under way following a full-throated endorsement of the bill from President Donald Trump, who is planning to mount an intense lobbying effort to get the measure quickly out of the House and to the Senate.

"Today's markup is a critical step to providing all Americans with affordable, patient-centered health care that is tailored to their needs," Brady said.

Democrats are however complaining that the hearings are taking place before the Congressional Budget Office has a chance to "score" the House legislation, a process that will provide answers on how much it will cost and how many people it will cover.

Rep. Richard Neal, the top Democrat on the Ways and Means panel, cited the lack of the report as a reason the effort should be postponed. "To consider a bill of this magnitude without a CBO score is not only puzzling and concerning, but also irresponsible," Neal said.

Democratic Rep. Lloyd Doggett said the GOP bill had been kept "as secret as Donald Trump's tax returns."

"It is surprising that you're so determined to hide your panacea," the Texas congressman told his Republican colleagues.

Democrats are preparing to flood their colleagues across the aisle with a long list of amendments to the legislation. While most, if not all, of the amendments are expected to fail, Wednesday offers supporters of Obamacare an opportunity to force Republicans to take formal positions on thorny health care issues.

One Democratic committee aide said the intent is to "make some Republicans take some really tough votes" that are "revealing" about GOP's priorities on health care.

The bill has already drawn criticism from rank-and-file lawmakers, powerful conservative groups and key senators. Trump vowed Tuesday to throw his full support behind the effort, saying he is "proud" to support a GOP-authored plan to replace Obamacare and told members behind closed doors that he would back it "100%," according to sources in a meeting between Trump and House Republicans at the White House.

But he warned lawmakers of the high-stakes nature of the effort, citing a potential electoral "blood bath," a member present said.

The Energy and Commerce Committee, headed by GOP Chairman Greg Walden and Democratic ranking member Frank Pallone, has jurisdiction over Medicaid. The Ways and Means Committee has jurisdiction over tax credits.

Both issues are central to the ongoing debate on overhauling the current health care system.

The GOP legislation unveiled Monday would get rid of Obamacare's individual mandate and put in place refundable tax credits for individuals to purchase health insurance. It also proposes restructuring Medicaid and defunding Planned Parenthood.

The bill looks to preserve some of the more popular elements of Obamacare, including protections for people with pre-existing conditions (though insurers would be allowed to charge higher premiums to individuals whose coverage has lapsed) and letting children stay on their parents' insurance plans until the age of 26.

GOP leaders have said that the bill will not be brought to the floor until the CBO scores the legislation.