House Republicans were warned by a conservative senator Thursday morning that without major changes to their Obamacare repeal bill, it will unquestionably fail in the upper chamber.

The warning came from U.S. Sen. Tom Cotton (R-Ark.), a rising conservative star, who let loose a string of tweets on Thursday morning about the repeal of the hated Affordable Care Act, aka Obamacare.

“What matters in long run is better, more affordable health care for Americans, NOT House leaders’ arbitrary legislative calendar.”

“House health-care bill can’t pass Senate [without] major changes,” Cotton tweeted. “To my friends in House: pause, start over. Get it right, don’t get it fast.”

Cotton’s tweet is ominous, because the Republicans have a slim majority in the Senate: 52 seats out of 100. If just three senators vote nay on the repeal bill, it is dead in the water.

Cotton explained further why he wants a slowdown.

“GOP shouldn’t act like Dems did in [Obamacare],” Cotton said. “No excuse to release bill [Monday night], start voting [Wednesday]. With no budget estimate!”

Cotton was referring to the Democratic rush in 2009 to pass health care reform, later known as Obamacare. In the middle of the Great Recession, former President Obama’s quest ate up much of the year’s agenda, putting the wounded economy on the back burner.

Cotton also seems worried about the “score” of the Congressional Budget Office. Usually, lawmakers wait for a CBO estimate of what a bill will cost. But White House press secretary Sean Spicer told reporters on Wednesday that the CBO made major mistakes in its estimates of Obamacare in 2010.

“What matters in long run is better, more affordable health care for Americans, NOT House leaders’ arbitrary legislative calendar,” Cotton concluded.

House Republicans have been under increasing pressure to rid the nation of Obamacare and fulfill a promise made repeatedly over the last seven years. Republicans voted to repeal Obamacare many times, including a vote in late 2015 that won the support of every single congressional Republican.

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The U.S. House Ways and Means Committee approved the GOP plan early Thursday morning. Cotton’s tweets followed, according to The Hill.

The bill is considered so important, the U.S. Energy and Commerce Committee worked through Wednesday night, and was still working as of Thursday morning, according to The Hill. The committee was judging possible amendments to the repeal bill, named the American Health Care Act.

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House Republicans would like to get the bill to the floor and approved before they leave for the Easter recess.

House Speaker Paul Ryan (R-Wis.) defended the House calendar Wednesday night on “Tucker Carlson Tonight.”

Ryan said he is committed to repealing Obamacare and pulling off tax reform in their “200 day agenda.” But House Republicans are taking heat for only being in session for eight days in April.

Ryan told Carlson that in the 200 days planned for major reforms, the House will repeal and replace Obamacare, overhaul the tax code, rebuild the military, pass an infrastructure bill, repeal regulations, repealing the Dodd-Frank financial bill and more.

House Republicans likely suspected they would not please everyone. But House leaders, including Ryan, seem taken aback by the criticisms of the repeal bill. President Donald Trump, who supports the House bill, has also gone on a “charm offensive” to woo conservative activists and lawmakers who are on the fence. On Wednesday night, he had dinner with conservative Sen. Ted Cruz (R-Texas).

On the Senate side, at least four senators said they would not support a bill that pulled back the expansion of Medicaid within Obamacare. All four Republican senators are from states that agreed to the Medicaid expansion.

They are Sens. Rob Portman of Ohio, Shelley Moore Capito of West Virginia, Cory Gardner of Colorado, and Lisa Murkowski of Alaska.

And from the conservative wing of the GOP in the Senate, U.S. Sen. Rand Paul (R-Ky.) reintroduced the 2015 repeal bill, which he called a “clean repeal” of Obamacare, removing all aspects of the law and allowing the GOP-controlled Congress to then add back in parts it wants to keep.

“Clean repeal” of Obamacare is popular among conservatives, but House Republicans warn it opens up the filibuster option for Democrats.

House Republicans want their first step in repealing Obamacare to be through “budget reconciliation,” so Democrats cannot shut down debate using filibuster. Ryan has said his repeal bill is only the first step in untangling Obamacare from the law books.