Less than a week after House Republicans journeyed to the White House to celebrate passing the GOP health bill, Senate leaders are working behind closed doors on a substantially different package.

Republican senators said they are eyeing major changes to the bill that House members are defending during their one-week recess this week. Republicans hold just a two-vote majority in the Senate and know they can count on no support from Democrats.

“Trumpcare is just a breathtakingly irresponsible piece of legislation that would endanger the health of tens of millions of Americans and break the bank for millions more,” said Senate Minority Leader Chuck Schumer (D-NY).

Senate Republicans plan to use a legislative maneuver known as reconciliation to pass a healthcare bill with a simple majority instead of the 60-vote threshold required for most legislation. But there are some potential deal-breakers even among Republicans. The House bill’s approach to Medicaid and to protections for people with pre-existing conditions are flashpoints for many senators.

The House bill would not only roll back the Affordable Care Act’s Medicaid expansion, it would cut more than $800 billion over 10 years from Medicaid and cap its federal funding for the first time. Many senators have problems too with a provision added to the House-passed American Health Care Act that would allow insurers to charge more to people with preexisting conditions if their states opted out of provisions in the ACA barring such decisions. In the House plan, the states that opted out would have to set up “high-risk pools” to absorb some of the costs of caring for people with pre-existing conditions.

Sens. Susan Collins (R-ME) and Lisa Murkowski (R-AL) have also been on record opposing the provision in the House bill that would cut off federal funding for Planned Parenthood.

The Senate is likely to take its time putting together a package, and wants to have an updated score from the Congressional Budget Office before proceeding. House leaders were heavily criticized for passing a revised version of their health bill without a new estimate on how much the bill would cost and how many people would stand to lose coverage under it. The CBO projected in late March that an earlier version of the House bill would result in 24 million fewer people having insurance in 2018 than under current law.

Senate Majority Leader Mitch McConnell (R-KY) downplayed differences of opinion about the House-passed health bill and said congressional Republicans are unified on repealing the ACA.

“The status quo is unacceptable,” McConnell said last week. “The pain caused by Obamacare is real for millions of Americans. We must repeal and replace this failed law.”