Four Republican senators say they cannot vote for the party’s plan to replace the law commonly known as Obamacare, but not because it’s too tough. They say it doesn’t go far enough.

Rand Paul of Kentucky, Ted Cruz of Texas, Mike Lee of Utah and Ron Johnson of Wisconsin penned a statement to colleagues saying the bill unveiled Thursday falls short. Without at least two of their votes, a bill cannot pass.

“Currently, for a variety of reasons, we are not ready to vote for this bill, but we are open to negotiation and obtaining more information before it is brought to the floor,” the joint statement said.

“There are provisions in this draft that represent an improvement to our current health care system, but it does not appear this draft as written will accomplish the most important promise that we made to Americans: to repeal Obamacare and lower their health care costs,” according to the statement.

Other senators have expressed doubts about parts of the bill, but they are yet to stake out opposition as starkly as Paul and company.

Read:Senate Republicans take the wraps off their health-care draft

Republicans need 50 votes to pass their replacement for Obamacare, more formally known as the Affordable Care Act.

Although corralling enough votes will prove dicey, Republicans are under tremendous pressure to repeal Obamacare. If they fail, they could alienate a sizable portion of their conservative base and undermine their own ability to govern. President Trump would also suffer a political blow.

Trump has already tried to cajole Paul to back the Senate bill, and he’s expected to ratchet up the pressure. Yet if the Republican leadership moves to satisfy the concerns of its most conservative members, it risks losing support from Senate moderates such as Lisa Murkowski of Alaska and Susan Collins of Maine.

Read: Why did these generic drugs’ prices jump as much as 85%?

The Republican-controlled House of Representatives faced a similar divide earlier in the year, but the party eventually cobbled together enough votes for passage (and a Rose Garden celebration), and now the onus is on the Senate.