Even if the House passes the GOP bill to repeal and replace Obamacare on Thursday, as Republicans hope, their colleagues in the Senate say the bill will face substantial changes in the Senate.

House GOP leaders set up a Thursday vote on the bill in the hopes of finally passing it. Senate Republicans have started sifting through the legislation and are already targeting areas they would seek to change.

"I know the members of the Senate have lots of amendments," said Sen. John Barrasso, R-Wyo., a medical doctor and member of the GOP leadership. "This is going to be an open process on the Senate floor, so we're working to get to yes."

A few GOP lawmakers are backing alternative plans, which further guarantees the final product will have to be a compromise between the two chambers.

"There are undoubtedly going to be some changes," Senate Finance Committee Chairman Orrin Hatch, R-Utah, who will play a key role in writing the compromise, told the Washington Examiner.

Republican agreement is essential. The GOP plans to pass the bill using a budgetary tool that blocks a Democratic filibuster. But they'll need almost all of their 52 lawmakers to vote for it in order to win a simple majority needed for passage.

Passing the House bill in its current form, Hatch warned, "would be difficult to do."

The House bill's essential component is flexibility. It allows states to waive Obamacare mandates, while preserving tax credits and protections for people with pre-existing conditions.

Sen. Tim Scott, R-S.C., said he's not thrilled with the House plan to preserve tax credits for earning up to 850 percent of the poverty level who want to buy insurance. He'd rather spend the money to bring down costs for older consumers, who under the AHCA would pay more.

"I'm not completely in line with that," Scott told the Washington Examiner. "We have to squeeze that down so we can give that money to folks between the ages of 50 and 64."

Many senators this year had been largely tuned out of the details of the evolving House bill, often telling reporters they are waiting to see what their partners across the Capitol can pass, or if they can pass anything at all.

But a few have been actively engaged, including Sens. John Thune, R-S.D., a member of the GOP leadership, and Sen. Marco Rubio, R-Fla.

Rep. Carlos Curbelo, R-Fla., who was undecided on Wednesday, said those two senators have promised to make changes he is seeking if the bill makes it to the Senate, including more money for older consumers who may be charged higher insurance premiums.

"I've gotten some strong assurances from Sen. Thune's office and from Sen. Rubio's office that there would be opportunities to enhance the bill to improve accessibility of resources for those nearing the age of retirement," Curbelo said Wednesday.

Two other Republican senators, Bill Cassidy of Louisiana and Susan Collins of Maine, have drafted their own measure that differs from the House bill in several ways. It includes a provision, for instance, allowing states to maintain Obamacare or create a state alternative. States would be able to auto-enroll residents with a Roth Health Savings Account, and a basic health insurance and pharmacy plan.

Cassidy, who is also a physician, told the Washington Examiner he will push to include Cassidy-Collins provisions in the Senate version of the bill to repeal and replace Obamacare.

"If the House passes something, then the Senate clearly will respond to the House," Cassidy said. "If the House does not pass something, then the Senate will have to respond with something of its own."

This week, worried Republican senators began to focus on the House bill, in part because the congressional calendar is running out and some fear the House GOP won't come up with the votes to pass a bill. Failure in the House would jeopardize repealing Obamacare, which is the number-one Republican campaign promise of the past decade, and it could hobble the rest of the GOP's ambitious agenda.

"I hope they pass it," Senate Majority Whip John Cornyn, R-Texas, told the Washington Examiner.

Republicans face a time limit if they want to send a bill to President Trump without help from the Democrats.

The budgetary tool, known as reconciliation, hinges on the 2017 fiscal year. It expires once Congress takes up the fiscal 2018 budget, or at the latest on Sept. 30, which is the end of the fiscal year.

Cornyn said he believes if the House sends over a bill this week, senators could complete their work and pass their own version of a repeal and replace bill relatively quickly. "It will take weeks," Cornyn said.

There has been limited coordination so far between the House and Senate. As House Republicans have drafted the AHCA, they have consulted with the Senate Budget panel to determine if their bill will satisfy parliamentary rules allowing the use of reconciliation.

Senate Minority Leader Chuck Schumer, D-N.Y., warned again on Wednesday that some of the provisions in the GOP bill, including one allowing states to waive Obamacare's pre-existing conditions coverage, may run afoul of the rules governing reconciliation.

"Even if the new version of TrumpCare passes the House, its chances for survival in the Senate are small," he said.