Senate changes likely for Republican health care bill Senate skeptics voiced concerns about the House version of the bill.

 -- While Senate Republicans say they are pleased with the progress the House of Representatives is making on its Obamacare replacement bill, there are still several major roadblocks ahead if the legislation makes it to the other side of the Capitol.

First, several Senate Republicans who opposed the first version of the House bill remain skeptical. Second, the Senate would likely have to change the bill significantly, perhaps in ways that might displease House Republicans when the two sides seek to reconcile the different versions.

When the House leadership released its original health care proposal in March, at least eight Senate Republicans said they had fundamental problems with the bill. The conservative Sens. Rand Paul, R-Ky.; Mike Lee, R-Utah; and Ted Cruz, R-Texas, all tweeted that they would oppose anything short of a full repeal of Obamacare. They also said the House bill's tax credit structure to help people pay for coverage amounted to a new entitlement.

Four other senators — Cory Gardner, R-Colo.; Lisa Murkowski, R-Alaska; Rob Portman, R-Ohio; and Shelley Moore Capito, R-W.Va. — from states that expanded Medicaid under the Affordable Care Act, were concerned about the proposal's lack of protections for expansion beneficiaries.

Sen. Susan Collins, R-Maine, who opposed previous GOP efforts to strip federal funding from Planned Parenthood, also expressed opposition to the previous version of the House bill.

Still others, like Sen. Tom Cotton, R-Ark., said it did not do enough to address rising premiums and deductibles and told the House to go back to the drawing board. "The House should continue its work on this bill," he said in a March statement. "It's more important to finally get health care reform right than to get it fast."

House Republicans' new health care bill differs from the first one they proposed because it would remove a guarantee to cover people with pre-existing conditions. Under the new plan, states would have the option to request waivers from that requirement as long as they have another coverage option, like high-risk pools, which some experts have warned will raise costs for that cohort.

None of the holdout senators have to vote on the exact wording in the House version because the Senate would most likely change the bill before its own vote. The question is whether drastic changes would be necessary to get the skeptics on board.

For the Senate to pass a version of the Obamacare replacement bill, the two chambers would likely have to reconcile differences in what's known as a conference committee. It's not clear whether House Republicans who voted for their version of the bill would sign on to changes requested by senators and vice versa — a perennial concern for any legislation that goes through conference committee.

No Senate Democrats are expected to cross the aisle and vote for a replacement health care bill, meaning Senate Republicans could afford to lose only two votes from their 52 members and still pass the bill. Vice President Mike Pence would cast a vote in the event of a tie.

Cruz, who rejected the first House bill, suggested he would be open to using the new version as a springboard in the Senate but added, "I have a number of concerns, and I think many senators do."

"If the bill does pass the House, then it will come to the Senate, and it will be incumbent on us to continue to work to make it better," he added.

A spokesperson for Portman said the senator wants to see a better transition process for changing the way Medicaid is financed and more help for those receiving treatment for opioid addiction.

Capito, who opposed the House bill's Medicaid provisions, said through a spokesperson that the new version does not address her concerns. "She will work to improve the bill if it comes over to the Senate," her spokesperson said.

Aides for the other holdout Senate Republicans have not yet returned ABC News' request for comment.

In any event, the process is not likely to be swift. Senate Majority Whip John Cornyn, the chamber's second-highest-ranking Republican, told reporters Wednesday he thought the process of reconciling the two bills could take weeks.