WASHINGTON, Sept 23 (Reuters) - U.S. President Donald Trump on Saturday blasted Senator John McCain for dealing a possibly fatal blow to the latest Republican attempt to dismantle Obamacare.

According to a new independent analysis, the bill awaiting a Senate vote could lead to 21 million fewer Americans having health insurance.

McCain, an Arizona Republican who is being treated for brain cancer and cast a crucial “no” vote to defeat a similar bill in July, said on Friday that he could not “in good conscience” vote for the proposal authored by Republican Senators Bill Cassidy and Lindsey Graham.

“He campaigned on Repeal & Replace. Let Arizona down!” Trump wrote about McCain on Twitter early Saturday morning.

Senate Majority Leader Mitch McConnell has said he will schedule a vote on the Graham-Cassidy bill by Sept. 30, the last day when it could pass in the chamber with a simple majority of 51 votes instead of the 60 typically required.

The bill would take federal money spent on the Medicaid program for the poor and disabled, as well as subsidies to help individuals buy private insurance, and deliver it to the states in block grants.

The non-partisan Congressional Budget Office, which analyzes legislation, has not had time to assess the Graham-Cassidy bill before the expected vote. McCain, a close friend of Graham, said he could not support it without knowing how much it would cost, its effect on insurance premiums, and how many people would be affected.

But the Brookings Institution said on Friday that the Graham-Cassidy bill could leave at least 21 million fewer Americans with health insurance by 2020 to 2026. The Washington think tank has generally been supportive of Obamacare, which is formally known as the Affordable Care Act.

The coverage estimate “likely understates” reductions in insured Americans because it is not clear how states would use the money or if they would face obstacles in setting up new programs, according to Brookings researchers.

“What is clear, however, is that the legislation would result in very large reductions in insurance coverage,” they wrote.

CBO estimates of previous Republican proposals showed they could lead to more than 20 million fewer insured Americans. This complicated passage in the 100-member Senate, where Republicans hold 52 seats and Democrats are unanimously opposed to repeal-and-replace measures.

Republicans need at least 50 votes to pass the bill, relying on Vice President Mike Pence to break a tie. Republican Senator Susan Collins of Maine has said she was leaning against the Graham-Cassidy bill, as has Senator Rand Paul of Kentucky. Kansas Senator Jerry Moran and Alaska’s Lisa Murkowski are undecided.

After blasting McCain, Trump on Saturday singled out Paul and Murkowski.

“I know Rand Paul and I think he may find a way to get there for the good of the Party!” Trump wrote on Twitter.

Regarding Murkowski, he tweeted that in Alaska: “Deductibles high, people angry!”

The insurance industry, hospitals, consumer activists, the AARP advocacy group and organizations such as the American Medical Association, American Heart Association and American Cancer Society all oppose the bill.

Shares of some health insurers closed higher on Friday after McCain announced his opposition. Centene Corp ended up 1.6 percent, while Humana Inc gained 0.2 percent and Aetna Inc rose 0.1 percent, reversing earlier losses. (Additional reporting by Susan Cornwell; Editing by John Walcott and Lisa Von Ahn)