By Jon Keller

BOSTON (CBS) — As we head into the weekend, it looks like the last-gasp Republican effort to repeal Obamacare is in big trouble.

With every Democrat set to vote no on the so-called Graham-Cassidy bill that would trash Obamacare, throw billions of dollars to the states and let them rebuild their healthcare systems from scratch, the GOP needs at least 50 of the 52 Republican senators to vote yes. (In that case, Vice President Mike Pence would cast the deciding vote.)

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Two Republicans have now said they’ll vote no – Rand Paul of Kentucky and John McCain of Arizona. Senators Susan Collins of Maine and Lisa Murkowski of Alaska are said to be on the fence.

And if in fact Graham-Cassidy goes down the drain, the first poll results on public reaction to the bill help explain why.

Public support for Graham-Cassidy. (WBZ-TV graphic)

The numbers are dismal, with only 24% support for the bill overall. But what’s really surprising is what Trump voters are saying about the key force behind this measure, the insistence of Republicans on keeping a promise they and the president have repeatedly made. As Mr. Trump put it at a recent rally: “Congress must do its job, keep its promise, live up to its word and repeal and replace Obamacare , you have to do it.”

Last night, Iowa Senator Chuck Grassley even came right out and said he doesn’t like the substance of the bill, yet he’s backing it because of that campaign promise.

But it’s not as if the Trump base is clamoring for approval of Graham-Cassidy.

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Trump voters and their support for the latest Obamacare repeal effort. (WBZ-TV graphic)

According to new numbers from Public Policy Polling, only 47% of Trump voters approve of that bill, while 17% disapprove and a whopping 36% are unsure.

And they also don’t approve of doing this in a rush. By a 48 to 40% margin, Trump voters want congress to get that Congressional Budget Office analysis of the bill’s impact before voting on it, an analysis the CBO says it can’t have ready before the deadline for a simply-majority approval expires under Senate rules on September 30.

So why are so many Republicans so concerned about voter retribution if they don’t repeal Obamacare?

The loudest voices are the ones that often get heard the most, and right-wing talk radio, for instance, has made this a litmus test.

But the new poll finds that while 42% of Trump voters would be more likely to support their members of Congress if they back Graham-Cassidy, 30% say it wouldn’t make a difference, and 20% say it would make them less likely to back them.

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So a majority of 2016 Trump voters are saying this is far less than a top priority for us. And that undercuts the political calculus that was propelling this bill in the first place.