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So far, five Republican senators have said they cannot vote for the Affordable Care Act repeal as written: Mr. Heller, whose concerns are with the bills benefit cuts, and four hard-line conservatives who say the bill is too generous.

Mr. Heller did not rule out ultimately voting for a version of the bill, leaving the battle for 50 votes ahead of a Senate showdown still very alive. But his denunciation of one of the pillars of Mr. Trumps agenda gave fresh hope to Democrats that they may be able to torpedo the measure.

Strikingly, there has been little in the way of advertising from the right pushing Republicans to support the bill. But Mr. Hellers criticism infuriated allies of Mr. Trump. America First Policies, a super PAC aligned with the White House, was gearing up to spend $1 million worth of advertising in Nevada aimed at making the senator change his mind, said a person familiar with the groups plans.

Mr. Hellers quick denunciation offered a morale boost to the Democratic Party after a trying few days of recriminations about why they lost a special congressional election in Georgia on Tuesday, the latest in a series of demoralizing defeats they have suffered this year.

A comparison of the Senate health care bill with the Affordable Care Act.

Its an all-hands-on-deck moment, said Anna Galland, the head of MoveOn.org, an advocacy group firmly on the liberal wing of the party. We are unified out of urgent, building-is-burning-down necessity. And health care is by far our top priority.

Even Hillary Clinton, who has been increasingly outspoken after a period of postelection restraint, blasted on Twitter: Forget death panels. If Republicans pass this bill, theyre the death party.

Scrambling to halt or at least slow the Senates repeal effort, a range of Democratic and progressive leaders said Friday that they intended to intensify pressure on Republican lawmakers.

Liberal groups have already organized protests against the bill, and Senator Bernie Sanders, independent of Vermont, plans to lead a campaign-style tour this weekend through West Virginia, Pennsylvania and Ohio, three states with Republican senators that also expanded Medicaid under the Affordable Care Act. Planned Parenthood, which would be defunded under the Senate bill, has been running television ads targeting Mr. Heller, as well as Senators Jeff Flake of Arizona, also up for re-election next year, and Shelley Moore Capito of West Virginia.

And in recent days, potential Democratic challengers have stepped forward against both Mr. Heller and Mr. Flake, the two Republicans most likely to face tough races in 2018. In Nevada, Representative Jacky Rosen has signaled she is likely to run against Mr. Heller, and Randy Friese, an Arizona state representative and trauma surgeon, said he was leaning toward challenging Mr. Flake. Mayor Greg Stanton of Phoenix, who is a Democrat, is also believed to be considering a campaign.

Senate Republicans crafted their bill behind closed doors, drawing considerably less news attention than House Republicans, who formally drafted their version of the legislation in open sessions. But Democrats believe the coming week represents their best and perhaps final chance to thwart repeal of the health law.

This is the one opportunity we have to shine a light on this legislation, and we will do it day and night, said Senator Chris Van Hollen of Maryland, projecting the bills passage a jump ball.

The Senate Democratic campaign arm, which Mr. Van Hollen leads, plans to increase its spending this week on internet ads focused on the health measure in Nevada and Arizona, as well as Texas and Florida, which also have Senate races in 2018. The Democratic Senatorial Campaign Committee is just one of a series of liberal groups that are airing spots online and on television and radio to pressure up-for-grabs Senate Republicans into opposing the overhaul.

Senators Lisa Murkowski of Alaska and Susan Collins of Maine, two centrist Republicans who at times defy their party, are also facing lobbying to vote no.

Mr. Heller may best reflect the tensions within the larger Republican Party. He is a popular but vulnerable lawmaker from a purple state that is full of rural conservatives but is increasingly shaped by a rising minority population in and around Las Vegas, its population center. And while Mr. Heller has called for repealing the Affordable Care Act a promise that thrills the Republican base he has also, like many of his colleagues, consistently reassured voters that their health coverage would improve under a Republican alternative.

A number of Mr. Hellers colleagues who fit that broad description could face backlash, in 2020 or beyond, if they vote for a bill that proves deeply unpopular as a matter of law. Along with Mr. Sandoval, several other Republican governors have voiced similar criticism of the bill, including Charlie Baker of Massachusetts, Larry Hogan of Maryland and John Kasich of Ohio.

Mr. Kasich shares a home state with Senator Rob Portman, a Republican who was just re-elected to a second term in November, and has not yet taken a stance on the legislation.

Standing with Mr. Heller, Mr. Sandoval offered a robust case for protecting those who have received insurance under the Medicaid expansion an effort that could prompt some Senate Republican hard-liners already uneasy about the bill to walk away. These are our friends. These are our families. These are our neighbors, Mr. Sandoval said of the 210,000 Nevadans who obtained Medicaid coverage under the Affordable Care Act. These are folks who are worth fighting for.

The health debate is not playing out just in the Capitol and in those states that are home to swing senators. In Virginia, where a coming election for governor in November is the Democrats next electoral priority, Lt. Gov. Ralph Northam castigated his Republican opponent this week for failing to take an unequivocal stance on the Senate bill.

Mr. Northam, who won his partys nomination for governor in a contested primary this month, attacked the legislation as immoral and expressed incredulity that Ed Gillespie, the Republican nominee, said he needed more time to study it.

If they do repeal the Affordable Care Act, its literally going to put hundreds of thousands of Virginians at risk of losing their coverage, Mr. Northam said, adding, I dont think they have a clue what theyre doing in Washington right now.

Mr. Northam, a pediatric neurologist, said health care would be one of the central themes of his campaign, warning, Everybody should be worried.

The bill is also expected to face resistance from mayors, including some Republicans, during a meeting of the United States Conference of Mayors in Miami Beach this weekend. Mayor Mitch Landrieu of New Orleans, the incoming president of the organization, said the Republican bill was a nonstarter that plainly fell short of the standards Mr. Trump and Republicans set for themselves.

Jonathan Martin reported from Washington, and Alexander Burns from New York. Maggie Haberman contributed reporting from Washington.

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A version of this article appears in print on June 24, 2017, on Page A1 of the New York edition with the headline: Key G.O.P. Senator Says He Opposes Health Care Bill.

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