They scored a tactical victory when President Donald Trump made an unexpected concession to Democratic leaders Chuck Schumer and Nancy Pelosi, ceding an upper hand to the minority party in upcoming negotiations. Reveling in the positive reviews, Trump appeared poised to pursue more bipartisan deals, including one to enshrine protections for DACA recipients. And on Capitol Hill, senators seemed to be moving toward a rare agreement to strengthen the health care market.

But just like that, Democrats are suddenly on the back foot: Defending Obamacare -- again -- from a last-ditch GOP effort to repeal the law

Just when it seemed that the GOP had moved on from its repeated and failed attempts to gut the health care law, what was deemed until recently a long-shot proposal from Sens. Lindsey Graham and Bill Cassidy is gaining rapid steam on Capitol Hill.

"I take it very seriously," Schumer told reporters Monday. "The message to the American people is: This bill is worse than the last bill. It will slash Medicaid, get rid of (protections for) pre-existing conditions. It's very, very bad."

Veteran Democratic pollster Geoff Garin, who has surveyed public opinion on the Affordable Care Act, said the latest move amounted to the GOP digging a "deeper hole than Republicans have already dug for than themselves on health care."

"I think Democrats in the Senate assumed that with all the other urgent pressing business facing the Senate, (Senate Majority Leader Mitch) McConnell would be more rational and focus on things that need be done as opposed to something that makes very little political sense at all," Garin said.

The legislation, which would repeal individual and employer mandates and create a block grant program for states, is closing in on 50 "yes" votes from Republicans in the Senate.

McConnell intends to bring the bill to the floor as soon as the math is there, as leadership against an end-of-the-month deadline to use a budget reconciliation vehicle to pass the bill with only GOP support.

Democrats quickly began sounding the alarm bell, trying to rally supporters of the law for yet another counteroffensive.

"We must take #GrahamCassidy health care repeal bill seriously and speak out NOW," New Hampshire Democrat Maggie Hassan tweeted on Sunday. Her colleague, Connecticut's Chris Murphy, also sent out a similarly grave warning: "This is happening," he wrote . "Drop what you are doing to start calling, start showing up, start descending on DC. Game on."

Off the Hill, a coalition of activist groups that bombarded Republican senators during their last several attempts to roll back Obamacare ramping up their attacks. After a series of intense and draining fights earlier in the year, capped off with a memorable victory when Sen. John McCain sunk the GOP's "skinny repeal" gambit, the rapidly escalating push has caught some off-guard.

"I felt like Dems were caught flat-footed last time but we still managed to pull something off. I think the next 24-48 hours are critical for resistance types," said one Democratic operative focused on digital outreach.

"The social-to-office calling pipeline needs to rev up again," the operative said. "There's a social media to media ecosystem that drives people to action on this where left activists get loud online, elected (officials) get loud offline, and the media cover it -- which drives media conversation, which drives social conversation."

The drumbeat, online and in organizing circles, is building. The familiar message, again: Flood Republican lawmakers' phone lines.

"(The GOP) is obviously playing a game here," said Charles Idelson, communications director for National Nurses United, a key union backer for the Democratic single-payer campaign. "They think they can rush this through before people are mobilized to oppose it."

Organizers insist they never took their foot off the gas pedal, even after the Republican "skinny repeal" bill failed in late July. With the September 30 deadline on the horizon, one last bid always seemed likely. But almost everyone, Republicans included, were doubtful that Graham-Cassidy would gain real support -- and momentum -- in the Senate.

"We've been on alert, on the lookout, for another attempt. The alarming thing is how quickly Graham-Cassidy has turned from a pipe dream into a very serious threat," said Ben Wikler, MoveOn.org's Washington director.

Wikler's Twitter account is one of the handful of hubs for organizers. Along with allies like Topher Spiro, vice president for health policy at the Center for American Progress, they have spent the last week cataloging developments and -- increasingly over the last 72 hours -- calling the rank-and-file back to the barricades.

"The resistance movement has grown accustomed to the zombie walker (bill)," Wikler said. "Trumpcare keeps on dying, and keeps on crawling its way out of the grave. The millions of people who've been engaged over the past nine months are ready to throw themselves back into the battle and we're moving all of our chips into knocking this beast back down."