A bipartisan group of senators sketching out an infrastructure proposal expanded their base of support Wednesday, even as they continue to haggle over how to pay for billions of dollars in new spending in line with President Biden’s vision for a massive overhaul of the nation’s public works system.

The initial framework, written by the likes of Sens. Mitt Romney (R-Utah), Kyrsten Sinema (D-Ariz.), Rob Portman (R-Ohio) and seven other senators, falls far short of the sweeping infrastructure proposal that Biden has pitched, yet aims to try to satisfy the president’s hunger for bipartisanship.

But their efforts received a big boost Wednesday, when 11 more senators joined the original 10 and said they supported the still-unreleased blueprint of a deal. The group now includes 11 Republicans, nine Democrats and one independent who caucuses with the Democrats. All told, they account for a fifth of the entire chamber.

The show of support, the senators said, marked an attempt to prove publicly that their new endeavor can win broad approval on Capitol Hill, even as an increasingly agitated liberal coalition has raised alarms that the drawn-out bipartisan talks have been fruitless — and some conservatives have questioned the need for another massive burst in federal spending.

“We support this bipartisan framework that provides an historic investment in our nation’s core infrastructure needs without raising taxes,” the group of Democrats and Republicans said in the statement. “We look forward to working with our Republican and Democratic colleagues to develop legislation based on this framework to address America’s critical infrastructure challenges.”

Even as they rallied support for their plan, however, Senate Democrats huddled privately Wednesday to devise a path forward for trillions of dollars in additional spending in infrastructure improvements and other economic initiatives that may not make it into a bipartisan deal. Senate Majority Leader Charles E. Schumer (D-N.Y.) repeated his intention to pursue the party’s agenda on two tracks — brokering a compromise on infrastructure while advancing Biden’s other proposals using a process known as reconciliation, which requires 51 votes rather than the normal 60 for legislation to pass.

Exiting the meeting, lawmakers including Sen. Bernie Sanders (I-Vt.) signaled that their ambitions remain vast, reflecting Democrats’ desire to seize on their majorities to combat climate change, boost federal safety net programs and potentially tackle other reforms in immigration, prescription drugs and health care.

“We have an enormous amount of work in front of us,” said Sanders, chairman of the Budget Committee, adding that there’s also a desire among Democrats to “make sure that the wealthiest people and largest corporations in this country start paying their fair share of taxes.”

The flurry of negotiations on the president’s top domestic policy initiative came on a day when Biden himself was out of the country, continuing his first visit as president to Europe. Asked about the bipartisan proposal in Geneva, Biden told reporters he hadn’t seen it, adding: “I don’t know what the details are. I know that my chief of staff thinks there’s some room.”

In the meantime, Democratic senators who helped craft the bipartisan plan met Wednesday with Biden’s top aides — including Steve Ricchetti, counselor to the president; Brian Deese, director of the National Economic Council; and Louisa Terrell, the White House’s legislative affairs director — to update the administration on their efforts, according to a White House official.

“The White House team was grateful for the briefing from the Democratic senators involved in the infrastructure negotiations, and found it productive and encouraging,” White House spokesman Andrew Bates said in a statement. “They look forward to briefing the president tomorrow after his return to the White House, and continuing to consult with senators and representatives on the path forward.”

The bipartisan Senate plan calls for about $974 billion in infrastructure spending over five years, according to people who spoke on the condition of anonymity because they weren’t authorized to reveal the details. This amounts to roughly $579 billion in new spending, in addition to some redirected spending from other programs. Biden had originally requested more than $2 trillion in new infrastructure spending, but White House officials had signaled they were willing to consider a package closer to $1 trillion.

Administration officials have indicated both publicly and to Capitol Hill that Biden is willing to let negotiations play out a little longer but not indefinitely — while many conservatives will not be eager to accept much new federal spending, if any, on massive public works projects.

“You know, unfortunately, we see people on both sides who throw arrows,” Sen. Jeanne Shaheen (D-N.H.) said. “I think we have to keep going forward and trying to do what we think is the right thing to get something done for the country.”

The first meeting Wednesday included a wider array of lawmakers beyond the initial 10 Democrats and Republicans who had prepared the bipartisan compromise.

Among those newly in the room were Republican Sens. John Thune (S.D.) and Todd C. Young (Ind.), who exited offering new praise for the proposal. Democrats who attended the meeting who hadn’t initially endorsed the plan included Sen. John Hickenlooper (D-Colo.), Maggie Hassan (D-N.H.) and Richard J. Durbin (D-Ill.). Durbin and Thune are both key because they are the respective vote counters for their parties in the Senate.

“We know that on both sides we’re going to have detractors to this, and so I think it’s going to be important that as we secure support from Republicans, we secure the same from Democrats,” Sen. Lisa Murkowski (R-Alaska) said before the statement from 21 senators was released. “So that’s why we’ve been kind of doing the whole Noah’s Ark approach.”

Senate negotiators also expanded their efforts to include the House, inviting Reps. Josh Gottheimer (D-N.J.) and Brian Fitzpatrick (R-Pa.), leaders of the bipartisan Problem Solvers Caucus. After the meeting, Portman said senators plan to brief the fuller House caucus this week, reflecting their strategy of “going out in concentric circles now and bringing in more and more people.”

The House bloc had joined a similar coalition of largely moderate-leaning members last year in helping to break another logjam over more than $900 billion in stalled coronavirus aid. After the meeting Wednesday afternoon, Gottheimer and Fitzpatrick said their roughly $1 trillion, eight-year blueprint is similar to the five-year, similarly sized package that the Senate is assembling.

“This can get across the finish line,” Gottheimer said.

But some disagreements still separated the two parties’ lawmakers in negotiations — namely, how exactly to pay for an infrastructure package without violating each side’s political red lines. Democrats have pledged they won’t raise taxes on Americans making under $400,000, and Republicans have refused to budge in opposing any tax increase that unwinds the cuts they adopted in 2017.

In a bid to broker a compromise, Senate negotiators proposed a package of “pay-fors” that change the gas tax, tying it to inflation, while imposing similar, new charges on the owners of electric vehicles. Democrats, however, have balked at the idea, arguing it violates Biden’s 2020 campaign promise, and the indexing provision appeared unlikely to survive.

Portman on Wednesday defended the plan, stressing it is not a “new tax.” But he said that lawmakers “need to work with them on coming up with alternatives.”

The developments came as Senate Democrats on Wednesday laid the early groundwork to chart their own course on trillions of dollars in additional spending on infrastructure and the party’s vast additional economic priorities. Many party lawmakers have made clear they would not support any bipartisan infrastructure deal emerging in the Senate unless they could move their robust reconciliation package essentially in tandem.

“This is the moment that we have to start addressing issues that have been neglected for very, for a very long time,” Sanders said ahead of the meeting.

The Democratic strategy aims to quell a potential revolt among the left, which has grown impatient with the length of the talks — and antsy about the trade-offs made in the name of securing GOP support. But Republicans expressed skepticism about the timing and nature of the deal that Democrats seek.

“It would be impossible to get any sort of pre-agreement on reconciliation before striking this package,” Young said.