The Biden administration’s negotiations on an infrastructure bill have so far largely been between the president and members of the Senate who fall to his right ideologically, whether it be Republicans or a small group of moderate Democrats. But after months of negotiations, a shrinking price tag and the seeming elimination of some big-ticket Democratic campaign-trail promises, Congressional Democrats further to the political left are raising their own objections, threatening to expose the limits of Democrats’ thin majorities in both chambers, and divisions on how, procedurally, to try to pass key agenda items.

President Biden made a $2 trillion proposal at the end of March that included sprawling spending plans in areas including clean drinking water, high-speed Internet, housing, hundreds of billions in spending for care for elderly and disabled Americans and other areas. It was an ambitious plan, and it formed the starting point for Biden and his team, who have repeatedly said passing a bill is a must.

But Biden has also consistently said he’s willing to negotiate, rather than pursuing what would be the second Democrats-only spending bill of his presidency — or taking the more drastic step of calling on Senate Democrats to change the filibuster rules that require 60 votes to advance most other legislation.

That desire to come up with a bipartisan bill seems genuine; on the campaign trail, Biden regularly cited his 36 years in the Senate as important, relevant experience that he could bring to bear as president.

But it was also driven by the reality that passing a bill via the reconciliation process — which Democrats were able to do this year in passing a $1.9 trillion infrastructure bill without any Republican votes in either the House or Senate — requires securing a “yes” vote from all 50 Democrats in the Senate. And several have said that, at least for now, they want the Senate to pass a bill through regular order. In other words, until those Democrats become convinced that it’s impossible to get at least 10 Republicans on board with a bill, a second reconciliation bill isn’t on the table.

Biden abandoned negotiations last week with one group of Republicans, led by chief negotiator Sen. Shelley Moore Capito (R-W.Va.). Another group of senators, which includes a few of the moderate Democrats who, so far, insist Biden should keep working on a compromise bill, is preparing to unveil a proposal about half the size of Biden’s original plan.

Sen. Joe Manchin III (D-W.Va.) has been the loudest Democratic voice calling for a bipartisan plan; some of his Democratic colleagues are less confident that Republicans are willing to vote for a bill that Democrats can also support. Whether its impatience, skepticism or both, Democrats have laid the groundwork for a potential reconciliation bill, even if they don’t have the votes in their own caucus to pass it yet.

“I’ve seen this movie before — it’s called the Affordable Care Act,” cautioned Sen. Richard J. Durbin (D-Ill.). “We waited a calendar year for bipartisanship, and it never, ever appeared.”

But beyond impatience, or skepticism that the requisite number of GOP votes will appear, other Democrats are starting to caution that they’re negotiating away too many of their preferred agenda items — including things they promised to voters last November.

House Speaker Nancy Pelosi (D-Calif.) seemed to want to remind Biden and Senate Majority Leader Charles E. Schumer (D-N.Y.) over the weekend that the House also has to vote on any bill they want to pass into law — and House Democrats might not accept what they see as a scaled-back bill that abandons key Democratic priorities.

“I don't know how we can possibly sell it … to our caucus unless we know there is more to come,” Pelosi said on CNN on Sunday.

And some of the more liberal members of her party have already shown a willingness to defect on votes; three House Democrats voted against a May bill that would have increased funding for security at the Capitol, while three more voted “present,” allowing the bill to just barely pass, 213-212.

One of the members who voted “present,” Rep. Alexandria Ocasio-Cortez (D-N.Y.), has been particularly critical of the removal of funding to combat climate change from infrastructure proposals, accusing Biden and Senate Democrats of “playing patty-cake” with Senate Republicans.

Sen. Martin Heinrich (D-N.M.) tweeted that “An infrastructure package that goes light on climate and clean energy should not count on every Democratic vote.”

Sen. Brian Schatz (D-Hawaii) tweeted, “Just a gentle, friendly reminder that the executive branch doesn’t write the bills,” after a Biden official said the final infrastructure plan could leave out key climate change proposals.

Manchin and a few other Democratic senators have dominated the headlines so far in Biden’s presidency. Manchin in particular seems to be wielding his vote as a tool to force his Democratic colleagues to try to negotiate bipartisan bills, and in some cases to move toward the center in negotiations.

But other Democrats realize that their votes could count just as much as Manchin’s — just as liberal House Democrats know the party’s margin in the House is thin enough for them to force the issue, too.

It could be a signal to moderates that the party’s liberals don’t want to see more of the party’s priorities negotiated away for a compromise that still might not get the requisite 10 GOP votes to pass in the Senate in regular order; or it could be a sign those negotiations have already gone further than they’re willing to stomach.

“What is being talked about … this is by and large something that could have been talked about 50 years ago,” Pelosi said of the reduced infrastructure proposals being considered by the bipartisan group of senators. But she wants to see more Democratic agenda items, like money to combat climate change, passed into law. “We’re talking about the future.”