The next two months could be the most consequential in Sen. Bernie Sanders’s more than 30-year career in Congress. For much of his time in the House, and then in the Senate, Sanders was a gadfly on the left, making speeches that received more attention off Capitol Hill than on it, and introducing bills and amendments that often fell short of the votes needed to pass.

Now, as chair of the Senate Budget Committee, Sanders is responsible for putting together a budget blueprint that, if approved by both houses and signed by President Biden, would represent the largest spending bill ever passed in the history of Congress.

The bill would come to the Senate floor under the procedure known as reconciliation. This rule allows the Senate to pass budget-related legislation by a simple majority, bypassing the normal rule that 60 votes are needed to advance legislation in the Senate. Since no Republicans are expected to vote for the bill, Sanders must come up with a package that can gain the support of all 50 Democratic senators, with Vice President Harris casting a tiebreaking vote if needed.

For the last several weeks, Sanders has been negotiating the details of the reconciliation bill with the White House, and with colleagues in both the Senate and the House. Sanders’s initial ask was for a $6 trillion package that would, among other provisions, lower the Medicare eligibility age from 65 to 60, make permanent the new child tax credit, and forgive much outstanding student loan debt.

When it became clear that neither President Biden nor centrist Senate Democrats such as Joe Manchin of West Virginia and Kyrsten Sinema of Arizona would support the $6 trillion program, Sanders rewrote his proposal to bring the amount down to $3.5 trillion, an amount that the White House will support. Several Senate Democrats said Sanders deserves credit for being flexible regarding the total, while at the same time proposing a larger package than many in Washington would have thought realistic at the beginning of the year.

Although many of the details of the Budget Committee’s $3.5 trillion plan remain to be worked out, Sanders has noted that it will include an extension of the child tax credit (although not a permanent one), an expansion of Medicare to include vision, dental, and hearing coverage, and hundreds of billions of dollars for environmental and green energy projects. Sanders also claims that the proposal is fully paid for, largely through rolling back some of the tax cuts for corporations and individuals with incomes over $400,000 that were signed by President Trump in the Republican Congress’s 2017 tax bill.

Passage of a $3.5 trillion reconciliation bill is by no means a done deal. Manchin and Sinema may balk at some of the provisions Sanders would like included. On the other side of Capitol Hill, members of the House Progressive Caucus may try to amend the bill to include additional spending, on the child tax credit, environmental programs or both. Such amendments, if passed, would make it harder to hold all 50 Senate Democrats in line behind the bill.

The Senate parliamentarian, who ruled against including a minimum wage increase in a reconciliation bill earlier this year, might also strip some provisions from the upcoming bill, on grounds that they are not clearly related to federal taxing and spending. Provisions that might fall into this category include those related to immigration, particularly maintaining the DACA program for immigrants who arrived in the United States as children.

Most of the “Dreamers” covered by this program are now young adults. The budget argument being developed by Sanders and his staff to persuade the parliamentarian is that regularizing the DACA recipients’ status would enable the Treasury to count on income tax and other receipts paid by them continuing throughout the entire time period covered by the reconciliation bill.

Eric L. Davis is professor emeritus of political science at Middlebury College.