WASHINGTON (AP) — With the Senate split 50-50, leaders of the Democratic majority are looking for ways to advance their priorities and President Joe Biden's agenda around the typical 60-vote threshold needed to overcome a filibuster by opponents.

This week, the Senate parliamentarian ruled that one tool, budget reconciliation, could be used more often than expected to pass certain measures with a 51-vote threshold. That potentially opens new opportunities for approving not only President Joe Biden's $2.3 trillion infrastructure package, as had been expected, but other legislation on a party-line vote without Republicans.

A look at the process and what's ahead:

WHY THE SENATE STALEMATE?

Democrats hold the majority in the evenly split 50-50 Senate because the vice president of their party, Kamala Harris, can provide as a tie-breaking vote.

But most major legislation requires 60 votes to advance, overcoming an objection from a filibuster, which can be waged by any senator who wants to halt action.

That's a tall order in the narrowly divided Senate, and a recipe for gridlock especially in partisan times.

Key senators want to change the filibuster rules, ending the 60-vote threshold they view as a throwback — a procedural relic of segregation before the passage of civil rights legislation.

But changing the filibuster rules has been difficult and requires the majority to be on board. Leading centrist Democrats, including Sen. Joe Manchin of West Virginia, have said they want to keep it in place.

BUDGET RECONCILIATION VS FILIBUSTER

Under the congressional budget process, certain measures regarding revenues, spending and the debt can be approved with a 51-vote threshold.

Congress has used this so-called budget reconciliation process before, more than 20 times since its was first unveiled in 1980.

Democrats used it to approve the Affordable Care Act, or “Obamacare,” in 2010. Republicans used it in 2017 to pass tax cuts.

Biden relied on it for party-line approval of his sweeping $1.9 trillion COVID-19 package last month when all Republicans voted no.

It can't be used for just any legislation, but provides a way for some budget-related measures to pass with a simple 51-vote majority.

WHAT DID THE PARLIAMENTARIAN SAY?

The Senate parliamentarian's opinion this week suggested the budget reconciliation procedure can be used more than once — not only on the annual budget, but on a subsequent budget revision.

Already Democrats have two budgets lined up this year, one for the current fiscal 2021 year that ends Sept. 30, and another for the coming fiscal 2022 year that starts on Oct. 1.

That opens the door to multiple opportunities for deploying the budget process on bills that would use the 51-vote threshold.

WHAT BILLS CAN PASS WITH RECONCILIATION?

While talks are swirling over ways to use budget reconciliation to advance immigration or Medicare legislation as soon as April, it is no sure-fire route.

Already elements of Biden's American Jobs Plan, the big infrastructure bill, were expected to use the reconciliation process this summer. Others could follow.

Approval would still will require Democrats to wrestle their slim majority to consensus, which is no guarantee in the diverse caucus made up of progressives like Sen. Bernie Sanders, I-Vt., and centrists like Manchin, presuming Republicans are opposed.

Some legislation won't qualify for the budget process, which requires that bills revolve around revenues, spending or debt.

Already, the parliamentarian rejected a proposal to raise the federal minimum wage to $15 an hour because it didn't fit budget rules.

Voting rights, gun violence and other policy measures may not be eligible.

VOTE-A-RAMA AND WHAT'S NEXT

If Senate Democrats do push ahead, the budget process can be long and cumbersome.

A budget bill or revision needs to be drawn up, debated in the committee and brought to the floor.

Typically there's a lengthy “vote-a-rama” process — an hourslong, all-night session of senators amending and debating the budget bill.

Only then can it be brought forward for a final vote.