Washington — A marathon vote series is underway in the Senate after the chamber debated President Trump's massive tax bill into the wee hours of the morning as Republicans work to pass the centerpiece legislation of Mr. Trump's second-term agenda.

The House narrowly passed the bill last month, and Senate Republicans have been working to put their mark on the legislation, treading carefully so as not to throw off the delicate balance in the lower chamber. The House will need to approve the Senate's changes to the bill before it can head to the president's desk for his signature. And lawmakers are trying to move quickly, with a self-imposed July 4 deadline to get the measure signed. 

The Senate worked through the weekend as the GOP nears a final sprint on the legislation ahead of the deadline. Titled "One Big, Beautiful Bill," the legislation includes increased spending for border security, defense and energy production, which is offset in part by cuts to healthcare and nutrition programs. The Congressional Budget Office estimated Sunday that the legislation would increase the deficit by nearly $3.3 trillion over the next decade.

Senate Republicans advanced the legislation late Saturday, with all but two voting in favor following hours of delay as the GOP worked to iron out last-minute details and dispel concern among holdouts. The vote on the motion to proceed stayed open for more than three hours as holdouts sought assurances from GOP leaders. Some tweaks were made to the bill before Republicans ultimately received enough votes to move forward.

Senate Democrats further delayed the legislation's path forward by forcing the bill to be read in its entirety, starting late Saturday. After nearly 16 hours, the Senate clerks concluded their reading of the bill on the floor, starting the clock on debate. Each side then had up to 10 hours for debate. 

The "vote-a-rama"

Following debate, and a break until the morning, the Senate began what's known as a "vote-a-rama" Monday in which senators may offer an unlimited number of amendments and force the chamber to cast vote after vote. Democrats are expected to use the opportunity to put their GOP colleagues on the record on a number of controversial issues ahead of the midterm elections.

But before the chamber could get to the amendment votes, senators had to address an outstanding disagreement over the current policy baseline, an accounting approach that would make it appear that extending the current tax policy would cost nothing. Senate Majority Leader John Thune maneuvered Sunday to allow the use of the current policy baseline, before Senate Minority Leader Chuck Schumer quickly appealed the move, requesting a vote as Democrats railed against it as the "nuclear option." The chamber voted 53-47, along party lines, to affirm the use of the current policy baseline on Monday. 

Schumer said that Democrats would bring "one amendment after the other" Monday, and began the process by offering an amendment to send the bill back to the Finance Committee to revisit some of its health care provisions. The chamber voted down the amendment in a party-line vote.  

Democrats proposed a number of amendments to attempt to roll back some of the bill's more controversial provisions. Sen. Ed Markey, a Massachusetts Democrat, proposed an amendment to remove the bill's provisions that he said would force rural hospitals to limit their services or close their doors. And Sen. Chris Coons, a Delaware Democrat, proposed an amendment to eliminate what he called "red tape" around Medicaid eligibility. Other amendments proposed by Democrats concerned cuts to food assistance and state provider taxes, among a number of related issues. The amendments fell short. 

Amid the slew of votes, anticipation swirled around a consequential amendment expected to be put forward by GOP Sen. Rick Scott of Florida later Monday. The amendment would significantly reduce the federal Medicaid expansion match made under the Affordable Care Act, barring new enrollees after 2030, in a move that would make the bill more palatable to some fiscal hawks. 

Thune has backed the amendment, calling it "great policy," and forecasted that it will get significant support among the Senate GOP. But whether it has enough support to be added to the bill remains to be seen. 

The path to passage

Senate Republicans have been pursuing the legislation through the budget reconciliation process, which enables the party in the majority to move ahead without support from across the aisle. With only a simple majority required to advance the measure, rather than the 60-votes needed to move forward with most legislation, Senate Democrats have few mechanisms to combat the bill's progress. 

With a 53-seat majority, Senate GOP leaders can only afford to lose support from three Republicans — and would then still require a tie-breaking vote from Vice President JD Vance. And although a number of senators who had expressed opposition to the measure ultimately decided to advance it Saturday, how they will vote on the measure in a final form remains unclear. 

Sens. Rand Paul of Kentucky and Thom Tillis of North Carolina were the two Republicans to oppose the bill's advancement Saturday, and are expected to oppose the legislation on final passage. Tillis, who announced Sunday that he is not seeking reelection, took to the Senate floor that night to outline his opposition to some of the bill's cuts to Medicaid, claiming "Republicans are about to make a mistake on health care" and arguing that the GOP is "betraying our promise."

"It is inescapable that this bill in its current form will betray the very promise that Donald J. Trump made" to target only waste, fraud and abuse in the entitlement program, Tillis said, claiming that the president has been "misinformed"

The North Carolina Republican argued that the July 4 deadline is an "artificial" one, saying Senate Republicans are rushing, while encouraging the chamber to "take the time to get this right" and align more closely with the House's Medicaid provisions.

But Senate GOP leaders are still moving ahead. Thune, a South Dakota Republican, delivered a defense of the bill on the Senate floor ahead of the vote-a-rama Monday, pushing back on criticism over Medicaid cuts, the impact on the deficit and the use of the current policy baseline.

"Let's vote," Thune said. "This is good for America."

When asked whether he's confident Senate Republicans have the votes to pass the legislation, the majority leader told reporters, "Never, until we vote."

Vance was on hand to break a possible tie vote Saturday, though his vote ultimately wasn't needed. Still, the vice president met with GOP holdouts in the majority leader's office Saturday as the White House put pressure on lawmakers to get the bill across the finish line. 

White House press secretary Karoline Leavitt said Monday that the president has "been in touch with lawmakers all weekend long to get this bill passed."

"The White House and the president are adamant that this bill is passed and that this bill makes its way to his desk," Leavitt said. "Republicans need to stay tough and unified during the home stretch, and we are counting on them to get the job done."

Meanwhile, Sen. Mark Warner, a Virginia Democrat, warned Sunday that the legislation would be a "political albatross" for Republicans, while suggesting that the bill could even lose support among the GOP, saying "it's not over until it's over."

"I think many of my Republican friends know they're walking the plank on this, and we'll see if those who've expressed quiet consternation will actually have the courage of their conviction," Warner said Sunday on "Face the Nation with Margaret Brennan."