Democrat Doug Jones' apparent victory over Roy Moore in Alabama's special election Tuesday will soon narrow the GOP's lead in the Senate to a slim 51-49 majority.
It also poses a range of potential complications for President Trump's agenda. Trump warned as much in tweets earlier this month.
"Democrats refusal to give even one vote for massive Tax Cuts is why we need Republican Roy Moore to win in Alabama," Trump tweeted. "We need his vote on stopping crime, illegal immigration, Border Wall, Military, Pro Life, V.A., Judges 2nd Amendment and more. No to Jones, a Pelosi/Schumer Puppet!"
Senate Majority Leader Mitch McConnell has said that Republicans will be able to get a tax reform bill on Trump's desk by Christmas. If that happens, Jones' victory probably will not affect the GOP's plan for a comprehensive tax overhaul.
That's because, pursuant to standing Senate parliamentary rules, Jones cannot be seated in the Senate until Alabama's secretary of state certifies the election results. The process requires all of the state's 67 counties to report their confirmed vote counts.
State officials have said the process is expected to be completed at the earliest by Dec. 26, four days after the Senate's current session is slated to end.
Until Jones is actually sworn in and takes office -- which Senate Majority Leader Mitch McConnell said likely will occur in January -- Republican Luther Strange will remain in the seat, and the GOP will retain its 52-48 lead in the Senate.
Because of Vice President Mike Pence's tie-breaking vote, Republicans can afford to lose two votes with that lead and still pass their tax reform plan under the Senate's budget reconciliation rules. (When Jones takes office, that margin will shrink to just one vote.)
Republican Sen. Bob Corker has already announced that he opposes the GOP tax plan, and was the only Republican senator to vote against the bill when it came to a vote in early December.
Republican Sen. Susan Collins has withheld support for the final “conference committee” version of the reform proposal unless it satisfies certain conditions. But even without her support or Corker's, the tax bill would still pass as long as it did not lose any additional GOP support.
However, the House and Senate have not yet agreed on a finalized tax bill, leaving some uncertainty as to the final vote as Christmas nears.
If Jones takes office before Republicans can put a final tax bill on Trump's desk, and if Collins or another Republican senator ultimately joins Corker in voting against the reform, the tax bill would fail.
There is also still the possibility that Jones will take office before the new year, because the federal government is only funded through Dec. 22. If Republicans are unable to reach a new short-term funding measure, Congress might need to convene over the holidays.
If that happened, Jones could be sworn in when Congress reconvenes, assuming Alabama officials have certified the results of the election by that point.
In all likelihood, though, the effect of Jones' victory will likely be felt more strongly in other areas of Trump's agenda.
White House press secretary Sarah Sanders said in November that the impact of Trump’s nominations to the federal judiciary would be felt for “decades and decades.”
But Jones' arrival in the Senate, which has the constitutional power to accept or reject the president's judicial nominees, might help Democrats blunt the president's judicial legacy.
On Tuesday, the Senate confirmed Leonard Steven Grasz of Nebraska to serve on the powerful 8th U.S. Circuit Court of Appeals in a narrow 50-48 vote.
Democrats had roundly criticized Grasz as unqualified, and with Jones in office, they might have been able to prevent him from taking the seat. (Luther Strange voted in favor of Grasz.)
Still, even with Jones in the Senate, Trump and Senate Republicans have the votes, and the vacancies, to shape the federal bench for generations.
As of Dec. 12, there are 17 more vacancies on federal courts of appeal and 120 on federal district courts, with seven nominees pending at the appellate level and 34 at the trial court level, according to a tally on the U.S. Courts government website.
The high number of remaining vacancies affords the GOP a chance to nominate judges who could meaningfully change federal law on issues as diverse as taxes, gun rights, religious freedom, abortion, and campaign finance.
Republicans, helped by an Obama-era decision by Democratic senators to permit the confirmation of judicial nominees below the Supreme Court level with a simple majority vote, have confirmed Trump's nominees to powerful federal circuit and district courts at an unusually rapid pace.
"Thanks to @SenateMajLdr McConnell and the @SenateGOP we are appointing high-quality Federal District and Appeals Court Judges at a record clip! Our courts are rapidly changing for the better!" Trump tweeted in November.
Carl Tobias, a University of Richmond law professor who focuses on judicial nominations, told Business Insider that the speed of the confirmations under Trump has been "very rare."
Most of these nominees have been confirmed by healthy margins. For example, U.S. Supreme Court justice Neil Gorsuch was confirmed by a 54-45 vote in April, and the Senate voted 53-43 to confirm Stephanos Bibas to the 3rd Circuit Court of Appeals in November.
Amy Coney Barrett also won Senate approval in October to the 7th Circuit Court of Appeals in a 55-43 vote.
But Jones' arrival to the Senate may impact the upcoming votes on some of Trump's embattled judicial nominees.
On Tuesday, Republican Sen. Chuck Grassley, who chairs the Senate Judiciary Committee, told Trump to “reconsider” the nominations of Jeff Mateer and Brett Talley to the federal bench, according to several reports. The full Senate has not yet voted on either nominee.
Mateer reportedly referred to transgender children as evidence of "Satan's plan," and Talley has been an outspoken supporter of gun rights. The American Bar Association has said that Talley is unqualified for a judgeship.
Immigration and border security
The short-term temporary funding measure keeping the federal government open until Dec. 22 does not resolve numerous points of budgetary contention, including debates over immigration reform.
Armed with a new senator, Democrats now have even more incentive to insist on a legislative solution to provide legal status to Dreamers, or illegal immigrants brought to the U.S. as children, as a condition for averting a shutdown.
Republican leaders have said that immigration “should be a separate process and not used to hold hostage funding for our men and women in uniform,” the Washington Post reported, while Democrats have sought to use their leverage to push various domestic agendas, including immigration reform.
Trump's planned border wall also faces a setback with Moore's loss. While the House approved $1.57 billion in border infrastructure funding this summer, the cash -- part of a $790 billion spending package -- always faced long odds of winning approval in the Senate.
With Jones' victory and a potential government shutdown looming, those odds get even longer. Democrats are set to gain even more leverage to insist that no money goes towards a border wall when Jones takes office.
“In general, that’s a nonstarter,” Democratic Senator Dianne Feinstein told the New York Times in July, referring to the U.S. government paying for a border wall.
Trump has indicated, however, that he believes Democrats can be cowed into approving the funds.
“Build that wall,” Trump said at an August rally. “Now the obstructionist Democrats would like us not to do it. But believe me, if we have to close down our government, we're building that wall.”
Democrats signaled that Trump and congressional Republicans would face significant consequences in the upcoming midterms if they pursued a shutdown.
“With the White House, House and Senate under one-party control, the American people expect and deserve a plan from Republicans to avert a catastrophic default and ensure the full faith and credit of the United States,” Democratic minority leader Nancy Pelosi said in an August statement. “With so much at risk for hard-working families, Republicans need to stop the chaos and sort themselves out in a hurry.”
Fox News' Chad Pergram contributed to this report.