First Read is your briefing from Meet the Press and the NBC Political Unit on the day's most important political stories and why they matter.

WASHINGTON — This week brought President Trump one step closer to his first major legislative victory of his presidency (though there are some last-minute obstacles to that tax bill). But it also appeared to all but close the door on his agenda for next year.

With their loss in Alabama on Tuesday, Republicans’ majority in the U.S. Senate will soon be reduced to a bare 51-49 majority, making it much more difficult to pass legislation appealing only to Republicans. The welfare reform Trump has talked about (“We’re looking very strongly at welfare reform, and that’ll all take place right after taxes”)? It’s hard to see it surviving in the Senate.

What about changes to entitlements like Medicare, which House Speaker Paul Ryan has said Republicans should target in 2018? Tough to see it happening, given that Senate Republicans couldn’t afford losing both Sens. Susan Collins, R-Maine, and Lisa Murkowski, R-Alaska.

Revisiting Obamacare repeal? No way.

Infrastructure? Maybe, but with Trump’s approval among Democratic voters in the single digits (and overall approval in the 30s), which Democratic senators are willing to risk backlash from their base in supporting anything with Trump’s name on it?

So after Washington ultimately resolves its battle over government funding — where Democrats have increasing leverage due to the need for 60 votes in the Senate — it’s very likely that attention turns away from Capitol Hill to the 2018 campaign trail.

Because it’s an election year, 2018 was going to be short on legislative activity. But we’re not sure enough people realize how Tuesday’s result in Alabama effectively shuts the door on it after January or February.

Tax bill hits a roadblock in the Senate

As for the tax legislation, here’s NBC’s Benjy Sarlin: “Just days before an expected vote, the sweeping Republican tax bill's fate was up in the air Thursday, with few details confirmed and key senators withholding support unless changes were made. Sen. Marco Rubio, R-Fla., announced he would oppose the bill unless it expanded a child tax credit to millions of lower income families by making a larger portion refundable against payroll taxes.”

“‘I want to support tax reform and it's important for the country, but I think this needs to be part of it,’ Rubio told reporters.”

Our take: It’s hard to see Rubio eventually voting against the final tax bill, but he’s definitely trying to exert his leverage here.

Trump administration’s shortened Obamacare enrollment period ends today

“As of last Saturday, 4.7 million people had signed up for health insurance on the federal exchanges set up by the 2010 Affordable Care Act, the Health and Human Services Department reports,” per NBC’s Maggie Fox. “That’s 650,000 more than had signed up at the same time last year.”

“But open enrollment ends sooner this year than in years past. Friday is the last day for people wanting to buy insurance that starts Jan. 1 on the federal exchanges. Some states that run their own exchanges have extended this period — Maryland extended the signup period for its exchange to December 22, while Massachusetts allows signups until Jan. 23. And so unless another 4 million people sign up in the coming days, total enrollment will likely fall short of last year’s total of 9.2 million who got health insurance on the federal exchanges.”

Fox adds, “The White House ordered HHS to scale back enrollment outreach and fought with insurance companies over subsidies, a battle that resulted in higher premiums in some places. And HHS also shortened the enrollment period for 2018, raising fears that four years of coverage gains would be lost. ‘They cut the enrollment period, they slashed the outreach and they are trying to do everything they can to give it death by a thousand cuts,’ said Wisconsin Rep. Mark Pocan, a Democrat.”

Arizona and Michigan both saw congressmen resign this month. But Arizona will have a replacement much sooner than Michigan will

With the rash of recent resignations on Capitol Hill, campaign reporters have had plenty of reasons to brush up on special election laws in states around the country. And those laws – and the decision of one governor — mean that one set of a departing congressman’s constituents will have representation in Congress for half a year more than another district’s, despite having representatives who resigned due to sexual harassment allegations in the very same week.

In Arizona, Republican Gov. Doug Ducey announced earlier this week that the special election to replace outgoing Rep. Trent Franks will be held on April 24, with a primary in February. He didn’t have much choice; Ducey was bound by state law that required that the special primary be held between 80 and 90 days after the vacancy, with a general election held between fifty and sixty days later.

But in Michigan, constituents in the 13th congressional district will have to wait until next November to see departing Rep. John Conyers replaced. Per Michigan state law, the governor has wide discretion when it comes to declaring the timing of both a special primary and special general election to fill a congressional vacancy, with no limits on how soon those elections must be held. Republican Gov. Rick Snyder could have, for example, scheduled those contests to coincide with election dates already on the calendar (there is a regularly scheduled election for some local offices in the state on May 7, 2018, and the state’s regularly scheduled statewide primary election day is August 7). But he instead chose to punt the election until the midterm election in November. That means 11 months without representation in Congress.

After criticizing it, Trump travels to the FBI

At 10:00 am ET, President Trump participates in a graduation ceremony at the FBI. The visit comes after he tweeted this earlier in the month: “After years of Comey, with the phony and dishonest Clinton investigation (and more), running the FBI, its reputation is in Tatters - worst in History! But fear not, we will bring it back to greatness.”