WASHINGTON — An end-of-the-year showdown that was once promised over delicate issues like immigration, health care and surveillance appeared to fizzle on Wednesday as key Republicans dropped their demands to shore up shaky health insurance markets and Democrats appeared to abandon their goal to force adoption of a measure protecting young undocumented immigrants from deportation.

Instead, Congress moved toward a one-month punt that would keep the government funded into January and once again put off policy confrontations.

As Republicans celebrated the passage of their tax overhaul at the White House with President Trump, the party’s leaders in the House and Senate worked behind the scenes to shape a stopgap spending measure to fund federal agencies and avert a government shutdown.

Only days ago, Senator Susan Collins, Republican of Maine, said she would vote for the tax bill because she had secured a promise from the majority leader, Senator Mitch McConnell of Kentucky, to include two health care provisions to stabilize insurance markets in an end-of-the-year spending measure. On Wednesday, she dropped that demand and said she hoped it would be passed in early 2018.