Congressional Democrats signaled Monday that they’re ready to block spending bills over President Trump’s planned border wall, raising the risks of a new government shutdown early in the new administration.

Adding more immigration agents to Homeland Security’s deportation agency, making Planned Parenthood ineligible for federal cash and changing former President Barack Obama’s policies on the environment, labor, veterans or consumer protections would also invite a shutdown, Senate Democrats said in a letter to GOP leaders.

Democrats are confident Republicans would shoulder the blame for a shutdown — though it’s a bigger risk for the minority party this year, when they have little control over the process other than the power of a filibuster in the Senate.

“If Republicans insist on inserting poison pill riders such as defunding Planned Parenthood, building a border wall, or starting a deportation force, they will be shutting down the government and delivering a severe blow to our economy,” the Democrats argued in their letter.

Republicans waved the warning aside, saying they’re focused on trying to write the bills needed to keep the government open beyond April 28, when current stopgap funding expires.

The Democrats’ letter is the fiercest note of resistance yet to Mr. Trump’s border wall, which was a major part of his 2016 presidential campaign, but which has proved more difficult to get going now that he’s in office.

Senate Minority Leader Charles E. Schumer and his lieutenants doubted a wall would work at stemming the flow of illegal immigrants, and they said it doesn’t sound like Mr. Trump has a concrete plan in mind anyway. Without that, they said, there’s reason to fear the money he asks for now would be wasted.

“The administration has not detailed how to 1) use eminent domain and other procedures to acquire land from private landowners and Native American tribes, 2) design the wall, 3) locate the wall, 4) construct the wall, and 5) require Mexico to pay for the wall given the president’s clear and repeated promises that he would force Mexico to do so,” the senators said in their letter.

Mr. Trump is scheduled to deliver his 2018 budget to Congress later this week, but Capitol Hill is still enmeshed in debates over fiscal year 2017, which is nearly halfway over — yet none of the 12 annual spending bills have been completed.

After Mr. Trump’s election last year, Congress and President Obama agreed to postpone decisions until the new administration could take over.

The House last week passed the 2017 defense spending bill, which now needs action in the Senate. Eleven other bills are also on tap, though Congress could combine them into one large “omnibus” bill or several bills.

If some or all of the bills aren’t signed into law by the end of the day on April 28, the government would go into a partial shutdown.

Democrats said Republicans agreed in 2015 to conditions for the debate, including an overall cap on spending and a balance of defense and domestic programs. Democrats also said the GOP agreed not to include any “poison pill” add-ons.

Yet spending bills are always an enticing place for the majority party to try to take care of priorities. In the past they’ve been used to roll back administration environmental regulations, alter major immigration laws or block the executive branch from new actions.

Republicans have long sought to strip federal funding for Planned Parenthood from spending bills, but Democrats for the last decade have had control of at least one political arm of government and have been able to block them.

A fight over Planned Parenthood nearly sparked a government shutdown in 2015, when Republicans controlled both the House and Senate but Democrats had the White House.

This year the bigger fight could come not over money the GOP wants to strip out, but money it wants to include.

Mr. Trump’s budget director has said he’ll ask for a small down payment to get his border wall started in the 2017 spending bill and another chunk in the 2018 budget. But the real money will start to flow in the 2019 budget, Office of Management and Budget Director Mick Mulvaney said last week.

He said estimates ran anywhere from $8 million a mile to $25 million a mile for construction.

As for Mr. Trump’s vow to make Mexico pay for the wall, the administration has given up on that for now, saying taxpayers will foot the bill and Mexico will be forced to pony up later. Top Republican Sen. Mitch McConnell, however, dismissed that idea last week, saying Mexico won’t end up paying.

Mr. Trump has also signed an executive order calling for 5,000 more Border Patrol agents and 10,000 more Immigration and Customs Enforcement agents to be hired — and Democrats said money for the ICE agents would likewise spark a shutdown showdown.

Immigrant rights groups praised Democrats for their shutdown threat.

“We salute them for their leadership. They are standing up to Trump’s immigration budget, because they rightly believe that our tax dollars should be spent combating poverty, expanding access to health care and promoting public safety — not on separating American families and building a stupid wall,” said Frank Sharry, executive director of America’s Voice, a leading advocacy group.

Activists have vowed resistance to Mr. Trump’s immigration plans.

One congressman, Rep. Luis V. Gutierrez, Illinois Democrat, met with ICE officials Monday to argue against the deportation of a woman with six children and an American citizen husband. Mr. Gutierrez said he was risking arrest by refusing to leave ICE’s Chicago offices.

Meanwhile, Senate Democrats announced a bill Monday to revoke Mr. Trump’s latest extreme vetting executive order, saying it’s discriminatory and counterproductive to the country’s efforts to influence allies.

Led by Sen. Dianne Feinstein, ranking Democrat on the Judiciary Committee, the new bill is succinct: It cancels the March 6 order that suspends the refugee program and puts a 90-day halt on most admissions from six Middle East and African countries.

“President Trump’s discriminatory executive order isn’t about strengthening our national security. It’s a transparent effort to fulfill his campaign promise to implement a ‘Muslim Ban,’ ” Mrs. Feinstein said in a statement.

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