Immigration activists demonstrate inside the Russell Senate Office Building on Wednesday. (John Moore/Getty Images)

“NEGOTIATIONS TO START now!” President Trump tweeted Friday, after signing a major spending bill that will keep the government open. In fact, the negotiations to which he was referring — on protecting the “dreamers,” undocumented immigrants brought to the United States as children who have integrated into American society — have been going for months, with agreement in several instances spoiled by Mr. Trump himself. Now that they have swept major spending questions off their agenda, lawmakers should stop waiting for the White House to be reasonable and work their will like a separate branch of government.

An early-March deadline to take care of the dreamers is approaching, and Senate Majority Leader Mitch McConnell (R-Ky.) promised to bring up a dreamers bill immediately after government funding questions were solved. Accordingly, the Senate will consider the dreamers issue in an open process next week, opening the floor to whatever package can attract 60 votes. This open process presents an opportunity to demonstrate where the Senate — and the country — really is. Those in the Senate who favor a “compromise” on the model Mr. Trump proposed — which demands a slew of hard-line restrictionist measures in return for clarifying the status of just a fraction of undocumented people in the United States — can bring it up and lose. A more reasonable trade of dreamers protections for more border security, perhaps even some wall funding, is the supermajority policy that should instead emerge.

Unfortunately, House leaders have not committed to such a free process. Minority Leader Nancy Pelosi (D-Calif.) delivered an unprecedented eight-hour floor speech and threatened to fight the recent budget deal if she did not get assurances that the House would follow the Senate’s procedural lead. Speaker Paul D. Ryan (R-Wis.) nevertheless swayed enough Democrats to vote for the budget deal by promising on Thursday that “we will bring a solution to the floor, one the president will sign.”

The first half of that pledge, which Mr. Ryan reiterated Friday morning, was comforting. The second half was disconcerting. Mr. Trump’s erratic record of “negotiating” on dreamers legislation, in which he twice killed real bipartisan deals after previously signaling support, should have already persuaded lawmakers to decouple their work from the president’s wayward, unpredictable preferences. Like the Senate, the House should show the president what an authentic majority bill looks like and dare him to veto it.

Throughout the recent dreamers debate, the president has gotten away with taking an absurd position. He claims he wants to help the dreamers, then insists he needs outrageous concessions to do so. When lawmakers balk at his demands, he blames them for hurting the dreamers. It is time Congress turned the tables. Lawmakers should give him an opportunity to help the dreamers, along with some concessions, and leave no doubt about who is hurting the dreamers if he still says no.