President Trump’s decision to unilaterally cut funding through a pocket rescission is leading one key Democratic senator to warn that he may not be able to sign on to any spending agreement this year, unless his Republican colleagues stand up for Congress’s power of the purse.

Senator Chris Coons, who serves on the Appropriations Committee, is one of the first moderate members of the Democratic caucus to signal a willingness to fight the president on this.

Mr. Coons, who has served in the Senate since 2011, is considered one of the most influential lawmakers in the chamber as a leading member of the Appropriations Committee. He also famously gets along with many of his GOP colleagues, unlike some other Democrats. 

During an interview with NPR’s “Morning Edition” on Tuesday, however, Mr. Coons was unable to hide his frustration with Senate Republicans. He says the president’s pocket rescission of $5 billion in foreign aid spending — which was appropriated as part of a bipartisan funding agreement — is a red line for him. 

“We’re gonna try hard to keep the government open and to pass appropriations bills, but bluntly, President Trump and his allies in Congress have already been shutting down whole parts of the government … through rescissions, and now through unconstitutional and dangerous pocket rescissions,” Mr. Coons warned Tuesday. 

The Delaware senator serves as the top Democrat on the appropriations panel’s subcommittee for defense spending, which is charged with writing the annual budget for the Pentagon and related programs. 

In the Senate, 60 votes are required to proceed to final passage of any funding agreement, meaning the majority leader, Senator John Thune, needs at least seven Democrats to go along with him to proceed, assuming all other Republicans are on board. 

“If they continue in those efforts, I won’t support keeping the government open,” Mr. Coons said Tuesday morning. 

If Republicans don’t do something to stand up for Congress’s Article I powers, he says, then Democrats will do everything in their power to grind the Senate to a halt in an act of protest. 

“We’re gonna stand up to his nominees, we’re gonna stand up to his overreach, and I’m not gonna go along with keeping the government open unless they change course,” Mr. Coons said. He listed a congressionally approved rescissions package that cut money for public broadcasting and USAID, along with last week’s pocket rescissions announcement, as his greatest concerns. 

“We’ve got a fight on our hands this month,” Mr. Coons said, noting that Senate Democrats will be debating their tactics “a great deal” over the course of the next month. The government is set to shut down on September 30 if no deal is reached. 

“It breaks the basic role of Congress,” Mr. Coons said of the president’s actions last week. “I’m hopeful that the courts will block these pocket rescissions, but my Republican colleagues also need to step up and make clear to the president: Congress is responsible for spending.”

“If they don’t join us in doing that, I don’t see any way forward,” he said. 

Mr. Coons argues that these rescission efforts are part of a broader movement by the president and his allies to whittle away at the role and responsibilities of Congress.

“He wants to break the guardrails, he wants to reduce or eliminate Congress’s power to challenge him, and we need patriots in the Republican caucus to stand up to him,” Mr. Coons argued Tuesday. “Otherwise, we are at real risk of just becoming shiny hood ornaments who meet in Washington, who speak, who debate, but who don’t actually constrain the president’s power.”

“Our framers wanted us to be a constraint on presidential power,” the senator said.