The Fix's Amber Phillips explains the tight deadlines Congress faces as they return from summer recess, and how President Trump's shutdown threat over funding his border wall and criticism of the debt ceiling "mess" threaten their agenda. (Jenny Starrs/The Washington Post)

The conflict between President Trump and Congress escalated a day after he threatened to shut down the federal government over funding for a U.S.-Mexico border wall as well as targeted opponents in Congress, aggravating tensions as a difficult legislative agenda looms.

Trump is now at odds not only with Democrats, who cemented their objections to funding the wall Wednesday, but also with Republicans, who must reconcile his brash rhetoric with the governing realities of Congress.

House Speaker Paul D. Ryan (R-Wis.) played down the prospect of a shutdown, telling reporters Wednesday that even if the wall debate remains unresolved, Congress probably would pass a stopgap extension of funding to prevent a lapse when the fiscal year ends on Sept. 30.

Other lawmakers chided the president for the attacks he fired off during a campaign-style rally in Phoenix on Tuesday evening — including indirect references to Arizona’s two Republican senators, Jeff Flake and John McCain.

“It’s entirely counterproductive for the president to be picking fights with Republican senators who he will need for important agenda items that they both agree on,” said Rep. Charlie Dent (R-Pa.). “Does he think that Democratic senators will be more cooperative than John McCain and Jeff Flake and Susan Collins? It doesn’t seem to make any sense.”

Billy Foster’s Texas ranch sits along the U.S.-Mexico border. He wants more security, but not a physical wall. (Zoeann Murphy/The Washington Post)

Republicans face a litany of high-stakes deadlines when they return to Washington after Labor Day: to extend funding for government agencies, raise the nation’s borrowing limit, and reauthorize programs for flood insurance and children’s health. GOP leaders also hope to begin an ambitious effort to rewrite the federal tax code in a bid to rescue their foundering legislative agenda.

“So I don’t think anyone is interested in having a shutdown,” Ryan said at a tax policy event in Oregon. “I don’t think it’s in our interest to do so.”

Trump and Senate Majority Leader Mitch McConnell (R-Ky.) moved to dispel reports of a relationship that has frayed through mutual frustrations — McConnell’s exasperation with Trump’s response to deadly violence at a recent white-nationalist rally in Virginia, and Trump’s annoyance about a stalled congressional agenda.

Both men issued statements Wednesday saying they are dedicated to working together on Republican priorities.

“We have a lot of work ahead of us, and we are committed to advancing our shared agenda together, and anyone who suggests otherwise is clearly not part of the conversation,” McConnell said.

White House press secretary Sarah Huckabee Sanders said that the two “remain united on many shared priorities, including middle-class tax relief, strengthening the military, constructing a southern border wall, and other important issues,” and that they plan to meet when the congressional recess ends after Labor Day.

The gesture of detente came after a New York Times report highlighted their rocky relationship and after Trump’s salvo in Phoenix — an ultimatum aimed squarely at Capitol Hill.

Senate Majority Leader Mitch McConnell (R-Ky.) and President Trump's relationship is fraying amid Trump's repeated public attacks and controversial statements. (Jenny Starrs/The Washington Post)

“Build that wall,” he said. “Now, the obstructionist Democrats would like us not to do it. But believe me, if we have to close down our government, we’re building that wall.”

At other points during the rally, Trump urged McConnell to ditch the long-standing Senate rules that give the minority party the ability to block major bills. He hinted that he would pardon former Arizona county sheriff Joe Arpaio, a divisive figure who has been found in criminal contempt by a federal judge. And without naming them, he referred to the opposition he has faced from Flake and McCain.

“I will not mention any names — very presidential, isn’t it?” he said, only to take aim at Flake as “weak on crime & border” in a tweet hours later.

The shutdown threat is a response to the leverage granted to the minority party in the Senate. Although Republicans control the House, the Senate and the White House, Democrats have enough votes in the Senate to filibuster any spending bill, giving them the power to make demands regarding what is and isn’t included in a funding package.

Democratic leaders reiterated their opposition to Trump’s proposal Wednesday. Senate Minority Leader Charles E. Schumer (N.Y.) and House Minority Leader Nancy Pelosi (Calif.) said Trump would be responsible if the government shut down over the impasse.

“If the President pursues this path, against the wishes of both Republicans and Democrats, as well as the majority of the American people, he will be heading toward a government shutdown which nobody will like and which won’t accomplish anything,” Schumer said in a statement.

Public opinion runs against the border wall, although most Republicans support it. Rasmussen Reports, a conservative-leaning firm, conducted an automated poll of likely voters late last month and found that a solid majority of Americans oppose building a border wall “to help stop illegal immigration,” with 37 percent supporting Trump’s proposal vs. 56 percent opposing it. That is similar to a poll conducted in February by the nonpartisan Pew Research Center that found Americans opposed the wall 62 percent to 35 percent.

Ryan said the border wall should ultimately be funded, reflecting the wishes of most congressional Republicans, including key conservatives who have rallied to Trump’s side. But he has refrained from engaging in Trump’s red-meat “build the wall” rhetoric, in what GOP aides described as an effort to avoid poisoning upcoming negotiations with Democrats.

“We do agree that we need to have the physical barrier on the border. We do need to have border control. We do need to enforce our borders,” Ryan said. “We completely agree on that.”

McConnell has avoided direct calls for a wall while supporting tougher border security generally. Although Trump claimed Wednesday that the two share that goal, McConnell did not mention the wall in his statement.

Aides also took note Tuesday that although Trump threatened to “close down our government” over the border wall issue, he stopped short of an explicit threat to veto any spending bill that did not include wall funding.

A veto threat could box in GOP leaders as they prepare to negotiate with Democratic leaders who have pledged never to support funding for the border wall. Trump clearly placed the wall at the center of those negotiations, increasing pressure on congressional Republicans to deliver.

Conservatives, however, already smarting from the GOP’s inability to pass health-care legislation, say the party is rightly feeling pressed.

“Obviously no one wants a government shutdown, but if Charles E. Schumer thinks it’s more important to hold up a bill that does what the American people elected their government to do, then let’s have that debate,” Rep. Jim Jordan (R-Ohio), a leader of the conservative bloc in the House, said Wednesday.

In recent years, when Democrat Barack Obama was in the White House and Republicans held one or both chambers of Congress, partisan demands over federal spending were hashed out among top leaders in closed-door negotiations. While leaders on both sides set out aggressive positions, they typically refrained from issuing hard ultimatums to preserve space to bargain.

The White House hasn’t decided whether Trump would agree to a stopgap spending bill, a senior administration official said, as he still needs to see details from lawmakers. Trump could block such a measure or agree to a short-term extension in the hopes that it would give lawmakers more time to agree on funding for a new wall.

Jordan said Wednesday that conservatives would be inclined to support an extension but are wary of any attempt to set up a high-stakes deadline ahead of the year-end holidays.

“Is this going to come due at December 23 at midnight? Usually when you do those things you sometimes get some things that aren’t the best for the taxpayer,” he said. “I think the timing’s important.”

During the presidential campaign last year, Trump vowed to force Mexico to fund construction of a wall along the U.S. border that he said could be up to 50 feet tall. Since the election, he has changed course, saying that Congress instead needs to spend taxpayer money to begin building new segments of the wall. Parts of the border already have a wall or fence.

On Wednesday, a committee that is raising money for Trump’s reelection campaign and the Republican National Committee sent an email to supporters calling on them to pressure Senate lawmakers that “the American VOTERS want this beautiful, impenetrable wall constructed” and asked them to digitally sign an “Official Build The Wall Petition.”

House Republicans voted last month to provide $1.6 billion in seed funding for the wall as part of a larger spending package. That bill is not expected to be taken up in the Senate.

Spending legislation that passed into law earlier this year did not include wall funding after Democrats refused to accept it. That impasse increased pressure on Republicans to deliver wall funding in future spending battles.

Dent, a senior member of the House Appropriations Committee, accused Democrats of being unreasonable in drawing a hard line against any wall funding — particularly, he said, because many of them supported a 2005 law mandating an expanded border fence. But he said he was concerned about the lack of specifics regarding what the Trump administration is planning for the border.

“It would be nice to know exactly what we we’re talking about and how much money,” he said. “If you’re asking me if it’s worth shutting the government over $1.6 billion, I think that would be a mistake. It would be very bad for the country, terrible politically, and completely counterproductive.”

Sean Sullivan contributed to this report.