We heard rare roars from the cowardly Republican lions in the Senate this week, and this time they really meant it: The president’s threat to slap escalating tariffs on Mexico is stupid and wrong and they won’t stand for it. For the first time in his presidency, GOP senators threatened a veto-proof majority to block him. And even if a deal were struck by the Monday deadline, the standoff -- between Trump and Mexico, and Trump and Senate Republicans -- is set to last a long time.

Majority Leader Mitch McConnell knows any relief from a deal will only be temporary. Two and a half years into riding the Trump tiger and a year into the trade war that’s hurting red states, there are no agreements in sight, and no end in sight. Trump may not know that yet, as he wakes each day to clobber the short term, come what may. But McConnell has a uniquely honed long view. What he can see down the road ahead to Election Day is the president’s fondness for tariffs and other drastic threats, and the majority he leads in peril.

After steep losses for Republicans in the midterms last November, smart minds like McConnell knew that the record turnout presaged another potential record, expected to be far larger, at the ballot box next year. Unlike in 2018, when Republicans in the Senate had the best map since 1938 and only picked up two seats, next year could be grim.

Trump’s approval rating is in dangerous territory in key states Republicans are defending, including Iowa (-12) North Carolina (-4) Colorado (-14) and Arizona (-6), and recent polls have former Vice President Joe Biden beating him in Texas and by double digits in North Carolina where Sen. Thom Tillis is also vulnerable.

But ever since the dust cleared on the midterm election data, Trump has done all he can to rile his voters instead of persuading new ones. First he shut the government down, then he threatened to shut down the border, then he declared a national emergency. He’s now threatened tariffs on Mexico. At the detonation of each of these explosions, McConnell has sought to stop Trump, without success.

Nearly a year ago McConnell refused to put legislation giving Congress more authority on the imposition of tariffs -- under national security criteria of Section 232 of the Trade Expansion Act -- up for a vote on the Senate floor saying, “We're not going to be, in the Senate, passing a bill preventing the president from what he can legally do under current law.” At the time McConnell also hoped Trump would not drag out a trade war. “I think the president’s pragmatic and I think he will, at some point, figure out whether this is working or not working, and do the right thing for the country,” he told Politico at the time.

But when most members of Congress, including committee chairmen, learned about the Mexico tariffs by tweet last Thursday evening, Chuck Grassley -- the Senate Finance Committee chairman -- immediately characterized the action as “a misuse of presidential tariff authority and counter to congressional intent.” Americans for Prosperity called the tariffs “the largest tax hike in modern history,” and insisted “it’s time for Congress to do its job,”

McConnell can put trade authority bills on the floor and senators can vote to disapprove the president's declaration under the International Emergency Economic Powers Act, but Trump’s latest showdown could shut the door on the only deal he has in the pipeline -- the USMCA. Despite acting Chief of Staff Mick Mulvaney’s declaration that tariffs on Mexico in an immigration fight and prospects for passing a new NAFTA through Congress “are absolutely not linked,” that's not what the Mexican government thinks. Republican leaders had estimated the only window for USMCA passage would have been by the August recess.

Meanwhile, Republicans and Democrats and the Trump administration are supposed to be working through a funding deal, legislation to lift the spending caps in the Budget Control Act to avert the sequester kicking in and an increase in the debt ceiling -- all by Sept. 30. Trump shouldn’t count on McConnell, or any Republicans up for reelection this cycle, to back him up on another government shutdown or any attempts to look for leverage in a debt ceiling standoff that risks even the appearance of default.

At the same time, there will be voices around Trump urging him to listen to McConnell even less. Conservatives Richard Viguerie and Craig Shirley wrote last month for Fox News that, post-Mueller report, “the next serious challenge Trump faces will be Senate Majority Leader Mitch McConnell and the GOP swamp that he leads.” The two criticize the Kentucky senator for rejecting “good potential nominations,” like those of Ken Cuccinelli for Homeland Security secretary. They warned that “McConnell and his weak-kneed colleagues are again failing to understand the mood of the American people.”

It’s safe to say that by the time incumbent GOP senators get through their primaries next year, including McConnell, they could also start looking for some distance from the president. If the president's approval rating is mired in the 39%-41% range, with women and college-educated and suburban voters and independents all still trending toward Democrats, the majority leader’s patience could wear quite thin.

As RCP polling analyst Sean Trende wrote this week, “[T]he GOP majority is in more danger than most analysts believe.” He cited a reliable simulation created in 2014 that factors in the president's approval rating, whether any given Senate race features an incumbent and whether there is a “highly problematic” candidate in the race. (Roy Moore is still threatening to run again in Alabama!) Using this simulation, Trende estimates Trump’s current approval rating in the low 40s is “basically the breaking point for the GOP holding the Senate,” as their projected loss would be two seats, while a net loss of three would mean losing the majority. If the president's approval dips to 40% or below, the loss of seats could reach four or more.

President Trump may continue to risk the GOP Senate majority, but he will soon learn that McConnell intends to remain majority leader, whether the president wins reelection or not.