At some point in 2019 (if not sooner) a Republican Senator may walk into the Oval Office and say to President Trump: "Mr. President, we don't have the votes," at which point the Trump presidency will end in a resignation or a conviction in the Senate.
This scenario actually occurred forty-three years ago this summer when Republican Senator Barry Goldwater walked into the Oval Office and told Republican President Richard Nixon that they didn't have the votes in the Senate to save his presidency.
Following impeachment in the House, a trial takes place in the Senate. Conviction requires two-thirds of the Senate and by my count there are already twelve senators who have shown a willingness to take on the president when they believe he is in the wrong.
If you add that to the forty-eight Democrats in the Senate (who have shown no inclination to work with this President), Donald Trump could be six votes away from conviction in the Senate.
Of course this assumes that the forces now in motion continue on their same trajectory and result in an impeachment vote. They are: the investigations into the Trump campaign; evidence of weakness in the Republican base ; historical trends indicating a possible Democratic takeover in the House; and, last but not least, defiance in the Senate. 
This last trend should be particularly worrisome for the president. Article I of the Constitution gives them the last word on the presidency. And yet instead of making friends in the Senate, Trump has done exactly the opposite.
After the Senate failed to pass his Obamacare replacement, Trump took to Twitter to denounce them as "fools" and "total quitters." That could not have gone over well with the senators who opposed him along the way. One of them, Senator Susan Collins (R-Maine), arrived home to an impromptu gathering of supporters at the Portland airport who applauded her vote against the president's replacement of Obamacare.
Senator Lisa Murkowski (R-Alaska) was defiant after being called out by the president, saying " No second thoughts at all. None," after her vote against the president on health care. Senator Mike Lee (R-Utah) has been none too pleased with the efforts to repeal Obamacare, insisting that it " does not go far enough in lowering premiums for middle-class families."
But perhaps the most high profile opposition to the president came from Senator John McCain (R-Ariz.), who returned to Washington from his hospital bed to cast the dramatic and final vote killing the Republican replacement for Obamacare. After Trump, during the presidential campaign, ridiculed McCain's seven years in a prison camp in Hanoi, the Arizona senator showed he is clearly not afraid to take on the president.
Another Republican senator from the west, Dean Heller (R-Nev.) also felt free to criticize the President and vote against him on several key issues. Senators Rob Portman (R-Ohio) and Shelley Moore Capito (R-W.Va.) have been vocal in their opposition to the president's budget—especially the proposed cuts in drug treatment programs.
Capito threatened to lead "a bipartisan group of my colleagues on the Appropriations Committee and in the Senate to reject those proposed cuts." They are part of the 18 Republican senators who voted against the Trump budget.
A second major setback for the president in the Senate was passage of a Russia sanctions bill that curtailed the president's freedom of action in adjusting sanctions—a clear signal that an overwhelming number of senators don't trust the president on Russia issues.
As a further reflection of that inter-branch distrust, there are two bipartisan bills in the Senate which would check the president's ability to fire the special prosecutor Robert Mueller looking into the Russia issue.
One is sponsored by Senator Thom Tillis (R. N.C.) and Senator Christopher Coons, (D-Del.) and the other by Senator Lindsey Graham (R-S.C.) and Senator Cory Booker (D-N.J.). Senator Graham has gone so far as to warn the president that firing Mueller would mark "the beginning of the end of the Trump Administration."
And then there is the man who used to be the president's closest friend in the Senate, Jeff Sessions, who endorsed Trump when no one else would and became his Attorney General only to suffer weeks of embarrassing insults from the President.
Senators such as Chuck Grassley (R-Iowa) have defended Sessions against the president, warning that if Sessions is fired there will not be a confirmation hearing for another attorney general this year.
Senator Rand Paul (R-Ky.) has been a consistent thorn in President Trump's side, repeatedly questioning his foreign policy appointments and insisting in an op-ed, "Make no mistake, no matter who is president or what their party is, it is my firm belief that the president needs congressional authorization for military action, as required by the Constitution."
Finally, Senator Jeff Flake, (R-Ariz.) wrote an entire book accusing President Trump of abandoning conservative Republican principles. Flake is facing a tough re-election race, and his book Conscience of a Conservative (the same title used by his hero Senator Barry Goldwater 57 years ago), is either a Hail Mary play, a genuine attack on what Trump has done to his party, or both. In it he writes, "Never has a party so quickly or easily abandoned its core principles as my party did in the course of the 2016 campaign."
These 12 Republicans have no fear of the president. You could probably add Senator Cory Gardner (R-Colo.), who is up for re-election in 2020 and whose state also voted for Clinton in 2016. The president needs to start making friends in the United States Senate.
 On average, since the Truman Era, a president's party loses more than 28 House seats in his first midterm election. (In 2018, Democrats need to pick up 24 seats to win back the House.)
Elaine C. Kamarck is a Senior Fellow in the Governance Studies program as well as the Director of the Center for Effective Public Management at the Brookings Institution. She is the author of " Primary Politics: Everything You Need to Know about How America Nominates Its Presidential Candidates ," " Why Presidents Fail And How They Can Succeed Again ," " How Change Happens—or Doesn't: The Politics of US Public Policy " and " The End of Government--As We Know It: Making Public Policy Work."