President Trump gathers with congressional Republicans in the Rose Garden in May. (Carlos Barria/Reuters)

If there's one thing both parties in Congress can agree on, it's this: President Trump, who bills himself as the ultimate dealmaker, is not a very trustworthy one.

That's an unpleasant reality for Republicans as they begin a make-or-break effort to try to rewrite the tax code. They have no margin for error, procedurally or politically, and they need a president who can help persuade reluctant lawmakers to get on board.

Instead, they have a president they don't trust to stick to his word. Trump's actions lately have solidified that.

As The Washington Post's Philip Rucker, Sean Sullivan and Paul Kane report, Trump called the GOP senator leading a bipartisan health-care deal one evening. The call left Sen. Lamar Alexander (R-Tenn.) feeling like he had the president's support. Ten days later, the president abruptly changed his mind.

In September, Trump hosted Democratic leaders at the White House and left them feeling as though they had the president's support on a deal to protect "dreamers." He muddied the waters on what he agreed to hours later. A month later, he closed the door completely.

In May, Trump celebrated House Republicans' version of an Affordable Care Act repeal in the Rose Garden, then behind their backs called it “mean.”

That type of ducking and twisting might have worked for Trump in the business world, but in Washington, it means he is entering one of the trickiest stretches of his presidency with a deficit of trust from Congress.

“You know, who knows where he’ll be? Maybe where he is this very second?” Sen. Bob Corker (R-Tenn.), a Trump critic, told The Post.

Trump's unreliability isn't just Congress's problem. His shaky dealmaking undercuts his own goals, too.

It makes it tougher for him to blame Congress for a lack of major legislative accomplishments: When Congress failed on health care, Trump blamed Congress. If Congress can't pass a tax bill, Trump probably will blame Congress. But that defies reality, Congress argues. Congress takes big steps only with a push from outside.

“Congress's de facto stance is gridlock,” Alex Conant, a GOP strategist and former aide to the presidential campaign of Sen. Marco Rubio (R-Fla.), told The Fix earlier this month. “Without leadership, Congress does nothing.”

It makes it very difficult for Republicans to get tax reform to his desk: The Republican Party is fractured nearly half a dozen ways — between conservatives and moderates, between moderates and Trump, between conservatives and Trump, sometimes between Trump's top aides and himself. And yet Republicans are trying to pass this bill with only Republican votes, which means they can only lose two dozen votes in the House and TWO votes in the Senate.

Republicans in Congress are going to need a unifying factor, which is a role they typically look to their party's leader to fill. Trump seems most comfortable playing the role of divider in chief.

It gives Democrats an opening to say "I told you so!": Speaking of divisiveness, Trump is handing Democrats a simple message with his repeated attacks on his party: Dysfunctional Republicans can't govern, even when they control all of Washington.

As I wrote in August, Democrats have a solid case to make on that front. After failing to deliver on a seven-year campaign promise to repeal Obamacare, Republicans and Trump fought about whether to shut down the government in September. Trump ultimately sided with Democrats on a deal to keep it open and raise the debt ceiling for three months.

History tells us presidents without a working relationship with Congress don't get a lot done: No modern president who has struggled in the first 100 days of a presidency has suddenly revved up in the next 100 days, or in the next couple of years, says Robert David Johnson, a presidential historian at Brooklyn College. Presidents can't execute their agenda on their own.

As I wrote at Trump's 100-day mark: Every day it gets more difficult to work with Congress. Every day is a day closer to the 2018 midterm elections, when vulnerable Republicans may not want to be seen compromising with an unpopular president.

If the president has proved anything, it's that he has no allegiance to any deal he makes. That makes members of Congress's lives miserable, but it also threatens to derail Trump's agenda.