Republicans have a majority in both the House and Senate, and there’s a Republican in the White House. So why does it seem the GOP doesn’t fully control the levers of Washington power?

The party hasn’t been able to repeal Obamacare, after all. An upcoming tax package remains a work in progress. Beyond that, the legislative outlook is hazy. Maybe they’ll get around to an infrastructure bill. But that’s far from a sure thing.

Like many US chief executives before him, President Trump is discovering that partisan dominance isn’t a magic button. There are numerous impediments to a party working its will in national governance, even if it has a congressional majority and holds the executive branch.

One is the particular interplay of the president’s personality and congressional leaders. But perhaps the biggest is the very nature of the US political system. There are only two major parties, meaning that by definition both will have numerous factions. That guarantees lots of colorful internal disagreement.

“Presidents have learned the hard way they can’t always count on their parties supporting them,” says Brian Balogh, an associate professor of history at the University of Virginia and co-host of the podcast “BackStory with the American History Guys.”

Of course, as far as party leaders are concerned, unitary control is still a lot better than the alternative. House and Senate majorities, combined with the Oval Office, have produced some of the most productive periods in US history, as far as passage of major laws is concerned.

Passing bills is just plain hard

The New Deal began with a historic spate of legislation passed by a Democratic Congress and signed by Democrat Franklin Delano Roosevelt in his first 100 days or so in 1933. A second New Deal in 1935 and 1936 produced major additions such as Social Security and rural electrification.

Lyndon Johnson used large Democratic majorities won in 1964 to enact his Great Society, including Medicare, the Voting Rights Act, and other milestone bills.

With those events as context, it’s easy to believe that anytime one party bestrides Washington, big things should result. But that’s just not the case. History is also full of times when presidents and congresses of the same party just couldn’t get in synch.

“Unified control is not a silver bullet. There are a lot of barriers to a party working its will,” said Sarah Binder, a senior fellow at the Brookings Institution and a political science professor at George Washington University, at a National Press Foundation seminar earlier this year.

One barrier is obvious to anyone who has spent time in D.C. – passing bills is just hard.

National legislation is complicated business. Big bills attract lots of attention and comment and lobbying and pressure from constituents. This can weigh against what party leaders want to do. At the least, it slows the process down.

“These things look very simple ... when you are looking in from the outside. As we know, being here, these are difficult things to get done,” says Sen. John Boozman (R) of Arkansas, interviewed on his way to an evening vote.

A second barrier is the variable nature of the relationship between Capitol Hill and the White House. Democrat Jimmy Carter famously had a difficult time working with a Democratic-controlled Congress, in part because Speaker Tip O’Neill and some top Carter staffers did not get along.

Republican Herbert Hoover had the same problem. He had little experience working with Congress, and the conservatives of his party viewed him as suspiciously progressive. Thus the GOP-controlled Congress paid their party’s president little heed.

Fault lines within the GOP

But perhaps the biggest reason that one-party control isn’t overwhelming is that the parties themselves aren’t homogeneous. In European democracies there are lots of parties for people of all political persuasions – Greens, Social Democrats, Conservatives, Radicals, etc. In the American democracy most of those groups cram into the two major parties around which the nation’s political life revolves.

Democrats and Republicans are coalitions. That creates fault lines and lots of opportunity for partisan infighting.

“These factions are what has done in the best intentions of presidents of both parties,” says Brian Balogh of the University of Virginia.

Southern conservative Democrats went along with the early stages of FDR’s New Deal, for instance, but by 1937 they weren’t happy with where the Democratic administration was going. They started to put a brake on things, legislatively speaking, infuriating the president.

As a result FDR in 1938 tried to purge such Southern conservatives as Sen. Millard Tydings (D) of Maryland and Sen. “Cotton Ed” Smith (D) of South Carolina by directly supporting more liberal Democrats in primaries. The effort flopped. Of 10 conservatives targeted by FDR, only one lost.

“The others returned to Washington even more antagonistic toward the President. In addition, many other Democrats resented the President’s meddling in local affairs,” writes William Leuchtenburg, a professor emeritus of history at the University of North Carolina, in an essay on FDR for the Miller Center of Public Affairs.

Republican President Dwight Eisenhower, for his part, entered the White House with a paper-thin GOP majority in both House and Senate. He needed votes from supporters of Sen. Joseph McCarthy (R) of Wisconsin, an inflammatory disrupter whose irresponsible allegations of Communist influence were tearing apart much of the US government.

Eisenhower hated Senator McCarthy but felt constrained from counterattacks due to the nature of his party coalition at the time.

“A lot of people believe Eisenhower should have spoken out more publicly against McCarthy ... but he didn’t do that precisely because he was worried about holding together his party,” says Balogh.

A thin GOP margin of control

Now it’s President Trump’s turn to discover the challenges of trying to deal with the army of cats that is a modern US political party.

That hasn’t gone fabulously so far. True, Senate Republicans stood behind new Supreme Court justice Neil Gorsuch. But the attempt to repeal Obamacare was a disaster, as tea party conservatives and more moderate Republicans have entirely different visions of what a GOP replacement for the Affordable Care Act should look like.

Looking forward, Trump and GOP congressional leaders need to keep in mind that their majority is thin – maybe not paper-thin, but not much thicker than an L.L. Bean catalog.

With a 52 to 46 edge in the Senate (two senators are Independents), the GOP has little room to maneuver on big bills. It needs to attract eight Democrats and/or Independents to pass legislation subject to filibuster, a daunting prospect given current levels of partisan animosity.

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Perhaps with the artificial 100-day deadline passed, pressure for quick action will abate. A slower approach might result in more party unity.

“You’ve got to build a coalition and get stuff that you can actually get enacted," says Sen. John Thune (R) of South Dakota, the third ranking member of the Senate GOP leadership, interviewed Monday on his way into a party event. "I think the lesson coming out of a lot of this is that we want to make sure we get it right; it’s better than getting it fast."