Canadian Prime Minister Justin Trudeau is on a roll.

On Tuesday, he cheered women's rights advocates by lifting the country's ban on using foreign aid money for abortion services. He's reversed the policy of his predecessor and enthusiastically welcomed 25,000 Syrian refugees. He implemented a tax cut for middle-class Canadians, cutting the second tax bracket from 22 percent to 20.5 percent, and raised the top rate from 29 to 33 percent for Canadians making more than $155,000. Most significantly for the nation's poor, he's consolidated and boosted its programs for families with children by creating a single, simple cash benefit offering up to US$4,935 per child under 6 and US$4,164 per child ages 6 to 17.

What explains this? Is Justin Trudeau just an unusually awesome leader, bringing the kind of sweeping change that's sometimes evaded President Obama? Not really.

The difference is that Trudeau is a prime minister, and it's vastly easier for prime ministers to get their agenda passed than it is for presidents. And because left-of-center people tend to like it when the government does stuff and passes bills, that has an asymmetric benefit.

Parliamentary government produces more left-wing policies than presidential government — and Americans of a left-of-center tilt ought to be thinking of ways to move the country toward parliamentary rule.

Parliaments mean more government spending

The main argument supporters of parliaments make is that parliamentary systems tend to collapse into dictatorship a lot less often than presidential systems. This point was most influentially made by the late Yale political scientist Juan Linz, who noted in 1990, "Aside from the United States, only Chile has managed a century and a half of relatively undisturbed constitutional continuity under presidential government — but Chilean democracy broke down in the 1970s."

Despite some critics, the finding that presidential systems increase the risk of democratic breakdown is borne out by the historical record, even when you take into account the fact that presidential democracies were more likely to sprout up in countries with histories of military coups.

That argument has bipartisan appeal, but there's another reason for liberals and leftists in particular to favor parliamentary government: It tends to lead to more government spending, and in particular more redistribution, than the alternative.

"Empirically, presidential systems are associated with lower government spending and lower redistribution," Claremont Graduate University's Melissa Ziegler Rogers writes in her book The Politics of Place and the Limits of Redistribution. Indeed, economists Torsten Persson and Guido Tabellini provide considerable evidence for this proposition in their 2003 volume, The Economic Effects of Constitutions.

They find that presidential regimes tend to have smaller governments, to the tune of 5 to 8 percent of GDP. For the US, that means a reduction of $911 billion to $1.46 trillion per year. They also find that presidential regimes have significantly lower welfare spending; on its own, this finding isn't very robust, but it is, and grows larger in size, when you restrict the sample to older, developed democracies like the United States.

Persson and Tabellini further divide up regimes into majoritarian systems — those like Canada, the UK, and US with winner-take-all elections — and those with proportional representation, like Germany, New Zealand, and most Scandinavian countries. Majoritarianism also shrinks government and welfare spending, by similar magnitudes as presidentialism.

The record is pretty clear: Parliamentary governments with proportional representation spend more, including spending more on welfare, than majoritarian presidential governments like that in the US.

The problem is that presidential systems make it too hard to pass popular laws

But is this caused by differences in government systems, or just correlated with them? It's hard to tell, but there's solid reason to believe this has something to do with veto points.

When Trudeau wanted to increase welfare state spending by creating a big new entitlement for families with children, he put it in the budget, and within months checks are going to start going out. His party has a parliamentary majority, the Senate almost never rejects legislation, and the House of Commons features incredible party discipline. If Trudeau wants a child benefit, it's law.

Similarly, the UK now has a soda tax more or less because Chancellor of the Exchequer George Osborne (basically the UK's Treasury secretary equivalent) got up one day and said, "There should be a soda tax," and so it became.

By contrast, in the US, for something to become law it has to make its way through committee markup in the House, and pass the full House, and get marked up in the Senate, and pass the Senate, and arrive at a compromise version in conference committee, and have the conference report pass the House, and have the conference report pass the Senate, and have the president sign it. Also there's little party discipline in either the House or Senate, and the Supreme Court strikes stuff down more than the judiciary in Canada or especially the UK does.

When it's so much harder to pass anything, why wouldn't it be harder to pass expansions of the welfare state in particular? Yes, it's true that there would be fewer formal barriers to Republicans, say, repealing Obamacare if the US had a parliament. But they still wouldn't. Government benefits are really popular, and tend not to get repealed due to the intense outcry that would ensue. Republicans had majorities in both houses of Congress in 2005 but still couldn't pass Social Security privatization because public opposition was so intense.

The University of Konstanz's Michael Becher identifies this as the mechanism through which countries with parliaments develop bigger welfare states than countries with presidents. "Redistribution is lower in presidential than in parliamentary democracies because the separation of executive and legislative power under presidentialism reduces the frequency and bargaining power of left governments," he writes.

In other words, when left parties win in parliamentary systems, they win everything, and can readily pass legislation. When they win in presidential systems, it's rarer that they control each branch, so they have fewer opportunities to expand the welfare state.

It's not just Becher. Plenty of other analyses have argued that the increased number of veto points in presidential systems leads to lower spending. Dartmouth's Lisa Baldez and John Carey have even suggested that one cause of presidential governments, such as in the case of post-Pinochet Chile, is an active desire by elites to restrain spending and welfare state development.

More abstractly, parliamentarism is also better for accountability. Currently, US voters vote for Congress and the president largely based on the performance of the economy. But the House, Senate, and president's separate and interlocking powers make it hard to know who actually deserves the blame for the economy being bad or good. Was the economy garbage in 2010 because of Barack Obama's policies? Or because Republicans filibustered them and reduced their scale? Or because he didn't stack the Fed with doves? Who knows! In practice, people just vote against the incumbent when the economy is bad and for her when it's good, leading to more or less random outcomes.

Parliamentary systems give their governing coalitions full control over levers influencing economic outcomes. If the economy's bad in 2019, the next time Canada is slated to have an election, Justin Trudeau will probably lose, and he probably should lose for not doing more to turn things around. There's real accountability there.

Parliaments are also less likely to produce what Johns Hopkins political scientist Steve Teles calls "kludges": ugly, ill-designed policies that are meant to satisfy veto players, especially in Congress, rather than solve a single problem cleanly and clearly. If you're a prime minister with a majority and no veto points, there's no real reason not to propose a simple, straightforward child benefit program like the one Trudeau proposed.

But in the US, a committee chair over here might want to keep one of the child assistance programs being integrated, one over there might demand an unrelated concession, and so on, so the process mucks up policies until they're ugly and inefficient.

How to get to parliamentarism in the US

Amending the Constitution to get to full parliamentarism is probably impossible. Our national reverence for the document, and the concept of separation of powers, is too great. But there are some in-between steps we could take. My preferred method would be to amend the Constitution to line up House, Senate, and presidential terms, so that each had an equivalently long term (four or six years) and every House member, senator, and president is up for reelection at the exact same time. That would probably reduce the incidence of divided government substantially. You could throw in a provision saying that either half of Congress can pass a vote of no confidence and trigger new House/Senate/presidential elections within 90 days.

A more utopian idea would be to simply junk the Constitution, the way we junked the Articles of Confederation, and write a new one from scratch. Perhaps some speaker could declare the House sovereign, order the Senate and presidency dissolved, and write up a new constitution abolishing the Senate and presidency entirely and establishing the speaker of the House as head of government. If Paul Ryan is looking for ideas on how to handle a Donald Trump victory, he could do worse than this.

Another tactic would be to try to get a state to adopt parliamentary government. As UCLA's Jonathan Zasloff has noted, there's nothing in the US Constitution prohibiting states from experimenting with government structure. Nebraska has a unicameral legislature, for example. That's progress, but why not go unicameral and abolish the governorship?

This might be tricky in states where fundamental constitutional amendments have to go through the state legislature; why would legislators vote to abolish their jobs for this brand new system? But in states with generous ballot initiative rules, it could be possible. Oregon looks like a particularly promising option.

It'll be a long slog regardless, but if one or two states were to adopt parliaments, and show the others how much better life is without gridlock and with coherent governance, maybe their peers will take the hint.

Justin Trudeau is pretty sure you won't move to Canada