President Trump meets with Senate Majority Leader Mitch McConnell (R-Ky.), second from left, Senate Minority Leader Charles E. Schumer (D-N.Y.), second from right, House Minority Leader Nancy Pelosi (D-Calif.) and other congressional leaders. (Kevin Lamarque/Reuters)

On the heels of President Trump and Democratic leaders’ deal last week, Congress and the president on Friday enacted a bill to suspend the debt limit, allowing the Treasury Department to issue more debt to meet its obligations.

Such votes are not unusual: Congress has raised or suspended the debt ceiling 79 times since 1960, and every president since Harry S. Truman has signed into law at least one such bill. What was unusual about last week’s votes was this: Democrats provided the lion’s share of votes for the bill — reversing a long-standing pattern. The burden of raising the debt ceiling has historically fallen on the party in power.

Here’s a look at why the parties traded places and why it matters.

This is what debt ceiling votes usually look like

In last week’s vote, only 60 percent of Republicans in the House and two-thirds in the Senate voted to raise the limit. Meanwhile, Democrats, the minority in both chambers, voted as a bloc in favor.

That’s precisely the opposite of what we see historically.

The two graphs below show the percentage of House and Senate Republicans and Democrats voting to raise or suspend the debt limit, averaged across all of the votes between 1953 and 2014. Members of the party out of power — here, the minority party during a period of unified party control of government — traditionally offer the least amount of support for debt limit increases.

When a single party controls both Congress and the White House, an average of 13 percent of a House minority party and a third of a Senate minority party vote to increase the debt limit. By contrast, overwhelming majorities (80 percent or more) of the party in power during unified control normally vote to raise the debt limit.

Moreover, it doesn’t matter whether Democrats or Republicans are in the majority during debt-limit increases. There have been very few differences between the parties when placed in similar circumstances.

Why do these divisions between the majority and minority parties arise? One reason is that raising the debt ceiling has never been popular. Opinion polls typically show the public disapproving of a debt-limit increase by 2 to 1. So debt limit votes provide the minority party a great opportunity to criticize the majority party.

When Democrats are the minority party, they typically use debt-limit votes to criticize Republicans’ deficit-increasing tax cuts. When Republicans are the minority party, they typically criticize Democrats’ spending.

Either way, the party out of power tends to force the majority to muster the votes to do the unpopular thing and raise the debt limit.

Why was it so topsy-turvy this time?

Republican leaders were hoping to pass a longer-term debt increase. They wanted to avoid another vote on the debt limit before the midterm elections and thought they could force Democrats to grant them a longer extension.

But Republican leaders faced resistance within their own party. Much of the resistance came from tea party Republicans elected after 2010 who had never before served under a Republican president. They want spending cuts in exchange for supporting debt-limit increases. Likewise, conservative groups such as Club for Growth and Heritage Action oppose any “clean” debt-limit increase — one that does not include broader fiscal reforms.

And so, once again, Republican leaders were struggling to advance must-pass bills such as the debt limit. When the party in power cannot do this, it cedes power to the minority.

Thus it was no surprise that Trump bypassed Republican holdouts and opted for the easy deal being offered by Democrats. Republican leaders were astounded. But as Rep. Mark Walker (R-N.C.), chairman of the Republican Study Committee, remarked: “If Republicans won’t work with the president … then maybe he goes and finds somebody who’s willing to.”

Perhaps Trump accepted a worse deal than was necessary. But a majority party that cannot muster the necessary votes is in no position to drive a hard bargain.

Republicans may learn that it’s better to support their own leaders on must-pass bills rather than force them to cobble together votes from the minority. House Speaker Paul D. Ryan (R-Wis.) has expressed hope that his party is just undergoing “growing pains” in transitioning to a “governing party.”

But if a decisive contingent of Republicans continues to withhold its support on critical votes such as the debt limit, Republican leaders will be weakened in their negotiations with both Trump and the Democrats. As House Minority Leader Nancy Pelosi (D-Calif.)  explained: “Here, the currency of the realm is the vote. You have the votes, no discussion necessary.”

Frances Lee is a professor of government and politics at the University of Maryland.