The big tents are shrinking. Republicans and Democrats used to take pride in calling their respective parties “big tents” with room for a diversity of views. That tradition has ended for Republicans. It may be on its way out for Democrats as well.

Some liberals are celebrating the fact that retiring Senate Democratic Leader Harry Reid of Nevada will soon be replaced by Senator Charles Schumer of New York. That’s because Nevada is a swing state. “By choosing leaders outside the party’s home turf,” Ari Melber wrote for Politico in 2010, “Senate Democrats up the odds that their standard-bearers will be distracted by close races — caught between the politics of their constituents and the national party.” Reid squeaked by in his 2010 reelection race with 50.3 percent of the vote.

That’s not likely to be a problem for Schumer. New York is a deep-blue state. Schumer won with 67 percent of the vote in 2010.

The main reason the tents are shrinking is that the number of swing states is diminishing. Congressional Republicans increasingly represent red states and congressional Democrats blue states. Thirty percent of Democrats in the House of Representatives now come from just two states — New York and California.

At the same time, there is pressure to “purify” the party leadership by making sure it represents reliable party supporters and won’t face pressure to make deals – that is, “sell out.” This has already happened to Republicans. Representative Steve Scalise (R-La.) got elected House Republican Whip in 2014 largely because conservatives demanded a Southern Republican in a leadership position.

Scalise explained his role in an interview with Politico: “My goal is to get leadership to bring more conservative policies to the floor. It’s not . . . to get conservatives to vote for something they don’t like.”

The shutdown of the Republicans’ big tent occurred in three stages. First, conservatives demanded a place at the table. They got one when Arizona Senator Barry M. Goldwater won the Republican Party nomination in 1964 by offering “a choice, not an echo.”

Goldwater lost big time to Lyndon B. Johnson, but conservatives gained influence in the Republican Party. Second, conservatives won control of the Republican agenda. That happened when Ronald Reagan got elected president in 1980.

In the third and final stage, conservatives banished dissenters from the table. That happened with the rise of the Tea Party in 2009 and 2010. Tea Party Republicans refuse to tolerate moderates — or even conservatives willing to make deals. They’re party poopers. The Tea Party brought down former House Majority Leader Eric Cantor (R-Va.) last year, and the long knives are now out for House Speaker John Boehner (R-Ohio.).

The Democratic Party went through stage one when South Dakota Senator George McGovern won the nomination in 1972. McGovern, too, lost the election big time, but liberals gained a seat at the Democratic Party table. It wasn’t until the election of Barack Obama in 2008 as president that liberals gained control of the Democratic agenda. The 2010 and 2014 midterms saw a massive purge of conservative and moderate Democrats. But it was not at the hands of liberals. Moderate and conservative voters threw Democrats out.

Now liberal groups are trying to “purify” the Democratic Party’s message, mostly by pulling likely nominee Hillary Clinton to the left. Liberal activists are urging Senator Elizabeth Warren (D-Mass.) to challenge Clinton for the Democratic nomination. Their argument? A letter just released by a pro-Warren group warns, “If we end up with a single Democratic candidate — and little or no debate in the primaries — those of us unlikely to support a Republican nominee will be left voting for a Democrat who may be opposed to the Republican agenda but is not necessarily a champion of the vision of change that millions of us seek.”

What’s wrong with offering voters a clear choice? Nothing, really, except that the Constitution makes it difficult for one party to govern on its own. It’s not like a parliamentary system, where one party can take over the government and simply pass its program.

To do that in the United States, a party has to control the White House, the House of Representatives and at least 60 votes in the Senate. That rarely happens.

Democrats did control everything for a year when Obama took the Oval Office in 2009. But they still had trouble passing healthcare reform.

Normally, the only way Washington can work is through compromise and deal-making. President Bill Clinton once said, “If you read the Constitution, it ought to be subtitled, ‘Let’s make a deal.’”

As the party tents shrink, the distance between them gets bigger. And it gets harder to make deals. The likely result? Perpetual gridlock, which can be resolved only by the least democratic institution of government — the federal courts.