Why are Senate Democrats unanimously supporting a government shutdown in order to benefit illegal immigrants (to state the issue most unfavorably to them)? The "Schumer Shutdown," as President Trump calls it, came even though many Senate Democrats were on record opposing a shutdown over legalizing Deferred Action for Childhood Arrivals recipients, even though they declined to vote for one on the last go-round, even though they are on record from the 2013 shutdown as opposing shutdowns over extraneous issues. It’s true that public opinion has moved away from Republicans in previous shutdowns: shutting down the government is generally unpopular, and Republicans tend to get blamed because (a) mainstream media always blames Republicans and (b) Republicans are the small government party. But there’s at least some polling showing Democrats getting hurt in the current circumstances, and yet every Senate Democrat went ahead and supported the shutdown anyway. What glue holds them together?

My answer, delving back into what is now history, is the 1986 off-year Senate elections. Going into that election, Republicans held a 53-47 majority in the Senate. President Ronald Reagan had job approval around 60 percent (it fell to just under 50 percent shortly after the election when the Iran-contra scandal unfolded). In the off-year election, the partisan balance changed little in House elections: the Democrats went in with a 253-182 majority and came out with a slightly larger 258-177 majority.

But the Senate elections were different. Democrats gained a net 8 seats, and came out with a 54-46 margin. In the 30 years that followed, Republicans have held majorities in the House more than half the time (1994-2006, 2010-16). But Democrats have held Senate majorities more than half the time as well (1986-94, 2001-2002, 2006-2014). Their 1986 victories were, in my view, critical to that success.

What’s fascinating is that seven of the Democrats’ pickups were by very narrow margins. Three seats changed hands by wide margins. In Florida, the outgoing Democratic Gov. Bob Graham beat Republican incumbent Paula Hawkins 55 percent to 45 percent, a 325,167-vote margin. In Maryland, Democrat Barbara Mikulski picked up the seat vacated by Republican Charles Mathias, 60 percent to 39 percent, a 237,841-vote margin. And in Missouri, Republican Christopher Bond picked up the seat vacated by Democrat Thomas Eagleton, 53 percent to 47 percent, a 77,988-vote margin.

The other seven Democratic pickups were won with between 50 percent and 52 percent of the vote, and the total Democratic popular vote margins in those states were just 137,741 out of 48.6 million votes cast. Six of the eight Democratic victories came in states that voted for Donald Trump in 2016: Alabama, Florida, Georgia, North Carolina, North Dakota, and South Dakota (The other two were in Nevada and Washington). And six of those seats are held by Republicans today, including Richard Shelby of Alabama, elected and re-elected as a Democrat in 1986 and 1992, who switched parties immediately after the 1994 election.

Among those Democratic pickup winners in 1986 were two future Senate Democratic leaders, who between them held that office for 22 years, from 1994 until 2016. In their different ways, both adapted the style of Maine Sen. George Mitchell, who succeeded Robert Byrd as majority leader after the 1988 election and held that office until he retired from the Senate in 1994. Mitchell combined a pleasant, seemingly reasonable demeanor with tough, take-no-prisoners partisan tactics. Tom Daschle, elected by a margin of 9,484 votes in South Dakota in 1986, and Democratic leader from 1994 until his defeat for re-election in 2004, continued in that vein. Harry Reid, elected by a margin of 14,349 votes in Nevada in 1986, didn’t bother as much with the seemingly reasonable demeanor.

Both Daschle and Reid fostered a sense of fellow feeling and mutual trust in the Senate Democratic Caucus, a sense often conspicuously lacking in their Republican counterparts. Creating such an atmosphere is a considerable political achievement, and I think we are seeing its effect in the Senate Democrats’ cohesiveness in the current shutdown. We’ll see how it works out for them.

In the meantime, the 1986 Senate races are a fine example of how just a small number of votes can make a great deal of difference over a long period of time.