Tuesday evening, I observed that the Republican Party now in thrall to the extreme far right of its base stands pinched in its own vise. About an hour later, the nation watched that vise pinch the life out of House Majority Leader Eric Cantor’s political career. The victory of college professor David Brat over the seven-term incumbent should be a wake-up call for Republicans to take their party back from the fringes. But it also should be one for Democrats.

In his inaugural address, President John F. Kennedy admonished the world’s newly free nations to not replace colonial rule with “a far more iron tyranny” because “in the past, those who foolishly sought power by riding the back of the tiger ended up inside.” This pretty much sums up what has been happening to the Republican Party since 2010.

The GOP establishment lurched hard to the right when three-term conservative Sen. Bob Bennett (Utah) went down in defeat at the state party convention in 2010. Four years later, a little-known tea party challenger with pocket change for campaign cash took out a member of the House leadership who harnessed and nurtured the anger of the far right. Cantor was as loyal to tea party concerns and priorities as he was to his omnipresent Gucci loafers. And yet his fealty didn’t protect him from the tiger he rode.

But Democrats celebrating the Cantor calamity better check their schadenfreude. There will be more Brats in Congress if they don’t show up at the polls in November, especially in key races that will determine the balance of power in the Senate. And there is a raft of polls and history showing they won’t.

The latest is a survey conducted by Public Religion Research Institute and the Brookings Institution released this week that shows a huge enthusiasm gap. President Obama’s most ardent supporters are less likely to be “absolutely certain” to vote in the November midterms. Take a look at the chart below.

Democrats love the president (75 percent), but barely half of them (51 percent) said they were “absolutely certain” to vote in November. African Americans in particular love Obama (75 percent). Yet just 49 percent of them said they were “absolutely certain” to show up at the midterms. Meanwhile, only seven percent of Republicans polled approve of the president’s job performance. But 68 percent of them said they were “absolutely certain” to cast a ballot. It’s data like this that have Obama warning the Democratic base over and over and over again against complacency.

The Post’s Dan Balz reported in April that Democratic Party officials are doing their best to reshape the midterm electorate along the lines of a presidential election year. Some might think that’s a fool’s errand, but it has been done. And it was done in Cantor’s home state. I’ve written about this before, so here’s the Cliffs Notes version. Terry McAuliffe lost the white vote by 20 percentage points, yet he became governor of Virginia by three points because African American voters came out in an off-year election (2013) in the same proportion they did in the 2012 presidential race.

If blacks were to replicate that feat in this fall’s critical Senate races, Democrats could hang on to the chamber. As The Post’s Aaron Blake reported in April, three states with the highest black populations — Louisiana, North Carolina and Arkansas — “are widely considered to be the most pivotal when it comes to the GOP’s hopes of winning the majority. These three races are expected to be the difference between a GOP majority and a Democratic majority — at least the way things look right now.” But that’s a big if.

Failure of Democratic voters to show up at the polls five months from now could mean a Republican-controlled Senate and a stalled presidential agenda for the next two years. Stalled is worse than gridlocked. Where gridlock assumes eventual movement, stalled is a nice way of saying “dead.” And just imagine the politically damaging nonsense (read investigations or even impeachment) a Republican-dominated legislative branch would pursue to tie down the lame-duck Obama in his last two years.

“A lot of the reasons that the president has not been able to move some of the things as fast and big as he’s wanted to move them is ’cause we slept,” a Democratic friend told me recently. “We won big in 2008. And we slept in 2010. And we got what we got. And we are still paying [the] price.”

Sure, it’s fun for Democrats to watch Republicans fight amongst themselves and hand their nominations to the fringe of their base. But if Democrats don’t vote in the numbers they need to in November, those folks who are more conservative than the ultra-conservative members already gumming up the works will come to Washington. If the threat of that is not enough of a wake-up call for Democratic voters, I don’t know what is.

Follow Jonathan on Twitter: @Capehartj