President Donald Trump speaks during a rally in El Paso, Texas on February 11, 2019. Nicholas Kamm | AFP | Getty Images

Vegas oddsmakers, shrugging off the House Democrats' midterm election triumph, favor President Trump to win a second term in 2020. So do Wall Street insiders. It's easy to understand why they would consider 2018 a temporary setback. Republican routs in 1994 and 2010 didn't keep Bill Clinton and Barack Obama from subsequently winning reelection. But a closer look at those lopsided midterms suggests a different conclusion. For three overlapping reasons, 2018 sent a stronger-than-usual signal about the upcoming White House contest.

Looking forward, not back

In 1994, House Republicans amassed much of their 54-seat gain on conservative ground. It deepened a political realignment that had already occurred at the presidential level, most dramatically among white Southerners. The 2010 elections reflected a similar dynamic, wiping out conservative remnants within the Democratic caucus. The 63-seat House Republican gain included 51 rural districts that Democrats had snatched amid the Wall Street crisis and unpopular Iraq War in 2008.

That made both midterms trailing presidential indicators. Last November looks different. In 2016, Trump edged Hillary Clinton among suburban voters, just as Mitt Romney had edged Obama four years earlier. Republicans approached 2018 confident that favorable district boundaries and strong economic conditions would safeguard their majority. Instead, Democrats smashed through those barriers for a 40-seat gain propelled by suburban voters, especially women. Overcoming traditional weakness in midterm turnout by their disproportionately young, non-white coalition, they captured 21 seats Trump won in 2016. "A big achievement – they managed to overcome their structural disadvantages," observes congressional elections scholar Gary Jacobson of the University of California-San Diego. "The same factors that inspired that turnout apply now."

Trump's consistent unpopularity

Jacobson was alluding to Trump's polarizing presence, which has repelled young, female, non-white and college-educated voters. His inability to inspire higher support distinguishes him from modern predecessors. Both Clinton and Obama began their presidencies with Gallup job approval well above 50%. They dipped below that level before their first midterms, but moved above 50% again approaching their reelections. Public regard for Trump has not been nearly so fluid. He remains the only president in the polling era never to reach 50% approval. "The inelasticity of people's views on Trump is important," notes Ruy Teixeira of the Century Foundation. Approaching last year's midterms, Trump languished underwater by a double-digit margin: 40% approve, 54% disapprove. His Gallup standing today: 39% approve, 57% disapprove.

A slowing economy

In 1992, Clinton rode economic discontent to the White House. The 1980s Reagan boom had given way to recession under President George H.W. Bush. Unfortunately for congressional Democrats, it remained unclear by fall 1994 how much things were improving. Fortunately for Clinton, the takeoff toward the late-1990s boom was clearer two years later.