ANALYSIS — Like other handicappers, I have noted that there are few signs that the national political divide, so apparent over the last three years, has started to crumble.

Trump voters are sticking with the president, while those who opposed him in 2016 generally have become even more vociferous in their opposition.

Given the closeness of the last presidential contest — and subsequent big Democratic gains in the House two years later — it’s hard to see 2020 producing a House wave for either party.

After all, only three House Republicans sit in districts won by Hillary Clinton in 2016, and most of the seats that flipped to the Democrats last year are in the suburbs, where Trump and the GOP are having serious problems.

In other words, there are few “easy” opportunities in the House for either party. But while electoral “waves” almost always refer to large changes in the House, the term can also apply to Senate and presidential outcomes where there are dramatic shifts.

In fact, the extreme partisanship we see, especially combined with the way our House districts are now drawn, limit the number of seats in the chamber that can conceivably flip, even in a partisan wave.

At some point, a competitive Senate seat becomes more likely to flip than an uncompetitive House race. We may be at that tipping point in this cycle.

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How a wave begins

Electoral waves generally happen under at least one of two circumstances.

They occur when turnout in one party drops precipitously, producing an electorate that dramatically favors the other party. Or, they can occur when swing voters, who normally divide evenly between the two parties, swing dramatically to one side, thereby producing an electorate that once again disproportionately favors one party over the other.

In 2004, for example, self-identified independents in the national exit poll split evenly, 49 percent for John Kerry and 48 percent for George W. Bush. 

Bush won the presidential election narrowly at the same time the GOP gained a modest three seats in the House. But two years later, in a wave election during Bush’s second midterm, the national exit poll showed self-identified independents breaking to the Democrats, 57 percent to 39 percent. 

Democrats gained 31 House seats that year. Four years later, during Barack Obama’s first midterm election, which produced a GOP electoral wave, the national exit poll showed independents breaking toward Republicans, 56 percent to 39 percent. 

When both partisan turnout and independent/swing voter preferences change at the same time (and in the same direction, of course), we tend to see larger electoral waves, as we did in 2010, when Republicans made large House (63 seats) and Senate (6 seats) gains.

Large gains are also possible (even likely) when the party on the defensive holds an abnormally large number of House and/or Senate seats that traditionally favor the other party.

What the polls say

While the 2020 election is still more than a year off, Republicans ought to be concerned about some early signs, both at the national and state levels.

Trump carried self-described independents in 2016, 46 percent to 42 percent, according to that year’s national exit polls, but the GOP lost them, 54 percent to 42 percent, two years later in the midterms.

Even more concerning, the Oct. 6-8 Fox News poll found the president’s approval among independents at 36 percent, with 61 percent disapproving of his performance.

In Minnesota, which Hillary Clinton won by only 1.5 points and where Trump’s campaign is likely to make a major effort, an Oct. 14-16 Star Tribune poll found the president losing to the three top Democrats anywhere from 9 points to 12 points in hypothetical ballot tests.

In Wisconsin, which Trump carried in 2016 by less than eight-tenths of a point, an Oct. 13-17 Marquette Law School poll found former Vice President Joe Biden leading him by 6 points, while Vermont Sen. Bernie Sanders was up by 2 and Massachusetts Sen. Elizabeth Warren led by a single point.

A Fox News poll conducted Sept. 29-Oct. 2 in Wisconsin found Biden leading by 9 points, Sanders by 5 and Warren by 4.

In Florida, an Oct. 14-20 University of North Florida poll found Trump stuck at 43 percent or 44 percent against four top Democratic contenders.

He trailed Biden by 5 points and Warren by 3. Trump won 49 percent of the vote in Florida in 2016, carrying the state by only 1.2 points.

In Iowa, an Oct. 13-16 Emerson College poll showed Trump essentially tied with Biden, Sanders and Warren in a state that he carried by 9 points, a serious problem for the president’s team if — and it is a big “if — the Emerson results reflect the actual strength of the candidates in hypothetical ballot tests.

It’s certainly possible that these national and state polls are misleading or flat-out wrong. Circumstances could change, either helping or hurting Trump, and both parties’ prospects won’t become clearer until the Democrats actually have a nominee.

But it’s equally unwise to be wedded to an assumption — e.g., we are headed for another squeaker in 2020 because the Trump and anti-Trump coalitions are largely immovable — that may ignore the possibility that modest defections from Trump combined with a significant change in the behavior of independents/swing voters (including suburban whites with a college degree) could produce substantial changes in the 2020 presidential vote and surprisingly substantial Democratic gains in the Senate.