President Trump gestures to supporters as he takes the stage for a campaign rally Friday in Charlotte. (Chuck Burton/AP)

This is the time in an election cycle when people who do politics for a living are nervous. They look for evidence, favorable or unfavorable, they can trust. They weigh what polls tell them against what their instincts tell them, or against what they believe can still happen. It is a time of uncertainty, even if a wave is coming.

On all the principal battlefields at play in the 2018 midterms — House, Senate and governors — the landscape is populated with races considered too close to call. Yes, strategists acknowledge that a few incumbents are already going to lose, but those races tend to be the exceptions. Instead, many races are in the toss-up category on the handicapping charts, including some that weren’t there a few weeks ago.

A political strategist who was once a senior official at one of the national party committees described his role in the closing weeks of an election as that of a money manager shifting funds from one account to another, doing triage as necessary on candidates who have faltered while seizing on races that have suddenly become opportunities.

Much of that late money goes into late TV ads, and there are plenty of funds available this year, particularly among Democratic challengers in House races. It is one of the factors that gives Democrats optimism that they can capture the House — that along with the number of Republican retirees and the quality of first-time candidates.

TV may help push some candidates over the line, but the real determining factor will be the mobilization of voters. Turnout, however, can be unpredictable. There are many examples over the years of candidates confident of their lead in the final days, only to be swamped by unexpectedly strong turnout by the opposition.

Democrats hope that dissatisfaction with President Trump will produce turnout that exceeds that of a normal midterm. Analysts tracking early voting have noted that, in some places, turnout is running close to that of the early vote in the 2016 presidential election. Whether that represents eagerness to cast ballots or a sign that people who normally sit out midterm elections are engaged this year won’t be known until the polls close.

Trump has been doing everything he can to make sure any surge in turnout in behalf of Democrats is matched as much as possible by turnout among Republicans. His campaign activity is impressive for its breadth and nearly nonstop pace. But even the president is not in control of events, as these past few days have shown, and he wasn’t happy about it.

Trump sent off a tweet Friday morning, hours before FBI officials announced they had arrested Cesar Sayoc and charged him with sending suspected pipe bombs to 13 people, all of whom the president has attacked publicly, often repeatedly. “Republicans are doing so well in early voting, and at the polls, and now this ‘Bomb stuff’ happens and the momentum greatly slows — news not talking politics,” the tweet read. “Very unfortunate, what is going on. Republicans, go out and vote!”

As with so much about this president, the tweet was remarkable, if predictable. At a time when the full apparatus of law enforcement at all levels was focused on the threat of political violence, when former high government officials and others were being targeted for attack, when the perpetrator was not publicly known to be in custody, the president could only see it the events through a narrow partisan and personal lens: What’s good or bad for me? He prefers that the news media focuses on the caravan of people moving up from Central America through Mexico.

Democrats are cautiously optimistic about taking control of the House, and Republicans agree that is more likely than less likely. That would produce a big change in Washington and a setback for the president. But is this a wave coming, or a tsunami? Or could Democrats still fall just short of the net 23 seats they need to win? It’s what keeps candidates and staffs wondering what more they can do.

What creates uncertainty about the House races is the fact that there are plenty of incumbent Republicans who are polling below 50 percent, never a good sign. There are also many, many contests within the margin of error in the polls coming back at this point, which means Democratic challengers still have work to do. In change elections, undecided voters usually break against incumbents rather than split evenly, and in change elections, one party gains a greater share of the toss-up races.

The Washington Post and the Schar School at George Mason University have been polling in 69 competitive districts, as compiled by the Cook Political Report. The most recent poll showed Democrats ahead in those districts by 50 percent to 47 percent — a sign of problems for Republicans, because those same districts went for the GOP collectively by double digits in 2016.

Meanwhile, the Cook Report teamed with the Manship School at Louisiana State University for a comparison of those battleground districts versus the nation as a whole. Those results were more bullish for Democrats. Nationally, the Democrats had an advantage of seven points, with a margin of error of plus or minus six points. In the 72 battlegrounds where there was oversampling, the Democrats’ advantage was 12 points, with a margin of error of plus or minus nine points.

The Senate is in another place, with Republicans — operating with an extremely favorable map — considered favorites to retain or expand their majority. Former House speaker Newt Gingrich (R-Ga.), speaking at an event hosted by The Post on Thursday, said the odds of Republicans emerging with 57 seats (they currently have 51) are greater than that of Senate Minority Leader Charles E. Schumer (D-N.Y.) becoming majority leader.

Gingrich said he wasn’t predicting a pickup of that many seats for the GOP, just that the uphill climb for the Democrats to win the majority appears that steep. Not surprisingly, Sen. Chris Van Hollen (D-Md.), who chairs the Democratic Senatorial Campaign Committee, disagreed with that assessment. Don’t count out Democratic incumbents in red states, he said.

Democratic pollster Peter Hart sent out a memo late Friday summing up the most recent NBC-Wall Street Journal survey. He quoted Chuck Todd, the moderator of NBC’s “Meet the Press,” as saying the results could be interpreted as showing a blue wave election, or as an election in which there has been a Republican surge of late, or one in which Republicans lose some seats but there is no wave.

“This is not a pollster dodge to avoid responsibility for a surprising result in November,” Hart wrote in his memo. “Rather, it is a reflection of how current voter attitudes send conflicting messages about this election.” The reason is that America is two countries right now, a blue America and a red America. “Partisanship marks all of the elections in the 21st century,” Hart wrote, “but the lines today are more sharply drawn across the entire political landscape.”

The NBC-Wall Street Journal poll found that only 18 percent of Americans believe the country is not mainly or totally divided, and 9 in 10 say it’s a serious problem for the country. But Hart noted that when people were asked who’s responsible: “Respondents reply that the fault lies with the other party.”

That’s the mood of the country 10 days out from the election, which is why some modesty in making predictions is in order. Conditions and geography favor Democrats in House races and Republicans in Senate races. Turnout patterns appear to favor Democrats, but they are not yet definitive. And at the center of the storm stands the president, determined again to upend the experts.