Sen. Claire McCaskill (D-Mo.) speaking last month to a group of supporters in Columbia, Mo. (Hunter Dyke/Columbia Daily Tribune/AP)
Opinion writer

Less than two years ago, conventional wisdom told us Donald Trump had transformed the political map: GOP strongholds in the South had joined with gains in the Rust Belt (Ohio, Michigan, Pennsylvania) and upper Midwest (Wisconsin, Iowa), and would give Republicans an electoral lock for years to come.

Then came 20 months of the Trump presidency. Senate races in Ohio and Pennsylvania, once thought to be pick-up chances for Republicans, are not really in play. Sen. Joe Manchin III (D-W.Va.) does not appear to be in danger. Democratic Sen. Tammy Baldwin of Wisconsin looks safe as well.

Instead, it is Democrats who are holding their own or are going on offense in red states such as Missouri, Montana and Indiana — though races there are nip and tuck. CBS polling reports:

Several incumbent Democratic senators are trying to win re-election in states that typically vote for Republicans, among them Claire McCaskill of Missouri and Jon Tester of Montana, who are both locked in close contests with Republican challengers. New CBS News Battleground Tracker polling [finds] Tester is up two points with likely voters over Republican Matt Rosendale, 47-45 percent in Montana, while in Missouri, McCaskill and Republican John Hawley are even in support among likely voters at 45-45 percent. Most supporters of all these candidates describe themselves as enthusiastic about voting this year. . . . Tester gains from getting more of the voters who say the president is not a factor. Jon Tester is helped by a majority 53 percent approval on handling his job as senator, and he enjoys a ten-point gap among registered voters on which candidate understands the needs and problems of Montana “a lot.”

Democrats also threaten to win open seats deep-red states such as Arizona, Tennessee — and maybe even Texas. This should remind us that experienced candidates usually beat inexperienced ones, and that likable ones usually beat unlikable ones.

Moreover, FiveThirtyEight reminds us:

There have been 114 opposition senators who have run in a midterm general election since 1982. Only four of the 114 (4 percent) lost. Most won by wide margins, with the average opposition senator beating the candidate of the president’s party by 28 percentage points. . . . In contrast, senators in the same party as the president running in midterm years — this will be Republicans in 2018 — lose fairly often. Of the 128 senators who fit this description, 25 (20 percent) lost re-election.

In other words, Democratic incumbents in red states may be safer than they seem, while red-state Republicans such as Sen. Ted Cruz (R-Tex.) may be at greater risk. In fact, senators running in states the opposition president won (e.g., Missouri, Montana, North Dakota) do very well in midterms (winning 93 percent of the time, FiveThirtyEight has found). In other words, the entire conventional wisdom that running in states Trump won by a big margin spells doom for Democrats is at odds with years of electoral results.

When you factor in the incumbent president’s very low approval ratings and unexpectedly strong Democratic recruitment (e.g., Rep. Beto O’Rourke in Texas, former Tennessee governor Phil Bredesen) Democrats’ prospects look sunnier than one might have believed, several considerations should be kept in mind.

First, discrete events matter in close races. For example, will Cruz act like a jerk if he has the opportunity to question Judge Brett M. Kavanaugh’s accuser?

Second, the economy is increasingly unimportant in the election. According to the latest Gallup poll, “A record-low 12% of Americans currently cite some aspect of the economy as the most important problem facing the U.S., down from 17% last month.” By contrast, concern about government itself is far-and-away the top issue, cited by 29 percent of voters. And it is in the realm of corruption, competence and functionality that Republicans may be at most risk in an election in which incumbent senators don’t bother investigating the president’s blunders, and his ex-national security adviser, campaign and deputy campaign manager and personal lawyer have all pleaded guilty or been convicted of crimes.

Third, Democrats have many strong gubernatorial candidates at the top of the ticket (e.g., in Florida) while Republicans have some ones who seem to be out of steam (e.g., Gov. Scott Walker in Wisconsin). That may also bolster Democratic Senate candidates.

Finally, many of these races are within the margin of error. In case anyone forgot the 2016 polling, a lead of a few points for one candidate is the same as a lead of a few points for the other, statistically speaking, when the lead is less than the margin of error. That means Democrats, against expectations, could win the Senate majority; however, it also means Republicans might be able to add a seat or two to their majority. Neither party has reason to be confident about the outcome.

UPDATE: New CNN polls deliver more bad news for Republicans. “The surveys show Democratic Rep. Kyrsten Sinema and former Gov. Phil Bredesen leading their Republican opponents for open seats Arizona and Tennessee, where sitting Republican senators are retiring. In Arizona, Sinema tops Republican Rep. Martha McSally by 7 points, 50% to 43% among likely voters, while in Tennessee, Bredesen holds a 5-point edge over Rep. Marsha Blackburn, 50% to 45% among likely voters there. Roughly 1 in 6 voters in each state say there’s a chance they’ll change their mind before Election Day.”