By Cokie Roberts and Steven V. Roberts

A young friend of ours who worked for Hillary Clinton in 2016 waved a big caution flag recently about the fall elections. The same complacency that doomed Clinton is starting to afflict Democrats again, she warns. Too many party operatives are convinced that a “blue wave” will sweep the Democrats to a majority in the House; the only outstanding question in their minds is whether they will win the Senate, as well.

Our friend is only one voice, but she’s worth listening to. She was on the ground in a key state when Clinton’s managers persistently ignored warnings from the field that their candidate was in trouble. Chill out, came the word from Brooklyn HQ; our data tells us we’re going to win. And what job do you want in the Clinton administration?

Democrats are fully capable of repeating the same willful self-delusion that sank them last time out. Is a “blue wave” possible? Absolutely. Is it inevitable? Absolutely not.

Several recent trend lines demonstrate the perils of complacency in Democratic ranks. President Trump’s approval ratings have edged slowly but steadily upward and stand at 43.2 percent in the latest average compiled by the website RealClearPolitics. That’s still low by historical standards, but he’s going up, not down.

One big reason is the economy. Unemployment has dipped to 3.9 percent, the lowest rate in more than 17 years, and a tax cut has bolstered paychecks for many workers. In the latest CBS poll, nearly two-thirds of voters rated the economy as good or fairly good, and a CNN survey found that 52 percent view Trump’s handling of the economy favorably.

At the same time, popular support has waned a bit for Special Counsel Robert Mueller’s investigation into possible ties between the Trump campaign and Moscow. The president has loudly and persistently denounced the probe as a “witch hunt,” and he’s having some effect. A Monmouth poll found that 54 percent of voters still feel Mueller is conducting a fair investigation, but that’s down from 60 percent just two months ago.

Trump’s core base of support shows no signs of cracking. Republican pollster Jim McLaughlin told The Washington Post: “That dynamic that elected Donald Trump that I thought was going to dissipate after 2016 elections is still there.”

In primaries this week, Republicans nominated two strong candidates — Patrick Morrisey in West Virginia and Mike Braun in Indiana — to oppose vulnerable Democratic senators and boost their chances of retaining control of that chamber.

Then there are the Berniecrats, now organized under the banner “Our Revolution.” They helped defeat Clinton in 2016 by dividing the party and deriding its candidate, and they threaten to do the same thing again by endorsing unelectable lefties like Dennis Kucinich, who lost a Senate primary in Ohio this week but gave the nominee, Richard Cordray, heaps of trouble.

Of course, there’s plenty of evidence in favor of a blue wave, starting with history. The party of an incumbent president almost always loses congressional seats in off-year elections, mainly because marginal voters who were excited by a popular candidate at the top of the ticket stay home two years later.

Without a doubt, Trump has galvanized the Democratic opposition, with new candidates and new contributors flooding into the political arena. The party has won critical elections in Virginia, Alabama and Pennsylvania, and a record number of Republican lawmakers are retiring instead of risking defeat.

“You need to understand what’s happening,” House Majority Leader Kevin McCarthy recently warned his GOP colleagues. “They’ve got money. They’ve got good candidates. They can speak well about issues. And they’re engaged.”

Democrats are focused primarily on suburban districts where college-educated women voters are increasingly disenchanted with the president. Many are traditional Republicans who backed Trump because they despised Hillary Clinton and were tired of eight years of Democratic rule. But now 56 percent of all women, and 59 percent of all white college graduates, tell the Quinnipiac poll that they are “embarrassed to have Donald Trump as president.”

Democrats are expanding their target list to include some rural districts where President Trump’s trade policies, which threaten to trigger retaliation from China against American farm products, could really hurt Republican candidates.

How these complex and competing forces play out in November is far from certain. History gives the Democrats a big advantage, but if they repeat the same mistakes they made in 2016 and take victory for granted, they could get blindsided again.

Steve and Cokie Roberts can be contacted at stevecokie@gmail.com.