Democrats thought it would be easier to beat Trump so they built him up during the primaries

Becket Adams:

In 2011, President Barack Obama's inner circle saw Donald Trump as a gift.

In Trump, they saw an opportunity to delegitimize Obama’s critics. They saw an opportunity to tarnish the GOP. Obama’s team chose to build up the Queens businessman, making him a political player.

“There was strategy," Obama campaign manager and senior White House adviser David Plouffe said in Obama: An Oral History.

"Lifting up Trump as the identity of the Republican Party was super helpful to us," he said of the decision for Obama to focus on the real estate tycoon at the White House Correspondents’ dinner. “The president went out in the briefing room to present his long-form birth certificate, [but] really to continue the dance with Trump."

"Our view was lifting Trump up at the White House Correspondents’ Dinner, you know, as kind of the example of the Obama opposition," Plouffe added. "There was a strategy behind the material and the amount of time we spent on Trump. Let’s really lean into Trump here. That’ll be good for us."

The funny thing is: This strategy doesn’t appear to have ended in 2011. Many in news and politics similarly built up Trump during the Republican primaries. Remember: As the GOP field winnowed from 17 to a handful, many even argued Trump was the most reasonable option remaining.

In August 2015, for example, New York Times columnist Maureen Dowd described Trump as “wickedly fun” and “wildly useful.”

“It’s always a pig in a poke. So why not a pig who pokes?” she asked.

The Washington Post’s Ruth Marcus wrote in December of that year that Sen. Ted Cruz, R-Texas, was “more dangerous than Trump.” She also said that the senator was more “ruthless and cutthroat.”

Left-wing pundit and former Clinton cabinet member Robert Reich agreed. So did progressive author Noam Chomsky.

Vox co-founder Matt Yglesias wrote in February 2016 that Trump was “running on a much less extreme agenda" than Sen. Marco Rubio, R-Fla.

New York magazine’s Jonathan Chait urged Democrats in February 2016 to “earnestly and patriotically support a Trump Republican nomination." Chait also wrote that “a Trump presidency would probably wind up doing less harm to the country than a Marco Rubio or a Cruz presidency. It might even, possibly, do some good.”

Times columnist Paul Krugman argued in February and April 2016 that presidential candidates Sens. Cruz and Rubio were likely more dangerous than Trump.
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Obviously, they have since changed their mind.  I think it began happening during election night as it became clear that Trump would win.  Their best-laid plans had blown up in their face.  They remind me of the Democrats in 1980 who thought Ronald Reagan could be easily defeated.