WASHINGTON — Dan Sena, the head of the House Democrats' campaign arm and architect of the Democratic takeover of the House, recalls being told the same thing each day for the past two years: "You have to win. You know this, right?"

Those were Sena's marching orders from his boss, Democratic Congressional Campaign Committee chair Rep. Ben Ray Lujan of New Mexico. And, he told NBC News, he got that message "every, single day."

Sena, the committee's executive director, says that the lessons learned from 2018, which have resulted in Democrats picking up at least 39 seats, can also apply to 2020 when the party will work to maintain their new House majority and perhaps even gain more ground in the midst of a presidential election.

He said the key to his party's victories, many in suburban districts that supported President Donald Trump in 2016, was eschewing ideological litmus tests and supporting "culturally acceptable" candidates who mirrored the districts they were running in.

"My job was to design the strategy to win the House in Trump's backyard," he said. He pointed to Ben McAdams, who won a hard-fought victory in Utah's 4th Congressional District, and Anthony Brindisi, who beat Republican incumbent Rep. Claudia Tenney in New York’s 22nd District, as examples of candidates who could relate to the more conservative electorate.

The party also supported candidates who didn't win, like Brendan Kelly in an Illinois district Trump won by 10 points in 2016. Sena said Kelly was “three steps to the right” of most of the party and not a favorite of the Democratic base. But he was the right kind of candidate for the district and even though he lost, Kelly received 45 percent of the vote, giving Sena hope the party can win the district in two years.

But the Democrats' success didn't happen without some bruises along the way.

During the primary process in many races, progressives were unhappy with the DCCC for not supporting the more progressive candidates. Sena said his job was to win back the majority and he said he knew that the most liberal candidates wouldn’t work in every district in the general election. “If you sort of upset the apple cart along the way, that’s OK,” he said.

He said that a successful Democratic presidential candidate in 2020 will be one that appeals to the broadest swath of voters, not the narrow base of the party.

"Their job is to win the general election," he said. "Winning the primary doesn’t necessarily win the general election." Sena says a successful candidate can campaign on progressive values, but in a way that inspires the country to do something big.

Another lesson Sena says the party can learn from 2018 is how to navigate the waves created by a president who dominates the political landscape. He notes that House candidates, especially those in Trump-won districts, were better off focusing on issues and topics other than Trump, rarely addressing the near-daily news about the president on the campaign trail or in campaign ads.

"It’s important to understand," Sena said, "when he shakes the snow globe it creates a ripple effect around the country. It helps us in some places and hurts us in others — but you ultimately don’t know where it’s going to hurt you until the very end."

He said he advised candidates to stay out of the character fights with the president, partly because he is better at it. But he also notes that Trump's supporters are apt to believe that attacks on the president are reflections of themselves and calling him names won’t sway them to vote Democrat.

"It has taken an enormous amount of energy at the very end to not swing at character," Sena said of the urge to confront Trump directly. He insists that's the best approach for the party heading into 2020 as well. "You are not going to win those voters if you make them feel like they are doing something wrong," he said.

And, he says there are plenty of policy distinctions to be drawn. "It has to be a conversation about what he is doing to the country. What does he do to the country? What does he do to your pocketbook? What does he do to your health care? And you attack him and take him on as the actual politician, which we did this cycle."