First Read is your briefing from Meet the Press and the NBC Political Unit on the day's most important political stories and why they matter

WASHINGTON — In 2016, Donald Trump won Pennsylvania’s 18th congressional district by 20 points, 58 percent to 38 percent. And in 2012, Mitt Romney won it by 17 points, 58 percent to 41 percent. But when Democrat Conor Lamb takes on Republican Rick Saccone in tomorrow’s special congressional election to fill the PA-18 seat, the race appears to be a pure 50-50 contest.Here are three reasons why the race outside of Pittsburgh is so competitive:

1. The Democrats’ enthusiasm advantage: From Kansas and South Carolina to Virginia and Alabama, Democrats — win or lose — have overperformed in most of the major races over the past year in the Trump Era.

KS-4 in 2016: Mike Pompeo 61%, Daniel Giroux 30% (R+31)

KS-4 in 2016 (presidential results): Trump 60%, Clinton 33% (R+27)

KS-4 in 2017: Ron Estes 53%, James Thompson 46% (R+7)

GA-6 in 2016: Tom Price 62%, Rodney Stooksbury 38% (R+24)

GA-6 in 2016 (presidential results): Trump 48%, Clinton 47% (R+1)

GA-6 in 2017 (initial round): Jon Ossoff 48%, Karen Handel 20%, Bob Gray 11%, Dan Moody 9%, Judson Hill 9%.

GA-6 in 2017 (runoff): Handel 52%, Ossoff 48% (R+4)

MT-AL in 2016: Ryan Zinke 56%, Denise Juneau 40% (R+16)

MT in 2016 (presidential results): Trump 57%, Clinton 36% (R+21)

MT-AL in 2017: Greg Gianforte 50%, Rob Quist 44% (R+6)

SC-5 in 2016: Mick Mulvaney 59%, Fran Person 39% (R+20)

SC-5 in 2016 (presidential results): Trump 57%, Clinton 39% (R+18)

SC-5 in 2017: Ralph Norman 51%, Archie Parnell 48% (R+3)

NJ GOV in 2013: Chris Christie 60%, Barbara Buono 38% (R+22)

NJ GOV in 2016 (presidential results): Clinton 55%, Trump 41% (D+14)

NJ GOV in 2017: Phil Murphy 56%, Kim Guadagno 42% (D+14)

VA GOV in 2013: Terry McAuliffe 48%, Ken Cuccinelli 45% (D+3)

VA in 2016 (presidential results): Clinton 50%, Trump 44% (D+6)

VA GOV in 2017: Ralph Northam 54%, Ed Gillespie 45% (D+9)

AL SEN in 2016: Shelby 64%, Crumpton 36% (R+28)

AL in 2016 (presidential results): Trump 62%, Clinton 34% (R+28)

AL SEN in 2017: Doug Jones 50%, Roy Moore 48% (D+2)

Let our news meet your inbox. The news and stories that matters, delivered weekday mornings. This site is protected by recaptcha

2. Scandal isn’t helping the GOP: The reason there’s a PA-18 special election in the first place is due to the resignation of anti-abortion Rep. Tim Murphy, R-Pa., who left his seat after allegedly encouraged his lover to terminate her pregnancy. "You guys can decide why the stakes are high nationally," Lamb said, per NBC’s Alex Seitz-Wald. "I think for our people, they feel very let down by people like Tim Murphy, who were hypocrites." And remember, parties that have to defend a seat vacated as a result of scandal typically have a hard time — see Democrats who lost Anthony Weiner’s congressional seat in 2011.

3. Lamb is viewed as the better candidate: Democrat Conor Lamb has outraised and outspent Republican Rick Saccone over the airwaves (although GOP outside groups have more than made up the gap). On the ground, NBC’s Seitz-Wald has noticed a lack of energy coming from the Saccone campaign. “Have never been to a political event quite as low energy as Rick Saccone's ‘GOTV Rally’ Friday night. No more than 2 dozen volunteers, many from out-of-state, who complained the locals were unenthused,” he tweeted. And even Republicans have admitted that Lamb is the superior candidate. “Candidate quality matters, and when one candidate outraises the other 5-to-1, that creates real challenges for outside groups trying to win a race,” GOP strategist Corry Bliss told Politico.

Add them all up — Democratic enthusiasm, GOP scandal and a superior Dem candidate — and you see why this race is so close, even in Trump Country. By the way, those same three features defined last December’s Alabama Senate race, and we saw how that race turned out.

It was a busy 2020 weekend

Our initial instinct isn’t to hype up a presidential race that’s still two years away, but there were several developments that were hard to ignore. First, Trump unveiled his 2020 slogan — “Keep America Great!”

Then, on “Meet the Press,” retiring Sen. Jeff Flake, R-Ariz., said that a Republican needs to primary Trump in 2020.

TODD: Do you think [Trump] needs to be challenged from somebody who espouses your views?FLAKE: Yes, I do. I do. I mean, it would be a tough go in a Republican primary. The Republican Party is the Trump party right now. But that's not to say it will stay that way.

And also on “Meet” yesterday, Sen. Elizabeth Warren, D-Mass., wouldn’t answer the question if she would serve a full term if she wins re-election in November.

TODD: If you win reelection this year are you going to pledge to serve a full six year term?WARREN: So look, I am not running for president of the United States. I am running for the United States Senate. 2018. Massachusetts. Whoohoo...TODD: I take it as a no you're not pledging to serve your full six year term if you win reelection?WARREN: I already told you. I have no intention of running for the United States — for president.

Trump backs away from proposal to raise the age for semi-automatic gun purchases

“President Donald Trump took the first step toward arming America's teachers on Sunday night, promising Justice Department assistance to help fund firearms training for school personnel,” NBC’s Alex Johnson writes. “The proposal would also seek to bolster firearm background checks, expand mental health programs and encourage military veterans and retired law enforcement officers to take up careers in education. But it doesn't include a proposal the president floated March 1 to raise the minimum legal age to buy semi-automatic weapons from 18 to 21, an idea the National Rifle Association vigorously opposes.”

Asked on “Today” why Trump has backed away on raising the age limit, Education Secretary Betsy DeVos — who had a rough interview on “60 Minutes” — responded, “The plan is really the first step in a more lengthy process. And the proposals the president has put forward really encompass a lot of things that are supported on a broad bipartisan basis.”

Why is Trump trying to make a nuclear deal with North Korea but kill the one with Iran?

The New York Times asks the question. “‘The ironies abound,’ said Robert S. Litwak, the director of international security studies at the Woodrow Wilson Center for International Scholars and the author of ‘Preventing North Korea’s Nuclear Breakout.’ ‘The man who wrote “The Art of the Deal” has staked out a position that the Iran deal was the worst one in history,’ he added. ‘And now he has to show that he can do much better, with a far harder case.’”

“[CIA Director Mike] Pompeo acknowledged that Mr. Trump, given his disparagement of the Iran deal reached by the Obama administration, will have to get a better deal out of Mr. Kim. ‘I think that’s the case,’ he told the host, Margaret Brennan, adding that he thought Mr. Trump would be negotiating from a greater position of strength. That is a debatable notion. Mr. Kim has driven the pace of this diplomatic effort so far, and American officials have conceded surprise at his boldness. And if Mr. Trump pulls out of the Iran deal, Mr. Kim may well wonder why he should negotiate with the United States if a subsequent president can simply pull the plug on any agreement.”

Sam Nunberg says he didn’t refuse to answer Robert Mueller’s questions

It was exactly one week ago when former Trump aide Sam Nunberg made those multiple TV appearances, initially saying he would defy special counsel Robert Mueller. And here he was yesterday, telling MSNBC’s Alex Witt that he answered all of Mueller’s questions.