He told reporters Monday:
"I will not be discussing anything relative to 2019 at all. I'm focused on this year. I'm focused on winning and playing hard like every single year. So if you guys have any questions after 2018, you can call [agent] Scott [Boras], and he can answer you guys. If you guys do talk anything about that, then I'll be walking right out the door."
Harper, 25, will likely become the highest-paid player in the history of baseball when he signs his next contract—whether it be with the Washington Nationals or another team. His production is borderline unprecedented for a player about to hit free agency in the arbitration era.
Harper hit .319/.413/.595 with 29 home runs and 87 RBI last season despite missing 51 games. He's recorded at least 20 home runs in five of his six MLB seasons and won the 2015 National League MVP Award. Only Mike Trout and Andrew McCutchen have a higher wins above replacement figure than Harper since 2012, per FanGraphs.
Washington is the National League East favorite, per OddsShark, and Harper said he wants to focus on making a World Series push in 2018:
"We're expected to win the East. We're expected to possibly win a pennant, and we're expected to possibly win a World Series. As a team, I think we're going to try to do the best we can and do everything possible to win ballgames. We've just got to take it one game at a time and win the East first. Because the Mets are tough. The Braves are good. Philly, Miami, whatever. Miami plays us pretty dang well. We've just got to go out there, play our game and see what we can do."
The Nationals have won the NL East in each of the last two seasons but have failed to make it out of the division series. The franchise has not won a playoff series since 1981. The former Montreal Expos moved to Washington, D.C., in 2005.