MIAMI — The penthouse has room for only one tenant, so the Mets’ message to the Nationals on Monday was essentially, “Get the hell out.”

After dismembering their NL East rival in a three-game sweep over the weekend at Citi Field, the Mets kept rolling with a 12-1 victory over the Marlins and sit alone atop the division for the first time June 19.

“We’ve been talking among all the players, not only the veteran guys, from now on every game is important,” Bartolo Colon said after allowing one run over eight innings. “Every game counts and we’ve got to take every game like it’s the last.”

The Mets (56-50) made this one a runaway and won their fourth straight. Michael Conforto’s first major-league homer, a three-run blast in the second, set the tone and Yoenis Cespedes finished with three doubles and four RBIs.

The Nationals lost 6-4 to the Diamondbacks and fell one game behind the Mets.

Curtis Granderson said the biggest concern was fatigue after playing a Sunday night game in New York and arriving to South Florida around 4 a.m. Monday.

“Regardless, I think this team did a great job of just not making any excuses,” Granderson said. “Realizing that we’re all tired, we’re going to find some rest at some point in time.”

Colon (10-10) allowed one run on seven hits with five strikeouts for his first victory since June 12. The right-hander had a 6.16 ERA over his previous seven starts, raising concerns he was cooked in the summer heat.

But the Marlins were befuddled by his two-seam fastball and offspeed pitches.

“That’s a big outing for Bartolo, because we need him,” manager Terry Collins said. “If you’re going to stay in the race, you’ve got to get outings. You’ve got to get innings from all your pitchers and he’s struggled lately and tonight was a good outing for him.”

In sweeping the Nationals three games at Citi Field over the weekend, the Mets scored only 10 runs total, but used dominant pitching from Matt Harvey, Jacob deGrom and Noah Syndergaard to finish the job. On Monday, they received a little bit of everything against a Marlins team that has been in disarray since April. Only exacerbating matters, the team’s top hitter, Giancarlo Stanton, has been on the disabled list for the past month with a broken hand.

It became a blowout late, with the Mets combining to score five runs in the sixth and seventh innings, much to the delight of a largely pro-New York crowd.

The Mets seized control with four runs in the fifth to take a 7-0 lead and knock out starter Tom Koehler in the process. Cespedes smashed a two-run double — just missing his first Mets home run — before Wilmer Flores delivered a run-scoring double that made it 6-0.

“That’s why we got [Cespedes],” Collins said. “We got him to be that guy in the middle of the lineup who drives in those big runs, so hopefully we keep getting guys on.”

Conforto’s first major-league homer gave the Mets a 3-0 lead in the second inning. Kelly Johnson reached on first baseman Casey McGehee’s error before Travis d’Arnaud doubled to right as Ichiro Suzuki appeared to get a poor jump on the ball. Conforto followed with a rocket into the seats in right-center.

“It’s the mix of the young talent that we have with the guys who have been winners in the past who have playoff experience,” Colon said. “You mix all that together and we can feed off the energy those guys bring and we can go out there and perform at a higher level.”