WASHINGTON — President Donald Trump said Monday that the work of repealing and replacing Obamacare has begun but characterized it as a more “complicated” process than anyone knew.

Trump and Vice President Mike Pence had promised during their campaign to immediately drop and create a replacement for the Affordable Care Act, a signature program by former President Barack Obama that Trump has denounced as costly and ineffective.

GOP leaders are working through the details of a repeal push.

“It’s an unbelievably complex subject,” Trump told a gathering of the National Governors Association at the White House. “Nobody knew that health care could be so complicated.”

Senate Minority Leader Chuck Schumer (D-N.Y.) pointed later Monday to what he said was infighting among Republicans on how to proceed on Obamacare.

“I believe the odds are very high we will keep the ACA. It will not be repealed,” he said at the National Press Center.

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White House Press Secretary Sean Spicer said many Americans are suffering, citing Obamacare’s “skyrocketing rates.”

Trump previewed his speech to a joint session of Congress Tuesday in his remarks to the governors, saying he would lower taxes, increase defense spending, drive down crime and improve infrastructure.

In discussing the country’s infrastructure, he speculated that the loose tiles in New York City’s tunnels could hurt someone.

“You take a look at the Lincoln Tunnel and the Queens Midtown Tunnel, and you’re driving and you see all this loose material that’s heavy ... And I say to myself every time I drive through, I say, ‘Man, I wonder how many people are hurt or injured when they’re driving at 40, 50 miles an hour through a tunnel and a tile falls off,’ ” Trump said.

MTA spokeswoman Beth DeFalco said in an email, “Not a single person has been injured by any falling tiles because no tiles are falling — they are being replaced by workers as part of an infrastructure project to repair the tunnel from Hurricane Sandy damage.”

Meanwhile, a spokesman for the Port Authority, which operates the Lincoln Tunnel, said in a statement that “the tile structures in the various Lincoln Tunnel tubes are intact, regularly inspected and pose no danger to the public. In the last 12 months there have been no reported incidents involving falling tiles.”

Also Monday, the president conducted a roundtable discussion with health insurance company CEOs, which a White House aide said later would help to inform his joint session remarks.

He was asked by a reporter whether a special prosecutor should be appointed to oversee investigations into the Trump team’s ties to Russia intelligence officials. Democrats and some Republicans have called for Trump-appointed Attorney General Jeff Sessions to recuse himself in the probe.

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Trump responded: “I haven’t called Russia in 10 years.”

Rep. Devin Nunes (R-Calif.), chair of the House Intelligence Committee, said Monday that there is no U.S. intelligence evidence of communications between the Trump campaign and Russia, saying, “We can’t just go on a witch hunt.”

The committee’s ranking Democrat, Rep. Adam Schiff (R-Calif.), countered that while there isn’t yet evidence, “it’s premature for us to be saying we’ve reached any conclusion about the issue of collusion.”

With Vincent Barone