WASHINGTON — President Donald Trump hosted a bipartisan dinner with six senators Tuesday at the White House to pitch his plan to lower taxes and overhaul the tax code.

His aides noted that he was again reaching across the aisle.

But Senate Minority Leader Chuck Schumer — the key Democrat with whom the GOP president struck a deal last week on the debt ceiling and funding the government — wasn’t invited.

The three Democrats in attendance — Heidi Heitkamp of North Dakota, Joe Donnelly of Indiana and Joe Manchin of West Virginia — were the only senators in their 48-member caucus to withhold their signatures from a letter to Trump that Schumer helped spearhead last month, urging compromise while staking out three conditions.

The three are up for re-election next year and represent states that Trump won in 2016.

“These senators aren’t leadership types; they’re members we believe will be willing to help on tax reform,” a White House spokeswoman told Newsday when asked why New York’s senior senator wasn’t invited.

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They were joined at the dinner by Republican Sens. Orrin Hatch of Utah, the chairman of the Senate Finance Committee, John Thune of South Dakota and Pat Toomey of Pennsylvania.

“I think most everyone can agree that Americans should keep more of their money than the government,” White House press secretary Sarah Huckabee Sanders said earlier in the day. “They spend it certainly far better than the government can. And I think that’s something that is a common goal that a lot of people want to come together on.”

She added that the gathering was a form of “moving that ball forward” on tax reform and said of the bipartisanship: “I think by nature of them sitting down with the president, that’s a very good first start.”

In a call with reporters Tuesday, Schumer said he believes the Democrats who didn’t sign the letter still “share the views” and believe in the stated conditions, including that tax law changes don’t hurt the middle class.

“They’re not for tax breaks for the top 1 percent, and they think it should not increase the deficit,” he said.

He expressed concerns about the opaqueness of discussions on Capitol Hill, where Treasury Secretary Steve Mnuchin and National Economic Council director Gary Cohn were meeting Tuesday behind closed doors with Republican lawmakers.

Schumer said of potential provisions that would affect New Yorkers: “I personally can tell you I will oppose any bill that tries to deal with the state and local deduction, as well as the mortgage deduction.”

Schumer’s spokesman said he did not want to speculate on why the senator wasn’t invited.

Sen. Kirsten Gillibrand (D-N.Y.) also wasn’t on the guest list, her office confirmed.

Thune, in remarks before the dinner, said the White House had decided who should attend. He called the meeting a litmus test and said Democrats could feel some ownership of the end product if they cooperated.

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“What we’re trying to do is sort of test the limits of whether or not Democrats are for any kind of a tax reform bill,” Thune said, “to find out where some of the more moderate-leaning Democrats in the Senate might be, and if there is in fact a way to get a bipartisan tax deal.”

Separately Tuesday, Huckabee Sanders said the president is scheduled to travel Thursday to Florida, which is recovering from Hurricane Irma.

Trump was expected to reach out to still more moderate Democrats on Wednesday. According to a source speaking on the condition of anonymity, he has invited to the White House members of the Blue Dog Coalition and bipartisan Problem Solvers Caucus, which includes Rep. Thomas Suozzi (D-Glen Cove).

With Tom Brune