A closed door meeting to decide on a strategy to stop Donald Trump is set to take place in Washington DC Thursday. And, according to reports, the high-level Republican operatives organizing the meeting will also discuss running a third-party conservative challenger, should Trump win the nomination.

It’s dawning on the Republican establishment that it is “increasingly necessary” to look at Trump as the party’s nominee in the November election, as pollster Ed Goeas wrote in a recent memo to the anti-Trump Our Principles PAC.

In that same memo, Goeas argues Trump has no chance of winning the general election in a match up against Hillary Clinton. It was written before Trump decisively defeated Marco Rubio in Florida, leading to the senator announcing the end of his campaign.

The delegate math is beginning to add up, and it shows that Trump is now likely to hit the 1,237 delegate target, though this might not happen until June. There’s a chance Rubio’s supporters will move en bloc to the other candidates, and that Trump wouldn’t earn any of his votes, but there is no certainty that will happen.

What else is there? A deal, brokered by GOP leaders, to bring the almost equally-disliked Ted Cruz and the sainted John Kasich together as a ticket. If not that, by the time either one or the other bows out it may be too late, and that would leave party elders praying for a Hail Mary — that is, Trump falls short and there is a brokered convention.

If Trump hits his delegate target, it’s almost certain a third party “conservative” candidate will stand, Stuart Stevens, Mitt Romney’s former political strategist, stated last night on NPR. Stevens said he will never vote for Trump.

“I think in a lot of states there will be a third party candidate,” said Stevens, adding that it is essential voters turned off by Trump come out in November and vote for down ticket Republicans. The party leadership is prepared to give up on the presidency.

It is worth digging into some numbers from Tuesday’s primary, not so much in Florida, but in Illinois.

This is a state where Cruz was predicted to do well — where he had spent most of the the preceding week, and where Trump’s campaign was supposed to be in a disarray after his state campaign chief stepped down.

There was some suggestion Trump’s numbers might be hurt by the scenes of scuffles and some fist fights at Friday’s canceled rally in Chicago. That, it seems, was not a problem for his voters, and in fact may have brought more out.

Taking into account that Kasich polled well, thereby cutting in to Cruz’s numbers, Trump swept up across the state, winning 52 of the 69 delegates.

It was clear from early voting there was going to be a big bump in the number of Republicans voting, particularly in Chicago, the rest of Cook County and its collar counties.

It was more than a bump – a full 100,000 more people voted in this Republican primary, in Cook County alone, than 2012. While some may have voted against Trump, a lot more backed him.  It is not known how many, but there is no doubt he received votes from Democrats in this open primary.

Hillary Clinton can also thank Cook County for delivering the narrowest of wins in her home state. The eight percent margin in the county was enough to win the state overall.

Delegates, both the 102 at congressional district and the 54 at state level, are pledged proportionally, meaning both Clinton and Sanders will have a close to equal share.

Likewise in Missouri, where the pair remained in a too close to call dead heat late into last evening. The 71 delegates will be split, likely 35 to 36.

With her wins in Florida, Ohio and North Carolina, Clinton was ahead 1,097 to 774 in pledged delegates as of late last night.

But Sanders is not going home any time soon. His energized supporters would never allow it, and he is far from finished hammering home a message that is resonating deeply.

Senior Sanders strategist Tad Devine spoke last night about making up the delegate advantage, and carving out a path to the nomination.

We’re only half way though primary season, he said, and Arizona, Idaho, Utah, Washington and Wisconsin are all ahead, important states for Sanders. Then New York at the end of April.

But Clinton, who won the popular vote in all five states yesterday, appeared confident.

“I want to congratulate Bernie Sanders for the vigorous campaign,” Clinton told her supporters, a seeming nod to his eventual dropping out of the race — though that might happen months from now.

But her point was well-taken: “Because of all of you, and our supporters across the country, our campaign has earned more votes than any other candidate, Democrat or Republican.”

Photo: Democratic U.S. presidential candidate Hillary Clinton shakes hands as she speaks to supporters at a campaign rally in West Palm Beach, Florida March 15, 2016. REUTERS/Carlos Barria