Even before their convention meets in Cleveland three weeks from now, many Republican leaders are already writing off Donald Trump and looking to 2020.  Some of the rivals the bombastic billionaire trounced in the primaries are dusting themselves off for a run against President Hillary Clinton four years from now.

Speaker Paul Ryan, who Mitt Romney and others in the GOP establishment begged to challenge Trump this year, took the unprecedented step of telling House Republicans to vote their "conscience" in November. Translation: I'm gonna hold my nose and vote for him but you don't have to.

Turning down that chance to run was a sign that he felt it was too late to get into the race; that doesn't mean he doesn't want the job, and some may consider him the putative frontrunner for 2020, but the competition will be fierce.  

Sen. Ted Cruz of Texas never stopped running after he dropped out; he just shifted gears and election target dates.  He has more campaign money in the bank than Trump at this point and raising more.

Sen. Marco Rubio of Florida last week announced he will to run for reelection to a job he told friends he hated because the party needs him to hold that seat in the GOP column in what could be a big Democratic year, and he was convinced it would help him stay in the national spotlight for the 2020 race.

Other candidates trounced by Trump this year are expected to try again in four years, including Wisconsin Gov. Scott Walker, Kentucky Sen. Rand Paul and Ohio Gov. John Kasich.

But for a number of losers, 2016 was very likely their last bid for the White House, like Rick Santorum, Mike Huckabee, Carly Fiorina, Jeb Bush, Bobby Jindal, Lindsey Graham, Ben Carson, Chris Christie and Newt Gingrich.

The latter pair are said to be campaigning hard to be Trump's running mate, a potentially hellacious and risky undertaking.

The GOP's historic weakness among female voters was exacerbated this year by the fact that a notoriously misogynistic Trump is running against the first woman ever nominated by a major party. I expect a number of women to challenge the nation's first female president, including South Carolina Gov. Nikki Haley, Sen. Joni Ernst of Iowa and New Mexico Gov. Susana Martinez.

Some new faces said to be thinking of running the next time include Sen. Ben Sasse of Nebraska, leader of the Never Trump movement; Sen. Mike Lee of Utah, Cruz's frequent wingman; Sen. Tom Cotton of Arkansas, the youngest member of the Senate; Sen. John Thune of South Dakota, and Indiana Gov. Mike Pence. 

Another is Sen. Bob Corker, chairman of the Foreign Relations Committee, a strong Trump supporter and foreign policy tutor, who is frequently mentioned as a possible running mate. 

The 2020 post-Trump GOP field can be expected to be as crowded as this year's if Clinton is president. There's never been a shortage of ambitious politicians or any stripe.

Former Sen. Rudy Boschwitz (R-Minnesota) once told me he was the only one of the 100 Senators not looking in the mirror each morning and see the person who should be President of the United States.  He was the exception because he was the only senator not born in this country.  He was born in Berlin in 1930, and when the Nazis came to power in 1933 his family fled to America.