(CNN) President Donald Trump continues to look in prime position to win his party's nomination for president in 2020. While Republicans such as John Kasich may run , a look at the numbers reveals they have a tough road ahead of them.

CNN's latest Iowa poll shows little room for a successful primary challenge in the first in nation caucus. Trump scored an 81% approval rating among registered Republicans. The vast majority, 67%, of Republicans also said that they would definitely vote to re-elect Trump.

Trump's numbers in the early states are a reflection of his national popularity with Republicans. Trump scored an 89% approval rating in the most recent monthly Gallup national aggregate.

Trump's approval ratings are currently well above what's historically been necessary to avoid a competitive primary.

Since 1952, 12 incumbent presidents could have run for reelection. Ten of those 12 have been renominated. The only two who weren't -- Harry Truman in 1952 and Lyndon Johnson in 1968 -- had approval ratings among their own party members of less than 55% right before the New Hampshire primary.

Just two other incumbent presidents were ever truly in trouble for renomination. Gerald Ford in 1976 had to fight off Ronald Reagan, while Jimmy Carter struggled with Ted Kennedy in 1980. Both Ford and Carter had approval ratings with their party members in the mid 60s just before New Hampshire voted in the primary.

George H.W. Bush was the only other president ever to face a credible primary challenge. He won every primary, though he barely finished with a majority of the New Hampshire primary vote against Pat Buchanan. Bush's approval rating with Republicans before the Granite State cast its vote was 72%.

Trump's approval ratings are well above all these incumbent presidents who faced renomination difficulty. Now could Trump's popularity among Republicans decline? Of course, it could.

Keep in mind though that Trump's monthly Gallup aggregate approval rating has never dipped below 79% among Republicans, even as his overall monthly approval rating has dipped as low as 36%.

Of more immediate importance, Trump's ability to hold onto the Republican electorate in the near future is likely to dissuade some Republicans from challenging him. For a primary challenge to be anywhere near successful, it will need to be launched fairly soon. Most high quality challengers though will likely take a pass on what looks to be a losing mission.

If that's the case, it could be that if Trump's numbers with Republicans ever do fall, there won't be a high quality challenger in the race to take advantage.