According to a very recent Des Moines Register poll, Republican voters in Iowa are split – 40% in favor, and 41% opposed – on the question of whether Donald Trump should face a primary challenge in 2020.

The remaining 19% in that poll simply can’t make up their minds on the issue. Forty percent is a sizable number of voters who would like to see a real race for the GOP nomination, even if they would still end up voting for Trump again in 2020.

Here are some more numbers on the feelings of Iowa GOP voters from the source:

Most registered Republicans think President Donald Trump is doing a good job, but they are split over whether another GOP candidate should challenge him for their party’s nomination in 2020, a new Iowa Poll shows.

The Iowa Poll, sponsored by the Des Moines Register, CNN and Mediacom, also finds 90 percent of registered Republicans want Trump to run a positive re-election campaign, focusing on the good things he’s done for the country. Just 4 percent want him to focus on attacking opponents — one of the president’s trademarks.

There’s a difference between wanting to see a primary challenge, and then actually asking voters whether they would vote for a challenger. So far, only moderate Massachusets Republican Bill Weld has actually formed an exploratory committee to test the waters for a primary challenge to Donald Trump, but no other Republicans are eager to get in the poll, for good reason.

Columnist David Byler, writing for the Washington Post, picks up the question, in relation to the poll, by asking whether Republicans would actually be willing to vote for a challenger if it meant the party would be weaker heading into a fight with Democrats next year:

Those numbers might seem like a Bat-Signal to Trump-skeptical Republicans who are eager for any sign that they could put up a real fight against Trump.

But that’s much easier said than done. A living, breathing primary challenger will have to take positions; attack a president who is popular within his own party (if unpopular nationally); and unite Republicans who want a primary challenge to Trump for very different reasons.

The best way to illustrate this difficulty is to look at Trump’s polling relative to his possible opponents.

According to the CNN poll, 82 percent of registered Republicans in Iowa rated Trump favorably while only 15 percent rated him unfavorably. Former Ohio governor John Kasich, one of Trump’s 2016 opponents, is already rated unfavorably by 28 percent of Iowa Republicans, with 27 percent rating him favorably. Larry Hogan, the highly competent Maryland governor who has expressed some interest in challenging Trump, was rated favorably by 4 percent of respondents and unfavorably by 12 percent. Bill Weld, the moderate former Massachusetts governor who has already formed a presidential exploratory committee, is also underwater, with only 4 percent approving and 15 percent disapproving. Favorability numbers can change, but Trump is starting in the driver’s seat and his opponents don’t have outstanding early numbers with which to challenge him.

In other words, there are no Republicans skirting the field right now with the same kind of popularity as Donald Trump within the party. Some voters may want to see Trump challenged, but ask them a follow-up of who should challenge him, and they probably won’t have any good answers.

It would be impossible for someone like John Kasich, or Bill Weld, for that matter, to actually convince a group of voters in a majority of primaries and caucuses to dump Donald Trump in favor of a new option for 2020.

Besides, the Republican National Committee, with Trump’s blessing, has been working to set up the field in 2020 to ensure that a primary challenge could not easily occur.

Maryland Governor Larry Hogan is a popular Republican in his home state, sometimes veering over 70% approval rating from his Democratic-leaning constituents. Still, in a highly-conservative GOP presidential primary, would he stand a chance at unseating a sitting Republican President? Highly unlikely.

The reason, according to Byler, for the 40% number wishing to see Trump challenged, is probably due more to personal preference than real-world political conditions:

Different people who want Trump to face “a primary challenger” are probably imagining different challengers. Someone who would prefer Sen. Ted Cruz (R-Tex.) to Trump might not prefer Kasich to Trump. Some of these voters are also probably imagining an idealized challenger who would cater to their exact preferences rather than a real person who has his or her own ideas and flaws. Everyone knows that guy who always complains about not having a significant other but then nitpicks and rejects every real person he goes out with. If some Trump-skeptical Republicans are thinking this way, then the anti-Trump base may be functionally smaller than those 40 percent numbers suggest.

If you were a die-hard Ted Cruz fan in 2016, you’re probably still a die-hard Cruz fan in 2020. In the depths of your mind, wouldn’t it be nice to see Trump falter and Cruz step up to give it another go in 2020? Some voters may be thinking along those lines with their preferred candidate from last time.

Jeb Bush voters might wish Jeb would challenge Trump. John Kasich voters might feel the same. If you total up those kinds of nostalgic sentiments within that 40% of voters seeking a challenge, you can probably explain most of it.

For Donald Trump to be successfully primaried in 2020 would take external forces beyond his control. Think something more along the lines of an indictment on business practices, or some bombshell from the Mueller Russian collusion probe.

Something that would not be easily ignored by GOP voters who would fear that the conditions could make Trump unelectable in 2020, so they decide to toss him during the primary and nominate someone new to fight the Democrats.

Outside of that scenario, Trump is sitting in a position which practically makes him immune to a Republican primary challenge. Right now.