Paul Ryan just said he would not accept the GOP presidential nomination at the convention

In a brief press appearance Tuesday, House Speaker Paul Ryan definitively ruled out accepting the Republican nomination for president at a contested convention this summer — and went further, explicitly advocating that the nominee should be someone who was actually in the race.

"Let me be clear. I do not want, nor will I accept, the nomination for our party," Ryan said. He later said that he would not allow his name to be placed into nomination.

Furthermore, he added that while it was not his job to tell the delegates what to do, "I believe you should only choose from a person who has actually participated in the primary" — and that they should adopt a rule making that clear.

"I believe if you want to be president you should run for president, and if we select a nominee, we should be selecting among people who actually ran for the job," Ryan said.

His announcement will disappoint establishment Republicans and many of the party's wealthy donors, who deeply dislike both Donald Trump and Ted Cruz and have hoped a contested convention could allow them to hand the nomination to someone else. (Billionaire Charles Koch was hoping Ryan could be the nominee, according to the Huffington Post's Ryan Grim and Sam Stein.)

It also calls attention to the difficulty of mounting a dark horse convention candidacy in the modern era, with modern democratic norms in play.

Technically, the delegates do have a great deal of power to swing the outcome. But the more they use that power, the more illegitimate their moves will appear to voters — potentially resulting in a serious backlash.

Indeed, a recent Vox/Morning Consult poll found that only 27 percent of Republican voters responded positively to the idea that someone who didn't run could become the party's nominee — and 63 percent responded negatively.

Ryan knows full well that Donald Trump and Ted Cruz have gotten the most votes. And in the modern era, that means voters expect that one of them will become the nominee.

If the convention had nominated him instead rather than respecting the voters' will, it could have torn the party apart — and ensured Ryan would be loathed by a great many of the GOP voters who backed Trump or Cruz. Why would he take on that risk, when he's still young and could have several more chances to make a real presidential bid?