Anyone on hand for the 2016 Republican National Convention, which got underway Monday, will see history made on a number of fronts. One of those fronts involves the sheer number people who won't be there to see that history made.

For months now, lawmaker after lawmaker has said they won't be in Cleveland for this week's proceedings. Since presidential election years are obviously campaign years, a few lawmakers here and there missing out is no big deal. But now that proceedings have begun, it's official: the sheer number of missing Hill Republicans -- including around half the Senate GOP -- who will be conspicuously absent is unheard of in modern times.

"This is extraordinarily unusual," said Robert David Johnson, a history professor at Brooklyn College.

"Most" of Senate Republicans on the ballot this fall won't be there, Andrea Bozek with the National Republican Senatorial Committee recently told The Atlantic's Michelle Cottle.  Twenty-four out of 54 Senate Republicans are on the ballot, so if we're to take her at her word, that's almost half the Senate Republican caucus that's skipping it. Also skipping the convention: two former Republican presidential nominees, Mitt Romney and John McCain, the past Republican president, George W. Bush, and some up-and-coming Republican female leaders like Reps. Mia Love (Utah) and Elise Stefanik (N.Y.).

Johnson was hard pressed to find a time when many lawmakers skipped the same one.

That's because conventions -- four days of the party elite and its biggest supporters together in one giant hall -- present an incredible opportunity for politicians to do what they do best: schmooze. There are donors to meet, political operatives in your state to befriend, national TV appearances to make. Conventions are the definition of see and be seen politics, a must-do for lawmakers who want to get reelected.

"In a normal convention, there's zero downside to attending," Johnson said.

This, of course, is no normal convention. Republicans are about to nominate the least-liked major party candidate in modern history, a candidate whose unpopularity could end Republicans' chances to take back the White House and drag down enough vulnerable Republicans up for reelection to take away their control of the Senate, too. If things get really really bad, control the House of Representatives might even be in play -- though that's more of a long shot.

At any rate, dozens of GOP lawmakers in Congress have decided the opportunity to schmooze just isn't worth being caught on camera with Trump -- or in the same room as him when he becomes their presidential nominee.

"I have to mow my lawn," said Sen. Jeff Flake (R-Ariz.), no fan of Trump's. Sen. Marco Rubio (R-Fla.) is expected to address the convention via video.  Sen. Steve Daines (R-Mont.) says he'll be fly fishing. Even Sen. Rob Portman (R-Ohio), who's in a tough reelection battle in the very state the convention is taking place in, isn't going to spend all of his time there. His team says he'll pop in and out.

When viewed through a historical context, their absence is telling. To the extent lawmakers have bailed in past conventions, it's been because of major party strife, Johnson said.

In 1856, some Southern Democrats bolted from their convention over a debate on slavery, helping deny the incumbent president, Franklin Pierce, re-nomination for the first time in U.S. history. In 1964, Democrats from Missouri and Alabama decided they didn't agree with the party platform and didn't attend. There are other examples peppered here and there, but almost all of them were only handful of lawmakers at a time -- nothing of this magnitude. (It's also worth noting that until 1968, GOP lawmakers played major roles in electing the president as delegates, kinda like the Democratic system now where party officials act as delegates, so they had even more incentive to stay.)

Johnson said the closest example to today would have to be the election that usually gets compared to this one: The 1964 Republican primary. There, conservative Barry Goldwater won the GOP nomination over the objection of the establishment. Some Republicans in the Northern and Midwestern states thought his anti-civil rights position would be untenable for their election hopes, so they decided not to attend the convention.

But once again, Johnson estimates that in '68 no more than 15 or so House and Senate lawmakers skipped the convention. In 2016, we're talking dozens and dozens of party leaders who don't want to be caught anywhere near it. Count poor attendance for the convention as just one more thing that's unusual about this presidential election.