When Virginia state Sen. Jennifer McClellan is sworn into the House of Representatives this evening, Congress’s lower chamber will be something it hasn’t been in a long time: full.

The last time the U.S. had a full House — with all 435 of its voting representatives seated — was nearly three and a half years ago. And even then, it wasn’t full for long. When Wisconsin Rep. Sean Duffy resigned on Sept. 23, 2019, to care for his newborn baby with a heart defect, he left a House that had had 435 members for all of six days. In the 1,261 days since, there’s been at least one empty seat in the House.

Though this has been an unusually long run in modern times, the timing and number of vacancies generally come down to chance. Members sometimes resign and leave office before their terms expire, whether for another work opportunity (inside government or outside) or because of a scandal. Less predictable circumstances, like a member’s health problems or even death, also affect whether the House is at capacity. The current Congress (the 118th) began with one seat empty because Virginia Rep. Donald McEachin won reelection in November 2022 but died later that month; McClellan was elected on Feb. 21 to fill his old seat.

And when vacancies happen to overlap, it can produce periods like the current run of musical chairs. Duffy’s resignation came less than a week after the House got back to full strength for the first time since January 2017 (about two and a half years). Wisconsin’s governor scheduled a special election for May 2020 to fill Duffy’s seat, but by the time of that contest, five other seats had become vacant due to resignations or deaths.¹ And while the 2020 general election held elections for 435 seats, two were left vacant at the start of the 117th Congress due to the death of Louisiana Rep.-elect Luke Letlow in December 2020 and an extremely close election in New York’s 22nd District that delayed the seating of Rep. Claudia Tenney. In the spring of 2021, appointments to President Biden’s administration opened up seats in Louisiana, New Mexico and Ohio, and a host of other resignations and deaths occurred — even in the last few days of the outgoing Congress in December 2022.

But one trend that isn’t necessarily down to chance may also be contributing to lengthier vacancies, which reduces the likelihood of having all 435 seats filled. States have been taking longer to hold special elections for vacancies than in the past — both because elections now need more lead time because of laws regarding overseas balloting and expanded mail voting, but also because of governors’ political maneuvering to delay filling a seat likely to go to the opposition party.

Before we get too comfortable, however, this full House is going to turn into a weaker poker hand pretty soon. That’s because Rep. David Cicilline of Rhode Island announced on Feb. 21 that he would resign effective June 1 to become CEO of the Rhode Island Foundation. It’s unclear when Rhode Island will hold a special election for Cicilline’s seat, but it might not be until later this year. If that’s the case, that would leave plenty of time for other members to resign — or for something unexpected to occur. One way or the other, though, it seems like our full House won’t last very long.