Every Saturday morning, we’re publishing “This Week in the 2020 Race”: a quick way to catch up on the presidential campaign and the field of 24 candidates for the Democratic nomination. Here’s our latest edition:

One candidate walks out, one candidate walks in

Three months to the day after he announced his presidential campaign, saying he was “burning the boats” behind him and would not run for re-election to Congress, Representative Eric Swalwell of California ended his presidential campaign and said he would run for re-election to Congress.

Less than 24 hours later, the California billionaire Tom Steyer — who had said in January that he would not run for president — said he would run for president, because the 2020 gods have decreed that the Democratic field shall not dip below two dozen.

Mr. Swalwell had centered his campaign on reducing gun violence, calling, among other things, for a mandatory federal buyback of assault weapons. But while gun control has been a much bigger issue in the 2020 race than in previous cycles, it was not enough to get Mr. Swalwell past 1 percent in the polls.