BOISE, Idaho — The population of American prisons is likely to rise for the first time in nearly a decade with President-elect Donald Trump’s promise to detain and deport millions of immigrants who are in the country illegally and his selection of tough-on-crime Sen. Jeff Sessions to the nation’s highest law enforcement post.

If so, one of the prime beneficiaries would be the private companies that operate many of the nation’s prisons. The stock market seems to agree.

A day after the election, CoreCivic Co., formerly Corrections Corporation of America, saw the biggest percentage gain on the New York Stock Exchange with shares climbing 43 percent.

Shares of Geo Group, another private prison company, also jumped 21 percent.

The federal prison population had been trending down for nearly a decade when the Obama administration announced in August that it would phase out its use of some private facilities.

The announcement followed a Justice Department audit saying private facilities have more safety and security problems than government-run lockups.

The policy change did not cover private prisons used by Immigration and Customs Enforcement, though federal officials have said they are considering phasing out private contractor immigration facilities.

Trump, however, said during his campaign that the nation’s prison system was a mess and voiced support for private prisons.

“I do think we can do a lot of privatizations and private prisons. It seems to work a lot better,” Trump told MSNBC in March, though he didn’t offer any details on what that might mean for the federal prison system.