Sessions famously appeared at a rally with Donald Trump early in the primary season, even putting on a Make America Great Again hat. But Sessions has also been a fierce defender of Senator Ted Cruz. When Cruz was struggling, it was Sessions who said Cruz had always been there in the fight against Amnesty, giving Cruz the credibility he needed to defend against Trump's charges of weakness on the issue. Cruz would not have won Iowa had Sessions (along with Steve King) not aggressively defended him.
In Texas, Cruz still leads in most polls and is expected to win. However, in the rest of the country, it is now a Rubio/Trump battle. And Alabama is one of the states voting on Super Tuesday.
Sessions may well have declined to endorse Trump if it was a close race in his state. But Cruz has already abandoned Alabama [Cruz collapse - Ted Cruz campaign gives up on Alabama, Pulls Out of State, Conservative Treehouse, February 25, 2016]. And Sessions's has given a kind of negative endorsement to Marco Rubio, suggesting anyone else is more worthy of support [Jeff Sessions Says No to Marco Rubio for President, WND, February 23, 2016]
Sessions needed to outright endorse Trump to stay relevant in the party. Sessions, more than anyone else, rebuts the absurd Narrative that Donald Trump is part of the "Establishment," even though he's challenging the powers that be on the most important issues. As Sessions observes, no one else comes close to his Trump's positions on trade [Jeff Sessions Praises Donald Trump's Answers To 'Sessions Test, by Julia Hahn, Breitbart, February 16, 2016] And Trump is hiring Sessions's key staffers.
If the likes of Rubio somehow take control of the Republican Party, Sessions will be rendered irrelevant. After Rubio and Paul Ryan pass Amnesty, the irrelevance will be permanent, not just for Sessions, but for all conservatism. Out of self-interest, if nothing else, Sessions had to act now.
The endorsement makes the war within the GOP explicit. Cruz, perhaps unfairly considering he's still winning in Texas, seems to have been rendered irrelevant. Rubio, the champion of the Donor Class, is surging in Virginia, holding four rallies and addressing as many as 20,000 people in one day. He's also within striking distance in Georgia. Most importantly, he's pledged to support the #NeverTrump campaign, essentially saying he will not endorse Trump if he wins. The anti-Trump forces are also explicitly promising to form a 3rd party to destroy Trump's chances at winning the election. The stakes could not be higher.
At a stroke, Trump secures the endorsement of perhaps the nation's most respected elected official on the immigration issue. He short circuits the absurd media attacks about Trump supposedly refusing to denounce the KKK. He most likely takes votes away from Cruz. And he with Sessions's powerful endorsement, he instantly rebuts all the claims Trump is not a "true conservative." For what it's worth, he even has more Senate endorsements than Cruz now.
For immigration patriots, this is a historic moment. Tuesday will determine whether the Republican Party will remain a collection of bagmen for corporate interests or whether it will be reconstituted as a nationalist, populist party.