The blonde-haired woman in the metallic Captain America body suit wasn't that impressed when someone joined Republican presidential front runner Donald Trump on stage at his North Alabama rally Sunday afternoon.

"Who is that?" she asked, holding her hand to her eyes to ward off the sun shining down on what organizers claimed was some 30,000 people who filled the municipal football stadium.

Those around her filled her in: Joining Trump on stage - and donning his own "Make America Great" hat- was Jeff Sessions, Alabama's junior Senator. It's the first time Sessions, a Republican, has endorsed a presidential candidate, Trump said, adding that that Senator is just the sort of person with whom he wants to work.

Sessions is the first U.S. Senator to endorse Trump, a political outsider who has courted controversy while amassing a groundswell of public support.

The importance of Sessions' endorsement has been noted before.

"Sessions' endorsement would carry a lot of weight. Sessions has more credibility with Trump's voters than Trump does," Mark Krikorian, executive director of the Center for Immigration Studies, told Politico in January.

Here's why Sessions' endorsement is a big deal for Trump:

Trump has landed THE immigration Senator

In 2014, Sessions was in line to take over as key of the powerful Senate Budget Committee. After an unlikely protest by Wyoming's Mike Enzi, who edged out Sessions in seniority, the Alabama Republican stepped aside and was given the chairmanship of the Senate Subcommittee on Immigration, Refugees and Border Security. In that role, he has become the leading voice against Obama administration plans to allow as many as 5 million undocumented immigrants to remain in the U.S.

When Trump began crafting his own hardline immigration plan, he turned to Sessions.

Even before endorsing Trump's candidacy, Sessions spoke favorably of the candidate's plan to put a halt to most immigration until more stringent criteria is put in place.

"This is exactly the plan America needs," Sessions said. "Crucially, this plan includes an emphasis on lifting struggling minority communities, including our immigrant communities, out of poverty, by preventing corporations from bringing in new workers from overseas to replace them and drive down wages."

Sessions' endorsement of Trump's plan and his candidacy is a stamp of approval for all those who want to the U.S. to take a hardline on immigration. It also lends credibility from a policy standpoint to Trump, who has been criticized for being long on talk and short on specifics.

For Sessions, Trump appears to be the answer to his fight not just on immigration but against the White House's trade policy as well. Sessions is a strong opponent of President Barack Obama's latest trade proposal known as the Trans-Pacific Partnership and he referenced that in Sunday's Alabama speech.

"We need someone to negotiate better," Sessions has said in reference to trade agreement.

He apparently thinks Trump is the man who can be that better negotiator.

It's a major blow to Ted Cruz

Sessions' name has been dropped in almost every Republican debate when immigration is mentioned, most notably by Trump challenger Sen. Ted Cruz. Earlier this year, Sessions indicated he was torn between endorsing a Trump and his fellow Senator.

That question apparently decided, Sessions' endorsement of Trump is a major blow to the Texas Senator whose campaign peaked with a win in Iowa and has suffered since. Cruz is trailing Trump and Florida Senator Marco Rubio in every poll except the one among Texas Republicans. In those polls, Cruz leads Trump by as much as 15 percent. Texas is the big prize for delegates on Super Tuesday and Cruz needs the momentum - and the numbers - that comes with a victory there.

Sessions' endorsement is not likely enough to swing Texas away from its home Senator. However, in a state that's on the front line of the immigration war, it could move Trump closer to Cruz and since delegates in the state are awarded proportionately, a strong second place showing would pad Trump's already large delegate lead.

It's all about credibility

Trump's campaign has been likened to a sideshow and in many ways it is. Sunday's rally in Madison came complete with people hawking "Make America Great" underwear and chants of "you're fired" to protesters in a takeoff of the mogul's reality show.

Sessions' endorsement, which comes just days after that of New Jersey Governor and former presidential candidate Chris Christie, lends of hefty dose of credibility among conservatives for Trump's White House bid.

While wildly popular among some voters, elected officials have seemed slow to endorse Trump. Before Sessions and Christie, Trump had picked up only three major endorsements from current elected officials: Maine Gov. Paul LePage, New York Rep. Chris Collins and California Rep. Duncan Hunter. A vote of confidence from a sitting U.S. Senator is a major win for Trump, who continued to heap praise on Sessions after the endorsement.

"That is so great. You know, (Sessions) is an incredible guy," Trump said.

Two more things...

In late January, Stephen Miller, Sessions' long-time communications director, left to join Trump's campaign as senior policy adviser. That change over can't be underestimated for either Sessions or Trump. Miller is the most visible link between the two.

Does this mean Sessions could be destined for a Trump White House?

Don't rule it out.

"I believe that with regard to lawfulness, immigration, and trade, good people, working for a strong president, can make a huge and positive difference," Sessions said just last week.