Less than a month after dropping his own White House bid, South Carolina Sen. Lindsey Graham on Jan. 15 endorsed former rival Jeb Bush while taking shots at the GOP's current front-runner, Donald Trump.

"I'm endorsing Jeb Bush for president this morning," Graham, considered a foreign-policy hawk and a member of the Republican establishment, announced on social media. The former Florida governor was "prepared to be commander-in-chief on Day 1," the senator assured.

Endorsement may help turn around polls

The backing from a GOP heavyweight such as Graham comes at a critical time for Bush, who has not been doing well in national polls recently. Bush feted the South Carolinian's decision as a move that would help his campaign "build momentum" and said he was "proud to have" his backing.

The brother of former President George W. Bush and son of former President George H.W. Bush immediately sought to use the announcement to encourage more voters to back his White House bid: On its website, the Bush campaign urged potential supporters to "join Lindsey Graham in standing with Jeb" and consider signing up for newsletters and making donations.

Bush's debate performance impressed Graham

Graham revealed his endorsement at a news conference in his home state, and he said Bush's strong performance during the previous night's Fox Business-sponsored presidential debate helped him make his decision.

"Last night I heard from Jeb Bush the right answer. He demonstrated somebody in my view who is ready on day one to be commander-in-chief," Graham said as Bush looked on.

GOP establishment moves against Trump

The senator's endorsement helps Bush solidify his position as one of most promising candidates within the GOP's establishment wing, which has become increasingly nervous that Trump could end up being the party's nominee. Both Bush and Graham have long typecast the real-estate tycoon as an irresponsible figure unfit to serve in the Oval Office.

"I can't think of a worse idea of how to fight and win this war," Graham said on Jan. 15 about Trump's proposal for a temporary ban on Muslims entering the United States. On a previous occasion, he had gone so far as to refer to the "Apprentice" star as a "jerk."

'Uninformed' about undocumented immigrants

Both Graham and Bush have also aligned in their criticism of Trump's often controversial comments on immigration, pointing out that Latinos will likely be a key electoral demographic.

"I think he's uninformed about the situation regarding the illegal immigrant population," Graham told CNN last year. "I think he has hijacked the debate. I think he is a wrecking ball for the future of the Republican party with the Hispanic community and we need to push back."

Bush, for his part, told the news channel that Trump was a "bully."

"Donald Trump's not going to be president of the United States by insulting every group on the planet -- insulting women, POWs, war heroes, Hispanics, disabled, African-Americans," he said. "You can't insult your way into the presidency."