That's seen as subtle knock at Trump and other GOP rivals, but — as we've seen so many other times over the course of this campaign — even the slightest of knocks doesn't go unnoticed by the New York real estate magnate.

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A few hours later,

:

No, your eyes do not deceive you — that is indeed Barbara Bush, the former first lady, literacy advocate and mother of another former president — wearing eye-black and shoulder pads.

And no, it's not a doctored photo.

She actually wore eye-black and shoulder pads in April 2014 in a photo shoot with Houston Texans player J.J. Watt as part of her campaign against illiteracy.

The latest Twitter tit-for-tat came on the same day that Bush criticized the Republican National Committee for dropping the National Review magazine as a co-sponsor of a late February debate. The GOP dropped the conservative publication from the debate on Thursday after it published a series of essays criticizing Trump and labeling him dangerous to the future of American conservatism.

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"I think that they would have all of these distinguished journalists and thought leaders of the conservative cause join together to say that Donald Trump is not a conservative — they're just telling the truth," he told Fox News Channel.

"It was the wrong thing to do. Donald Trump needs to defend why he was supportive of Hillary Clinton and Bill Clinton, why he gave money to the foundation, why he gave money to her campaign," he added.

Even as Bush has sunk in the polls, Trump continued to pick away at him, and he rarely gets through a campaign rally or television interview without taking at least one if not several jabs at Bush. Trump often prefaces these attacks with the disclaimer that he really shouldn't pick on someone who is doing so poorly in the polls.

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Trump has called Bush "low-energy," "so sad," a "sad sack," a "loser," "terrible," a "stiff," an "embarrassment" and "dumb as a rock." He has mocked Bush for spending tens of millions of dollars more than him, yet still not performing as well. And he has questioned why the candidate is going by "Jeb!" on the campaign trail instead of using his famous last name.

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Bush has relentlessly focused on Trump over the course of the last few months, believing that he can win over undecided Republican voters by raising doubts about the front-runner's lack of conservative bona fides. The former governor has seen a modest uptick in his polling in recent days in places like New Hampshire, South Carolina and Florida, but he still trails Trump by double digits.

In the past few weeks, Bush's campaign has released a series of ads and snarky videos attacking Trump, including one this week that forewarns of Trump becoming the Republican nominee, losing to Democratic front-runner Hillary Clinton and then conceding the race in a tweet. These ads seem to have enraged Trump, who has intensified his attacks on Bush.

In late December, the super PAC supporting Bush's campaign released an ad that featured a heated exchange the two candidates had during a GOP debate, during which Bush told Trump he cannot insult his way to the White House. Trump said at a rally in Hilton Head, S.C., on Dec. 30 that the commercial was "false advertising" because "I killed him in the debate."