Republicans are preparing a full court press on former Vice President Joe Biden’s mental fitness for office in the event that he wins the Democratic presidential nomination, GOP sources told the Daily Caller News Foundation.

Biden’s formerly rival candidates made a handful of references to the issue over the course of the campaign: former Housing and Urban Development Secretary Julian Castro made a veiled jab at Biden’s mental fitness before walking it back; New Jersey Sen. Cory Booker questioned Biden’s “ability to carry the ball across the end line without fumbling;” and Ohio Rep. Tim Ryan said Biden is “declining” and lacks “clarity” when he speaks.

But by and large, Biden’s fellow Democrats have been hesitant to press the issue beyond making passing comments.

Republicans, meanwhile, are preparing to highlight questions about Biden’s fitness if he becomes the Democratic nominee.

A Trump campaign official told the DCNF that Biden’s “capacity” will be an issue in a potential general election matchup, echoing language used by Democratic New York Rep. Alexandria Ocasio-Cortez.

President Donald Trump has repeatedly taken shots at Biden, saying the former vice president is “not playing with a full deck” and “has lost his fastball.” Trump’s campaign has already churned out videos highlighting Biden’s struggles.

WATCH:

Before Joe Biden jumped in the race, Barack Obama told him “you really don’t have to do this.” After months of watching Joe on the campaign trail, everyone can see why. Cory Booker put it best: “You listen to Joe Biden and you just wonder.” pic.twitter.com/qLosBxWM2f — Trump War Room – Text EMPOWER to 88022 (@TrumpWarRoom) March 4, 2020

Researchers at the Republican National Committee are keeping a running internal list of Biden’s verbal and mental miscues in preparation for the general election, an RNC source told the DCNF. (RELATED EXCLUSIVE: Hunter Biden Touted Connections Last Year While Pitching UCLA Law On Letting Him Teach Drug Policy Course)

Examples abound.

Biden trailed off Monday while citing the Declaration of Independence, telling a crowd: “We hold these truths to be self-evident. All men and women created by the, you know, you know the thing.”

WATCH:

You can’t make this up…

pic.twitter.com/Hq6dLUbXLC — Daily Caller (@DailyCaller) March 2, 2020

He told a South Carolina crowd on Feb. 25 that he’s “a Democratic candidate for the United States Senate” and that they should “vote for the other Biden” if they don’t like him. He also told a South Carolina crowd he is “looking forward to appointing the first African-American woman to the United States Senate.”

“We have to choose truth over facts,” Biden said in August 2019.

He claimed in the Feb. 25 debate that gun violence killed 150 million people — roughly half the U.S. population — in a decade.

Biden also said in a debate last September that he is “the vice president of the United States” and called Vermont Sen. Bernie Sanders “the president” of the country.

Biden, whose campaign didn’t return a request for comment, has also struggled with places and dates.

“I love this place. Look, what’s not to like about Vermont in terms of the beauty of it?” he asked a New Hampshire crowd in August 2019.

Biden said in December that the Obama administration took place in 1976, before correcting himself. He appeared to confuse the 2016 and 2010 elections in a Feb. 20 town hall.

He has also confused both German Chancellor Angela Merkel and former British Prime Minister Theresa May with Margaret Thatcher, who passed away in 2013 and whose reign as British prime minister ended in 1990.

Content created by The Daily Caller News Foundation is available without charge to any eligible news publisher that can provide a large audience. For licensing opportunities of our original content, please contact licensing@dailycallernewsfoundation.org.