Milwaukee. JD Vance is supposed to help Donald Trump win the Midwest this fall.

But almost immediately after the Ohio senator was announced as Trump's vice presidential pick on Monday, one thing became clear: Vance, a 39-year-old Republican with less than two years in Congress, is not well-known among many in his party, even in the swing states Trump hopes he'll deliver.

Michigan Republican Party Chairman Pete Hoekstra offered a blunt response when asked about Trump's pick minutes after it was announced: “We don't know him.”

"If he’s from Ohio, he understands our state and the other northern battlegrounds,” Hoekstra said, standing on the floor of the Republican National Convention. “But we haven’t had a chance to take his measure yet.”

Trump’s team now has less than four months to strengthen Vance’s profile in the states that matter most this fall in his 2020 rematch against Democratic President Joe Biden. Already, a collection of political foes -- Democrats and Republicans -- is working to fill the void by seizing on Vance’s inexperience in government, his nationalist views and his critical comments about Trump himself.

“I’m not sure he helps him in the campaign," said veteran Republican pollster Neil Newhouse, suggesting Vance may be better positioned to help Trump enact his agenda on Capitol Hill if given the chance. “He's not that well-known even in Ohio. ... This isn't a campaign pick. It's a policy pick, a governing pick.”

Republican strategist Kellyanne Conway, who served as Trump’s chief counselor while in the White House, had encouraged Trump to pick a different running mate in the weeks leading up to his announcement. Privately, she believed that Florida Sen. Marco Rubio or Virginia Gov. Glenn Youngkin would do more to help Trump win.

Vance, who quickly developed a reputation as a MAGA firebrand in his short time on Capitol Hill, earned modest applause when he entered the packed convention hall for the first time Monday as Trump's running mate. The Republican senator posed for selfies, shook hands and signed posters. Later in the night, the crowd was more excited as he greeted Trump -- who entered the room with a bandage covering his right ear, injured in Saturday's assassination attempt -- for the ticket's first public appearance.

Recent polling confirms the notion that most voters don't know Vance.

Just 13 percent of registered voters said they had a favorable opinion of Vance with 20 percent an unfavorable one, according to a CNN poll conducted in late June. The majority said they had never heard of him or had no opinion.

Trump's vice-presidential pick is arguably the most important decision of his 2024 campaign. Vance, who is literally half the 78-year-old Trump's age, and has the least political experience on a short list that included Rubio and North Dakota Gov. Doug Burgum.

Sensing an opportunity, Trump's critics in both parties quickly went after him.

“Almost any other choice might have expanded the map for them, but Trump needed a candidate who looked like him, talked like him, and thought like him. He needed a candidate who would grovel,” former New Hampshire Republican Party chair Jennifer Horn wrote on X. “JD Vance was the least experienced, least qualified, most obsequious, psychopathic, servile candidate on the list.”

But Trump made up his own mind based on a different set of criteria.

Trump especially liked Vance’s performance on television, where he has become a fixture on conservative media. The former president also likes Vance’s looks, saying he reminded him of “a young Abraham Lincoln.”

Trump is also hopeful Vance can draw from his life story growing up in Appalachia to help appeal to Midwestern voters. Vance has experienced poverty and addiction up close in a way that is uncommon among leading Republican officials.

Vance also had another advantage: his chemistry with Trump. The first-term senator has developed a strong rapport with Trump, his son Donald Trump Jr. and leading MAGA figures during his recent rise in Republican politics.

Vance is an Ivy League-educated author, former Marine and businessman. He is known for his aggressive questioning of Biden administration officials.

Biden's campaign hosted a conference call Monday denouncing the pick, focusing especially on his limited record on abortion and the economy and his support for Trump's efforts to overturn the 2020 election.

Vance previously said he would support a national abortion ban at 15 weeks of pregnancy. He also said he would not have voted to certify the 2020 election results, as former Vice President Mike Pence did over Trump's objections.

“I will certainly take that matchup any day of the week and twice on Sunday,” said Jen O’Malley Dillon, the Biden campaign chairwoman. “Because while Trump and Vance have an agenda focused on themselves and their wealthy donor friends, President Biden and Vice President Harris are fighting for the American people.”

One of Biden's greatest assets in his campaign against Vance might be what Vance previously said about Trump.

During the early stages of Trump’s political career, Vance cast Trump as “a total fraud,” “a moral disaster” and “America’s Hitler.”

“If you go back and listen to the things that JD Vance said about Trump ... he said some things about me, but see what he said about Trump,” Biden told NBC's Lester Holt in an interview Monday.

Vivek Ramaswamy, once considered a potential Trump running mate as well, described Vance as “a major asset” on the ticket whose evolution on Trump would ultimately help him connect with swing voters.

"He’s also somebody who can say, ‘You know what, in 2016, I may not have voted for Donald Trump either, but here’s why I am with him to the fullest today,’” Ramaswamy said.

But for now, Vance joins the Trump presidential ticket as a mystery to many voters and elected officials alike.

Georgia Gov. Brian Kemp said Vance was one of the few vice-presidential prospects that he “really haven't crossed paths with.”

“I don’t know that much about him," Kemp said.