Donald Trump narrowed his shortlist of Vice Presidential picks down to three candidates Wednesday.

According to Trump, the top-three contenders are New Jersey Governor Chris Christie, Former House Speaker Newt Gingrich and Indiana Governor. Mike Pence.

Trump is expected to name a running mate by Friday, just days away from the start of the Republican National Convention.

In the home stretch of his VP search, Trump continued to stir speculation and build anticipation as he prepares to make the biggest decision of his campaign to date.

“Donald Trump certainly has the bigger question mark because he has not been elected before to anything,” said Ray Sullivan, a veteran political consultant in Texas.

Republicans from across the country will gather in Cleveland to kick off the convention Monday. There, the GOP will presumably make Trump’s nomination official and formally approve the party’s presidential ticket.

“Donald Trump has to find somebody who is steady, credible and ready to governor on day one,” Sullivan said.

A big state governor, or a longtime U.S. Senator could give Trump a momentum boost coming out of the convention, Sullivan said.

Trump said he needs his VP to be an “attack dog” to go after Hillary Clinton and during on-stage auditions rumored contenders acted the part at a series of Trump events in recent weeks.

Sullivan said the GOP ticket needs experience and candidate who represents conservative values.

“Someone who has been around and who can help sure up the conservative base who is still a little bit nervous about Trump,”  said Sullivan. 

On the other side, Sullivan said Hillary Clinton faces a different problem as the party looks to balance the Democratic ticket.

Sullivan said Clinton could do one of two things—either pick a VP that energizes the Liberal base, “Or try to sure up that credibility gap that she has, a lot of people think she’s untrustworthy,” said Sullivan.  

A Texan, former Mayor of San Antonio, Julian Castro is reportedly still in the mix Clinton’s possible VP picks but her shortlist is much longer than Trump’s.

Castro currently serves as the U.S. Secretary of Housing and Urban Development and is the youngest member of President Barack Obama’s Cabinet.

If chosen, 41-year-old Castro  would be the first Latino candidate on a presidential ticket.

“Castro would help potentially bring along Hispanics,” Sullivan said Castro could broaden Clinton’s support “somewhat,” but not as much as other potential picks. “I think she’s really going to need to go to someone who can electrify her liberal base or really sure up her credibility problems,” Sullivan said he’s not sure Castro does either of those things. 

Sullivan worked on the Bush-Cheney campaign in 2000, Rick Perry’s two presidential runs and most recently, Jeb Bush’s primary campaign.

Sullivan said a nominee’s VP pick is a personal decision, a choice that “usually doesn’t matter” in elections. “But in a year like this where all conventional wisdom is out the window, where the candidates are both flawed in many ways,  that decision is going to be telling on both sides,” Sullivan said.

It is possible for delegates to try to challenge the Presidential or Vice Presidential nominees at the parties’ national conventions, although that is unlikely.