Tuesday’s primaries in California, New Jersey, and elsewhere solidified what was already known: Democrat Hillary Clinton and Republican Donald Trump would be their parties’ presidential nominees this fall. Although a primary remains in the District of Columbia on June 14th, for all intents and purposes the primaries are over, and the Clinton and Trump campaigns must move on to the next phase of election.

The next stage in a presidential campaign—following the end of the primaries and in the lead up to the party conventions—involves changes in staff organization, staff size, messaging, fundraising, state targeting, and more. However, one decision will weigh heavily over each candidate in the coming weeks: whom to select as the vice presidential nominee. In selecting a running mate, each candidate must look for different qualities and face unique challenges.

Long gone are the days of geographic balancing of the ticket. Presidential candidates, of late, have paid little attention to using the VP slot to target a key state. Instead, standard bearers look for a partner, someone he or she can work well with, and someone who offers a balance in demographics and/or experience.

Who will stand with Trump?

Donald Trump is an unconventional candidate in style, in approach, and in experience. He faces clear divisions in his party that were made worse by his recent controversial comments about the Mexican heritage of the federal judge presiding over a civil suit against him. If Trump wants any chance of winning the White House in November, he must unite his party and convince skeptical establishment Republicans that he can be trusted in the Oval Office.

Part of building that trust will involve Trump changing his behavior over the coming weeks. Yesterday, during a talk at the American Enterprise Institute, Senate Majority Leader Mitch McConnell launching a scathing rebuke of Trump’s recent behavior, noting, “It’s time for him to look like a serious candidate for president.”

Trump’s choice in a running mate may help that process along. Trump needs someone with significant government, policy, and electoral experience to bring to the table what he most notably lacks. He needs to select an individual who is a disciplined campaigner and fundraiser. His VP candidate must overcome some of the skepticism about Trump’s conservative bona fides. Trump would do well to select someone who is a member of the GOP establishment or someone well-regarded within the establishment.

Often, VP candidates are chosen to be attack dogs: someone who can get out on the campaign trail and eviscerate the opposing ticket on a daily basis. Trump doesn’t need that. He is a professional attack dog—in speeches, on Twitter, in interviews, and wherever else he speaks. No one will outdo him, and frankly, no one will get away with the type of rhetoric Trump utilizes.

Trump doesn’t need an attack dog; Trump needs a show dog. He needs someone who looks the part, knows all the right moves, and will ultimately be approved by the establishment of the party.

Of course, Trump has a huge problem. There are plenty of candidates who fit that mold: John Thune, Bob Corker, Bill Haslam, Nikki Haley, Paul Ryan, Marco Rubio, Scott Walker, and Rand Paul—the list goes on. Trump’s problem is not finding a candidate; it’s finding a candidate who will say ‘yes.’ All election season long, many Republicans—even while endorsing Trump—have distanced themselves from him and/or preemptively closed the door on being his running mate. In the wake of Trump’s comments about Judge Curiel, it will be even harder for him to put the right kind of candidate on his ticket.

What else Trump needs this for the general election

Perhaps as important as picking the right VP, Trump needs a political operation capable of running a national campaign and transforming Trump into the kind of candidate who is disciplined enough to look presidential. Effectively, Trump needs to build a campaign apparatus that fulfills all of McConnell, et al.’s wishes. McConnell has done this before. Often. He has seen what works and what does not, and he is a remarkably effective political operator. Looking at the Trump campaign—small staff, no communications shop, limited state organization, and meager fundraising—McConnell likely sees a November loss. He sees a candidate with messaging that is offensive and alienating and falsely gives Trump confidence that what worked in the primary will absolutely work in the general. McConnell sees not simply a sinking presidential ticket, but his own position as Majority Leader at risk.

Pushing Trump to be more professional and presidential will not be easy, but it is essential. A man accustomed to being the boss, is used to winning, and is only familiar with getting what he wants is now running to be the most powerful man on the planet. It will be a true challenge for campaign staff to tell him what to do and what not to do—and the past week has made that ever clearer.

Who will Clinton choose?

Hillary Clinton faces a largely different calculus in selecting her running mate. It’s true both she and Trump need to quell elements within their party that doubt their values and ideology, Where Trump needs to reassure Republicans that he is a conservative, Clinton may want to add progressive credibility to her ticket in an effort to appeal to the left.

Beyond establishing progressive bona fides, Clinton needs someone who can hit the campaign trail every day and deliver blistering attacks on the Trump ticket. In some ways, intentionally or not, Massachusetts Senator Elizabeth Warren has auditioned powerfully for that role, recently launching relentless and effective attacks on Trump via Twitter and in public speeches.

Clinton may also consider selecting someone who offers demographic balance to the ticket: someone male, someone significantly younger than she is to soothe questions about whether Democrats have a deep bench, a racial or ethnic minority, or some combination thereof. Alternatively, House Minority Leader Nancy Pelosi, while endorsing Clinton Tuesday morning, suggested Clinton should consider an all-female ticket, a choice that would make a historic candidacy even more noteworthy.

Part of Clinton’s VP calculus should include the fundraising prowess of potential candidates. Beating Donald Trump and trying to shepherd in a Democratic Senate (and perhaps House) will be expensive. Clinton has shown herself to be an effective fundraiser, as have Bill Clinton, Chelsea Clinton, and two key fundraisers who will soon be coming on board: President Barack Obama and Vice President Joe Biden. However, every dollar will matter in Campaign 2016, and the VP candidate will be expected to do his or her part. Fundraising training wheels may be okay for a congressional candidate, but Clinton’s running mate must be a pro.

Clinton has another opportunity—connected to a demographic balance on her ticket—that is unique to this campaign. She can use the VP slot as a means of baiting Donald Trump. Clinton will of course look for someone qualified, who she can work with, who can raise money, and who has broad appeal within the party. But once all of those boxes are checked she may lean toward someone who Donald Trump cannot resist attacking—not politically, but personally.

Over the past year, Trump has not only shown that he lacks the discipline of a presidential candidate, but lacks the self-restraint one expects of any individual in public. If Clinton finds a running mate who can, “get under his very thin skin,” it may push Trump to continue his self-destructive rhetoric.

Clinton could select another woman; a racial, ethnic, or religious minority; an individual with a physical disability; a veteran; or someone from the many demographic groups whom Trump has offended during the primary. Such a running mate launching attacks against him will push Trump to react in exactly the opposite manner Mitch McConnell and other GOP leaders have been begging him to go in.

That consideration may be risky for Clinton, and it may seem obvious in intent. However, if it works, if it sets Trump off, and if it changes the dynamics of the general election campaign, it may provide the ultimate payoff.