With the Republican and Democratic parties finalizing plans for their nominating conventions this month, presumptive U.S. presidential nominees Hillary Clinton and Donald Trump are in the early stages of searching for their vice presidential running mates.

“What they’re doing is floating these trial balloons or putting names out there to see what kind of positive or negative reaction they get, and that may help them decide by the convention just whom they are going to choose,” University of Maryland political science professor James Gimpel said in an interview with VOA.

Clinton, Trump Eye Vice Presidential Running Mates

With the Republican nominating convention set to start July 18 and the Democratic convention a week later, the Trump and Clinton campaigns are reviewing lists of potential partners who Gimpel said can “cover the turf and the extensive terrain of the campaign.”

Clinton, Trump Eye Vice Presidential Running Mates

The campaigns also are considering other factors that can strengthen their tickets, including whether the vice presidential hopefuls come from a swing state that could help decide the election, and whether they have the experience and skill to assume the presidency if necessary.

Clinton, Trump Eye Vice Presidential Running Mates

The ideal vice presidential running mate for Clinton’s Democratic ticket should be someone “who is above reproach and has no ugly skeletons that will fall out of the closet when some enterprising reporter starts digging around,” said Gimpel.

“Given all the questions that have been raised by her honesty and her integrity … she definitely needs to vet the person to make sure that they’re scandal free,” he added.

Trump has said he wants a Republican running mate who can help him navigate the political landscape in Washington and advance his legislative program through Congress, because he has never held elected office before.

Gimpel believed a strong advocate of social conservatism and free enterprise, “someone whose Republican cred is not in doubt,” would be a good strategy for the GOP ticket.

“If he can bring on board a very solid mainstream Republican who has never been anything but, that would seem to strengthen the impression of voters that may have doubts about him still.”

Those ultimately picked as vice presidential nominees have to be ready for what will likely be a grueling general election campaign that will intensify in late August.

In alphabetical order, here are two lists (one Democratic, the other Republican) of those whose names have been publicly mentioned as possible vice presidential candidates:

As the highest-ranking Latino in the House of Representatives, Becerra is a dependable Clinton ally. No stranger to Washington, he has served in Congress since 1992. Becerra is chairman of the House Democratic Caucus, which works with Democratic members of Congress to help achieve consensus.

Three years into his first term in the Senate, the former Stanford University football player and Rhodes Scholar is already in the mix. Clinton and her campaign aides are impressed with Booker, an African-American, and the energy he brought to her campaign in Iowa. At 47, Booker's relative youth could excite segments of the Democratic Party.

As a representative of a critical swing state, Brown has a voting record and an economic philosophy that are appealing to progressives, who generally support Clinton's Democratic rival, Bernie Sanders. Brown knows the Washington political scene, having served in the House of Representatives from 1993 to 2007.

The former mayor of San Antonio, Texas was invited by Barack Obama to join his Cabinet in 2014. Since he has been in Washington, Castro, a Latino, has developed a good relationship with Clinton. Castro's family is no stranger to public service. His twin brother is Representative Joaquin Castro of Texas.

With strong credentials on economic issues, Donovan could help offset Trump's advantage in the polls on matters involving the economy. Prior to his current position, Donovan preceded Julian Castro as secretary of the Department of Housing and Urban Development.

A progressive, Franken has the ideological background to appeal to Sanders' supporters. With his experience as a former "Saturday Night Live" comedian, he also has the improvisational verbal skills to counterattack Trump. After several years as a comedian and writer, he turned to liberal political activism.

As past chairman of the Democratic National Committee, Kaine is an experienced political strategist and fundraiser. He is one of only about 20 people in American history to have won elections as mayor, governor and senator. Some political observers consider him an early front-runner.