Now that they're both in the Senate, Paul and Harris are teaming up on another criminal justice reform idea they know is popular back home and believe can find bipartisan support: providing aid to people who can't afford bail.
"It literally is about do you have cash sitting at home that you can afford to write the check to get out versus, you don't, you're barely making ends meet every month," Harris, D-Calif., said.
But bills co-written by Democrats and Republicans barely move past the first round of headlines and news conferences these days - even if it's backed by an unlikely pair. Just ask Paul, who earned attention in 2014 for partnering with Sen. Cory Booker, D-N.J., to try changing the nation's sentencing laws and drug control policy. The combination of a short, mop-topped ophthalmologist from rural Kentucky and the tall, bald former big-city mayor was too good to be ignored. Their plans went nowhere.
And yet, Paul is trying again.
He and Harris argue that the requirement that defendants provide some cash to get bail and earn their release keeps hundreds of thousands of defendants behind bars awaiting trial on minor charges because they can't afford to get out. They call it one of the most inequitable aspects of the criminal justice system.
"I think there's a majority - there might even be 60 votes for some of these things on criminal justice reform," Paul said in a joint interview with Harris on Tuesday. "We have to push forward."
"A lot of what we're talking about is disparities in terms of how Americans are treated in the criminal justice because of their wealth or not," she said. "Poor people, working people are treated differently, particularly on this issue of bail."
Paul and Harris' Pretrial Integrity and Safety Act is a modest proposal that would set aside $10 million in federal grant money to begin encouraging more states to drop or curtail cash bail systems and consider other factors when sorting out if a defendant should be kept behind bars before trial.
Nationwide, roughly 47 percent of felony defendants with bonds remain jailed before their cases are heard because they cannot make bail. Harris's office said the problem affects more than 450,000 people nationwide.
But a growing number of large jurisdictions are changing their policy both to ease prison populations and to account for a defendant's financial situation. For years, Washington, D.C. has released suspects awaiting trial without requiring them to leave behind any money on a promise that they will return to court and meet conditions such as checking in with a pretrial officer or submit to drug tests.
And last week, Chicago became the largest city in the nation to allow judges to consider whether people do not have enough money to pay for their release, an important victory for bail reform advocates. Under the new rules, judges can no longer set bail so high that defendants can't afford to pay for their release. The decision followed a Chicago Tribune investigation that found as many as 300 prisoners sat in the Cook County Jail because they couldn't pay $100 to post bail.
"It's not just about race it's about poverty," Paul said, noting that support for reforming the criminal justice system cuts across states, parties and ideology.
In Kentucky, state lawmakers are debating whether to expand a felony expungement law that would allow people convicted of minor drug offenses to clear their records after 10 years. That comes after the Republican governor signed a bill last year that allows people convicted of nonviolent felonies to apply to expunge their records if they stay out of trouble for five years and pay a $500 fee.
Paul said he supports such bills because, "You can't work if you have this glaring criminal record out there that prevents you from finding work."
Three years ago this summer as buzz about a potential presidential campaign hit a fever pitch, Paul traveled to Guatemala to perform free eye surgeries for hundreds of impoverished people. The stage-managed political voyage helped reveal a rarely seen side of the senator and helped stoke speculation ahead of an ultimately unsuccessful 2016 presidential campaign.
Harris, just seven months into her first Senate term, is already enduring fevered speculation about her political future. In recent weeks, she has met with top-dollar Democratic donors who backed Hillary Clinton's presidential campaign and has consented to carefully-chosen interviews with niche outlets, including a live-audience tapping of "Pod Save America," a podcast popular with young progressives, and the release of a Spotify music playlist to Blavity, a website popular with Black millennials, timed to coincide with African American Music Month.
In their only joint interview to tout their new bill, Paul indirectly encouraged Harris to explore a presidential bid - but didn't endorse her. After all, he hasn't ruled another presidential campaign some day.
"I think on the Democrat side there's a huge opening for lots and lots of people to rise up to be that person," Paul said.
Harris dismissed speculation about the presidency - but didn't deny an interest. Asked why she thinks people are talking about her as a presidential candidate, she said, "I think people like to gossip."
"I came to D.C. hoping to have impact on real human beings and their lives. And sharing with this incredible body of people - these smart, powerful people - the views and the voices of folks that might not otherwise be seen or heard," she added.
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