In the latest chapter of their faux partnership, newly elected Democratic National Committee Chief Tom Perez and Sen. Bernie Sanders have announced a national tour focusing on core Democratic issues. It all sounds so family like, the idea of everyone pulling together for the same cause, but we shouldn’t be fooled.

With less than three and a half years until the 2020 presidential election, the Democrats are much more divided than what may appear. The problem: Tour co-star Bernie Sanders and his unwillingness to share the family jewels.

After the Democratic National Committee concluded its meeting with the selection of forMER Obama Secretary of Labor Tom Perez IN FEBRUARY, the outcome seemed unifying enough, especially since Perez nominated Sanders-backed Congressman Keith Ellison for a new Deputy Chair position, or the number two spot in the party leadership. But that wasn’t enough for Sanders, who steamED from his presidential nomination loss to Hillary Clinton last year as well his treatment by the party organization.

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Opinion: Stiffing California ratepayers won’t help us reach our climate goals Less than 24 hours after the meeting’s adjournment, Sanders showed the extent of his resentment. When asked whether he would share his email donor list with the DNC, Sanders refused to commit, saying that instead he would use his list to selectively support progressive Democratic candidates mirroring Sanders’ positions. His refusal is the first step of assuring a MORE divisiveness in the Democratic Party. That’s the last thing the party needs, given its loss OF the presidency as well as both chambers of Congress last year.

Ironically, since his DNC election a month ago, Perez has tilted increasingly to the progressive wing of the Democratic Party. He has praised the Women’s March for their activism, condemned Trump’s executive order on new labor rules, and lambasted the president for seeking to deny working Americans affordable health insurance—themes that should more than satisfy Sanders. Yet, Sanders has not shown any signs of collaborating with the party leadership.

Email and mailing donor lists are particularly critical to the Democratic Party. That’s because it relies on small donations by spirited donors as a cornerstone of its success. Barack Obama became the model for soliciting small donors via the internet in 2008; they accounted for $96 million, 22 percent of his fundraising effort. That kind of resource makes Sanders’ email list extremely valuable to the Democratic Party, and Sanders knows it.

The Democratic leadership shouldn’t be surprised. After all, Sanders established a precedent of sorts last year. As a registered Independent, he pursued the Democratic nomination. Independents have run for the presidency before, but never by attaching themselves to a major political party as the vehicle to the nomination. This precedent no doubt flummoxed the party establishment, who were wary of offending Sanders supporters. In fact, the only reason they didn’t balk outright at the odd arrangement was because they wanted to capture the money and energy from the Sanders wing once Clinton sealed the nomination. But for Sanders, the damage was already done. Sanders made it clear last August that he wouldn’t turn over his email list to the DNC, even as the Clinton campaign began to fray. He is continuing with that plan today, making it difficult for the Democrats to generate true unity other than symbolically.

The Democrats have their hands full with the 2018 elections. Of the thirty-three U.S. Senate seats up for grabs, they must defend twenty five. Of the thirty-seven governorships at stake in 2017 and 2018, the Democrats, control only nine. If the Democrats hope to make a serious challenge for the presidency in 2020, they must build their infrastructure now, and that means winning elections. Simply put, the party can’t succeed without truly unifying, an effort that must include the cooperation of Bernie Sanders.

Some of the problem has to with the compositions of the two major parties. Republican values tend to run from moderate conservative to libertarian. Democrats share a wider political spectrum, ranging from moderate conservatives to ultra-liberal. Because Democrats are so much more diverse than Republican, finding a candidate that can bring them together is a much more difficult task than for the Republicans. Which is why it’s important for the party to nominate someone just left of center, someone such as Hillary Clinton.

One of oldest concepts in politics focuses on the dilemma of winning a battle in the midst of losing a war. Should Sanders hold out with his email list, Democrats will encounter intra-party factionalism they cannot afford, given past defeats. Sanders may take solace in keeping his list, but his altruism may play a large part in preventing the Democrats from being competitive in 2020.

Family reunions have their charm, but they should never be confused with the divisions within.

Larry N. Gerston is professor emeritus of political science at San Jose State University and the author of “Reviving Citizen Engagement: Policies to Renew National Community.” He wrote this for The Mercury News.