In Maryland's June 26 Democratic primary, Cardin faces a challenge from the left by Chelsea Manning, the Army private who passed government secrets to WikiLeaks and wants to harness anti-establishment sentiment. Three lesser-known Democrats are also running, along with a Republican and a Libertarian.
Cardin, who filed his reelection papers Monday, has a long history in Maryland politics and is favored to win in November. He was first elected to the Senate in 2006 and served before that in the House of Representatives and in the State House in Annapolis. A 2017 Goucher College poll said 45 percent of Marylanders approve of his performance and 21 percent disapprove, with 34 percent holding no opinion.
Democrats outnumber Republicans nearly 2 to 1 in Maryland, but Simon said he believes voters are willing to stray from their party affiliation, citing the election of Republican Gov. Larry Hogan in 2014 and Robert L. Ehrlich Jr. (R) in 2002.
"There are no political parties in our Constitution. That guy over there," Simon said, pointing to a picture of George Washington on the wall of his office, "in his farewell address, his number one concern was that Americans would develop more loyalty to political parties than to their country."
Simon said he also is encouraged by the number of young voters who consider themselves independent. Almost 700,000 Maryland voters have declined to register with a political party, a 58 percent increase from a decade ago, state data show.
Without blaming Cardin specifically, Simon said he believes Democrats should have worked with Republicans on the tax bill that passed in December with only GOP support, pushing for changes that would permanently lower taxes for the working class, simplify the tax code and reduce the impact on the deficit.
Simon said he voted for Hillary Clinton for president in 2016, but wasn't enthusiastic about any of his choices. A native of New York, he earned a bachelor's degree from Brown University, got his MBA from the University of Chicago and moved to Maryland in 2002.
He is taking a leave of absence from his job as chief executive of Bronfman Rothschild to run for office, and said he plans to fund his campaign through a combination of his own money and outside donations. He is chairman of the board of the Greater Washington Community Foundation and previously served on the boards of Interfaith Works and the Capital Area Food Bank.
An earlier version of this story incorrectly spelled the name of candidate Neal Simon.