Why can't congressional Democrats deliver more on their promises? It's complicated.

Political scientists explain why congressional Democrats aren't making more progress on their priorities and President Joe Biden's big agenda.

In January, when President Joe Biden took office and Democrats secured both chambers of Congress, millions of Americans had high hopes that the laundry list of causes touted on the campaign trail would become reality.

They had promised action on voting, elections and policing reform, on immigration and infrastructure. They touted sweeping programs now in Democrats' social spending bills, addressing issues they said Americans care about most, from child care to climate change.

But this week's failure by Senate Democrats' latest effort to even start debate on a voting rights bill, their first piece of legislation to pass the House, is just the latest blow to Biden's campaign agenda and the vow Democrats made to preserve Americans' most fundamental right in the wake of the 2020 election's "Big Lie."

Many Democrats who expected more are frustrated.