Democratic Senators can make competent Presidents:

Democratic Senators who went on to be Presidents.

Harry S. Truman (MO) 1935–1945 1945–1953
John F. Kennedy (MA) 1953–1960 1961–1963
Lyndon B. Johnson (TX) 1949–1961 1963–1969
Barack Obama (IL) 2005–2008 2009–2017

Truman continued FDR's policies and supervised the end of World War II. His defeat of Dewey in 1948 allowed for the continued expansion of America's middle class.

Kennedy inspired the world, handled the Cuban Missle Crisis extremely well, and set America on course to put a man on the moon, all in under 3 years.

Johnson made the mistake of believing Republican nonsense about the "Domino Theory" and how the expansion of Communism must be stopped at all costs. Thus he threw America headlong into the horrible Vietnam War. Yet his domestic policies, including those of pushing for Civil Rights, including the Voting Rights Act, and his policies regarding the War on Poverty were truly great.

Obama rescued America from the Great Recession and gave us Obamacare.

Democratic Senators know how to get legislation through Congress. Those who go on to be Vice Presidents, also have a fundamental understanding of how the White House is supposed to work. I am very happy that our Democratic nominee will have so much Senate experience and 8 years of Vice President experience. He is the perfect antidote to a President who understands so little about how the government is supposed to work.

I don't need to fall in love with the Democratic Nominee. I just need him to do his freaking job. I may not have agreed with everything that every previous Democratic President who was a former Senator did, but every Democratic Senator who went on to become President at least knew how do to their freaking jobs, and none of them simply ignored a crisis or pretended that it would magically go away.