Lack of pre-nomination vetting combines with Democrats’ close scrutiny of several nominees.

Nearly three weeks into Donald Trump’s presidency, the Senate has approved just a handful of his cabinet picks in what has been the slowest pace of confirmations in decades. The process has been hamstrung in part by a lack of pre-nomination vetting by Mr. Trump’s transition team, according to a potential nominee and emails from the U.S. ethics office. Democratic senators have seized upon issues involving several nominees to slow down action on them, saying the nominees have complex financial records that require further examination. Tuesday, the Senate confirmed Betsy DeVos as secretary of education—just the fifth cabinet appointee approved thus far. Votes are expected this week on Jeff Sessions’ nomination for attorney general and Tom Price, who is slated to head the Department of Health and Human Services. The Senate may also act on the selection of Steve Mnuchin as Treasury secretary.

“The American people expect their senators and Congress to do our jobs and fulfill the duties to those who we represent,” Sen. John Cornyn (R., Texas) said Monday, urging quick action on Mr. Trump’s nominees. Several factors had suggested a smoother confirmation process for Mr. Trump’s cabinet nominees compared with previous presidential transitions. Republicans hold a majority of seats in the Senate, 52, and can now take advantage of a procedural change that Democrats made in 2013 that eliminated a 60-vote threshold on executive branch nominations. In addition, Mr. Trump’s staff has had access to government support and resources for transition efforts since August—far earlier than previous incoming administrations thanks to a law passed in 2010 to streamline the post-election process. That gave his transition team a head start on vetting personnel compared with previous years, though he shuffled that team shortly after winning the election and set the process back. The White House didn’t respond to a request for comment. Recent presidents have had most of their cabinet in place by the end of their second week in office. Former President Barack Obama had 12 of 15 cabinet officials at work 14 days into this administration, while George W. Bush had 13 out of 14. Before that, 13 of 14 of Bill Clinton’s cabinet departments had heads at the two-week mark, while 9 out of 14 of George H.W. Bush’s cabinet secretaries were approved.

Mr. Trump, by contrast, still has 10 open cabinet positions and numerous sub-cabinet positions unfilled 18 days into his administration and as he has moved forward with an ambitious agenda.

More From The Wall Street Journal (subscription required):