The top Democrat in the US Senate, Chuck Schumer, denounced what he called undue haste to confirm President-elect Donald Trump's nominations for various top posts, saying they needed more thorough vetting.
Hearings start tomorrow for the Republican president-elect's choices for senior administration posts, beginning with US Senator Jeff Sessions, nominated to be attorney general, and retired General John Kelly, Mr Trump's pick for secretary of homeland security.
Mr Schumer said Mr Trump's nominees, many of whom have extensive business backgrounds at companies such as Exxon Mobil Corp and Goldman Sachs, should be carefully scrutinised to be sure they avoid conflicts of interest.
"We're not doing this for sport. Democrats feel very strongly that pushing for a thorough and thoughtful vetting process is the right thing to do," Mr Schumer said in a speech.
Republicans, who control the majority in the Senate, are presenting the Democratic objections as political grandstanding, saying they moved quickly eight years ago to confirm Democratic President Barack Obama's cabinet picks, approving seven the day he began his first term.
Mr Schumer said some of Mr Trump's nominees have not completed a review process conducted by the US Office of Government Ethics.
Trump transition spokesman Sean Spicer said today that every nominee with a hearing this week has turned in the required paperwork.
"Everybody will be properly vetted as they have been in the past," said US Senate Republican leader Mitch McConnell, speaking in New York after meeting Mr Trump today.
He said he hoped to confirm six or seven national security appointees by the time Mr Trump takes office on 20 January.
Mr Trump needs to keep the 52 Republicans in the 100-seat Senate on his side to secure the simple majority votes needed for confirmations.
One of the more contentious hearings could be over Rex Tillerson, nominated for secretary of state.
Mr rump's stated desire for warmer relations with Russia comes at a deeply sensitive moment, after US intelligence agencies concluded that Moscow used cyber hacking and other methods to try to tilt the US presidential election in Mr Trump's favor over Democrat Hillary Clinton.
Republicans including US Senator John McCain and US Senator Lindsey Graham, veterans in foreign policy and security issues, have signaled concerns about Mr Tillerson's ties to Russia during his tenure as the chief executive of Exxon Mobil.
Transition officials expressed confidence that Mr Tillerson would win the pair's support.
Mr Sessions is also likely to face a thorough grilling, with critics arguing he obstructed civil rights protections in the past.
Democrats are likely to question nominees for positions related to national security about Mr Trump's proposal to build a wall along the US border with Mexico and his pledge to temporarily suspend immigration from regions deemed to be exporting terrorism.
The Trump team has put the nominees through mock hearings that are standard practice ahead of such events.
Questions have included the kind of everyday concerns that periodically upset nominees for high office, such as the price of a gallon of gas.
Each mock-up has also included at least one disruptive pretend protester, a transition official said.
Transition officials say they believe they could pick up Democratic votes from senators facing 2018 re-election bids in states Mr Trump won on 8 November, such as US Senator Claire McCaskill of Missouri.
Mr Trump voiced optimism about the process today, telling reporters at his office and residence in New York, "The confirmation is going great. I think they'll all pass."
Trump names son-in-law as senior adviser
Meanwhile, Mr Trump has appointed his son-in-law, Jared Kushner, to the position of senior adviser to the president, which is a potentially thorny choice in the face of anti-nepotism law.
The appointment of Mr Kushner, first reported by NBC, had been anticipated but it had been unclear what his official role would be.
Like Mr Trump, Mr Kushner is a New York-based real estate developer with a wide net of business dealings that could pose potential conflicts of interest.
Mr Kushner, who married Mr Trump's eldest daughter, Ivanka, in 2009, helped guide Republican Trump to victory over Mrs Clinton.
Mr Kushner, 35, emerged as an important voice early in Mr Trump's campaign and was involved in almost every aspect of it, from key personnel decisions to strategy and fundraising.
Mr Kushner spearheads his family's real estate development company, Kushner Companies, and is the publisher of the New York Observer weekly newspaper, which he acquired at age 25.
It was unclear how any Kushner appointment would be affected by a federal anti-nepotism law that prohibits a president from hiring family members to serve in his administration.
Mr Kushner is working with lawyers on how we would have to divest and distance himself from his family's business if he were to take a role in the Trump administration, the New York Times reported.
China's Anbang Insurance Group Co Ltd is in talks to invest in a project to redevelop a flagship New York City building owned by Kushner Companies.
The Times reported the deal on Saturday in an extensive article about Jared Kushner that detailed a November meeting between him and Anbang Chairman Wu Xiaohui just days after Mr Trump was elected president.