The colour is in tribute to women's suffrage, a century after they won the right to vote in the US. Speaker of the house Nancy Pelosi is among those wearing white.
The Nineteenth Amendment – which says all US citizens have the right to vote regardless of their sex – became part of the constitution in August 1920.
Several Democrats did this at last year’s address in a silent but powerful protest against gender inequality.
As well as wearing ‘suffrage white’, some in attendance have green Equal Rights Amendment pins on ahead of an expected House vote on the issue this month.
Others are sporting red-white-and-blue-striped lapel pins to highlight climate change.
Several notable Democrats are boycotting this year’s State of the Union, many of whom wore white last year.
Congresswoman Alexandria Ocasio-Cortez announced she would not be attending the ceremony because of President Trump’s “lawless conduct and subversion of the Constitution.”
“None of this is normal, and I will not legitimize it,” Ms Ocasio-Cortez posted on her Twitter account.
In addition to her Ayanna Pressley, Al Green, Earl Blumenauer, Steve Cohen and Joe Wilson have all said they are not attending.
Additional reporting by agencies