LGBTQ caucus is among the largest in Congress
Sen. Tammy Baldwin, others object to the policy rider
45 senators signed a letter issued to leadership on Thursday
'Not since DOMA' has 'an anti-LGBTQ+ policy been enshrined into federal law'
51 of 52 anti-LGBTQ riders were defeated
Challenges come despite bipartisan interest in addressing the problem
President Joe Biden has vowed to block homophobic, transphobic bills
