WASHINGTON (NEXSTAR) - Democrats have proposed taxing billionaires to help pay for the president's Build Back Better plan, which would invest in social programs and safety nets. "People who have tens of billions of dollars - particularly with appreciated stock - don't pay any tax on that appreciation," said Sen. Mark Warner (D-VA). Oregon Sen. Ron Wyden wants to start taxing tradable assets - like stocks - that increase in value but haven't yet been sold. Warner said billionaires frequently use loans from those holdings to avoid taxes. "If that's the way they finance their lifestyle, I view that as closing a tax loophole," said Warner. The new taxes would only apply to people with more than $1 billion in assets or who make more than $100 million in income for three consecutive years. California Congressman Pete Aguilar said the taxes raised would help pay for measures like free universal pre-school, child tax credits and about $500 billion to fight climate change. "The president's committed to pay for this piece of legislation," Aguilar said. But some say taxing the ultra-wealthy to fund President Biden's plan isn't a good idea. "It's another day and another really bad idea from Democrats to fund their reckless tax and spending spree," said Sen. John Thune (R-SD). Thune called the effort "desperate." "Markets go up, markets go down - if you had a year where your total value went down by 20% then what? Do you get a refund?" he said. Democrats are also considering other alternatives to pay for the president's plan. "Imposing a 15% minimum tax to make sure large corporations pay their fair share… creating a global minimum tax," said White House Press Secretary Jen Psaki.