But Congress is NOT trying to stop him. There is no bill on the table that would tell the president not to pay the debts, and such legislation would be legally unenforceable if it even existed.
Instead, this is just about the president seeking to expand his power to borrow against the nation's credit. Congress is skeptical of giving him that power, as it should be under our system of checks and balances.
Should the president refuse to pay the lawful debts, that would be grounds for impeachment in my view, so he really should stop threatening it.