Easy, the left spends its way out of existance. There's always pull and push forces on politics. Parties wax and wane.
The republic would be dead long before that. It would either be destroyed by civil war or be voted into a tyranny. Both are likely if the left invites another 40 million Latin Americans into their base. Revolution and protest are endemic in their cultural mindset. Huge protests happen every week over every issue imaginable in Mexico and points south. With a mere 10 million more Dem voters no amount of outrage could outvote them. And they, along with our own leftists, would vote everything they want from the national treasury.