Spend the political capital on a bill called "A pathway to citizenship for undocumented aliens who otherwise haven't done anything wrong" (Editor's note: I split the difference between 'illegal aliens' and the left's more obfuscatory term 'undocumented persons'). Spend the political capital on the debate and get it done.

They have. Proposals to do that have been frequently proffered, from time to time. But all have failed. Every effort at what has been called "comprehensive immigration reform" has foundered. Typically these proposals involve a path to citizenship for aliens who otherwise haven't done anything wrong combined with strengthening border security. But invariably, they generate a ton of political opposition from immigration hardliners in the GOP, who fundamentally oppose the idea of a path to citizenship being included in such a bill.

See, for example:

en.wikipedia.org - Border Security, Economic Opportunity, and Immigration Modernization Act of

....which garnered 14 Republican votes in the Senate, but died without getting a vote in the House. And that wasn't a "clean" pathway to citizenship bill - it had provisions to strengthen border security and ratchet up internal enforcement, but it wasn't enough to overcome fierce opposition from the GOP base against providing any pathway to citizenship.

So you end up with a patchwork of local regulations that seek to protect those who otherwise haven't done anything wrong - ie, sanctuary policies. They try to minimize the degree to which state and local governments (especially LEO's) collect immigration status information and/or voluntarily perform functions at the request of the federal government.