FRANKFORT, Ky. — Sen. Rand Paul, a tea party favorite and frequent antagonist of leaders of his Republican Party, is ready to test how much change voters want, both for their government and for the GOP.

Paul is set to begin his White House campaign today, kicking off the presidential run with a rally in downtown Louisville, then jetting to early nominating states with a pitch aimed at the libertarian corners of the GOP. Paul begins the 2016 race as the second fully declared candidate, behind Sen. Ted Cruz of Texas, but he could face as many as 20 rivals for the nomination before the lead-off Iowa caucuses early next year.

Along the way, the first-term senator is likely to challenge his fellow Republicans’ views on both foreign and domestic policy, as well as the nuts and bolts of how campaigns are run. Tech savvy and youth-focused, Paul is expected to be an Internet juggernaut his competitors will be forced to chase.